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Earth is also nominated for a Webby, for Best Science Website. <a href=\"https://vote.webbyawards.com/PublicVoting#/2017/websites/general-website/science\" target=\"_blank\">Vote here</a>.</em></p><p>In theory this is not the only Universe that might exist, and in many others, identical copies of us can be found.</p><p>The question is, how do we get there? </p><p>BBC Earth's Melissa Hogenboom goes on the hunt for her cosmic twin.</p><p><em>Melissa Hogenboom is BBC Earth's feature writer. She is </em><a href=\"https://twitter.com/melissasuzanneh\"><em>@melissasuzanneh</em></a><em> on Twitter. Video produced by Pierangelo Pirak; he is <a href=\"https://twitter.com/ppirak\">@ppirak</a> on twitter. </em></p><p><em>Join over five million BBC Earth fans by liking us on </em><a href=\"https://www.facebook.com/bbcearth\"><em>Facebook</em></a><em>, or follow us on </em><a href=\"https://twitter.com/bbcearth\"><em>Twitter</em></a><em> and </em><a href=\"https://instagram.com/bbcearth/\"><em>Instagram</em></a><em>.</em></p><p><em>If you liked this story, </em><a href=\"http://pages.emails.bbc.com/subscribe/?ocid=ear.bbc.email.we.email-signup\"><em>sign up for the weekly bbc.com features newsletter</em></a><em> called \"If You Only Read 6 Things This Week\". A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday</em>.</p>","BusinessUnit":"worldwide","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[{"Content":{"AssetImage":[{"Content":{"Copyright":"NPS Photo/Alamy","FileSizeBytes":3324488,"MimeType":"image/jpeg","SourceHeight":3159,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2l/56/p02l56j9.jpg","SourceWidth":5616,"SynopsisLong":"Delicate Arch in Utah, with the Milky Way in the background (Credit: NPS Photo/Alamy)","SynopsisMedium":"Delicate Arch in Utah, with the Milky Way in the background (Credit: NPS Photo/Alamy)","SynopsisShort":"Delicate Arch in Utah, with the Milky Way in the background (Credit: NPS 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Physicists have proposed several candidate forms of \"multiverse\", each made possible by a different aspect of the laws of physics.</p><p>The trouble is, virtually by definition we probably cannot ever visit these other universes to confirm that they exist. So the question is, can we devise other ways to test for the existence of entire universes that we cannot see or touch?</p><p>{\"image\":{\"pid\":\"p03n5pgh\"}}</p><p><strong>Worlds within worlds</strong></p><p>In at least some of these alternative universes, it has been suggested, we have doppelgängers living lives much like – perhaps almost identical to – our own.</p><blockquote><p> Giordano Bruno speculated that the Universe might be infinite </p></blockquote><p>That idea tickles our ego and awakens our fantasies, which is doubtless why the multiverse theories, however far-out they seem, enjoy so much popularity. We have embraced alternative universes in works of fiction ranging from Philip K. Dick's <em>The Man in the High Castle</em> to movies like <em>Sliding Doors</em>.</p><p>Indeed, there is nothing new about the idea of a multiverse, as philosopher of religion Mary-Jane Rubenstein explains in her 2014 book <em>Worlds Without End</em>.</p><p>In the mid-16th century, Copernicus argued that the Earth is not the centre of the Universe. Several decades later, Galileo's telescope showed him stars beyond measure: a glimpse of the vastness of the cosmos.</p><p>So at the end of the 16th century, the Italian philosopher Giordano Bruno speculated that the Universe might be infinite, populated by an infinite number of inhabited worlds.</p><p>{\"image\":{\"pid\":\"p03n5pbc\"}}</p><p>The idea of a Universe containing many solar systems became commonplace in the 18th Century.</p><p>By the early 20th Century, the Irish physicist Edmund Fournier d'Albe was even suggesting that there might be an infinite regression of \"nested\" universes at different scales, ever larger and ever smaller. In this view, an individual atom might be like a real, inhabited solar system.</p><p>Scientists today reject that notion of a \"Russian doll\" multiverse, but they have postulated several other ways in which multiverses might exist. Here are five of them, along with a rough guide to how likely they are.</p><p>{\"image\":{\"pid\":\"p03n5r5f\"}}</p><p><strong>The patchwork universe</strong></p><p>The simplest multiverse is a consequence of the infinite size of our own Universe.</p><blockquote><p> There must be worlds identical to Earth somewhere out there </p></blockquote><p>We do not actually know if the Universe is infinite, but we cannot rule it out. If it is, then it must be divided into a patchwork of regions that cannot see one another.</p><p>This is simply because the regions are too far apart for light to have crossed the distance. Our Universe is only 13.8 billion years old, so any regions further than 13.8 billion light years apart are utterly cut off.</p><p>To all intents and purposes, these regions are separate universes. But they will not stay that way: eventually light will cross the divide and the universes will merge.</p><p>If our Universe really does contain an infinite number of \"island universes\" like ours, with matter and stars and planets, there must be worlds identical to Earth somewhere out there.</p><p>{\"image\":{\"pid\":\"p03n5qky\"}}</p><p>It may sound incredibly unlikely that atoms should come together by chance into an exact replica of Earth, or a replica that is exact except for the colour of your socks. But in a genuine infinity of worlds, even that strange place must exist. In fact, it must exist countless times.</p><blockquote><p> The Universe began as an infinitesimally tiny point and then expanded incredibly fast in a super-heated fireball </p></blockquote><p>If so, then somewhere almost unimaginably far off, a being identical to me is typing out these words, and wondering if his editor is going to insist on radical revisions. <em>[nice try Phil - ed]</em></p><p>By the same logic, rather farther away there is an entire observable universe identical to ours. This distance can be estimated at about 10 to the power 10 to the power 118 metres.</p><p>It is possible that this is not the case at all. Maybe the Universe is not infinite. Or even if it is, maybe all the matter is concentrated in our corner of it, in which case most of the other universes could be empty. But there is no obvious reason why that should be, and no sign so far that matter gets sparser the farther away we look.</p><p>{\"image\":{\"pid\":\"p03n5rcw\"}}</p><p><strong>The inflationary multiverse</strong></p><p>The second multiverse theory arises from our best ideas about <a href=\"http://www.bbc.com/earth/story/20141106-why-does-anything-exist-at-all\">how our own Universe began</a>.</p><p>According to the predominant view of the Big Bang, the Universe began as an infinitesimally tiny point and then expanded incredibly fast in a super-heated fireball. A fraction of a second after this expansion began, it may have fleetingly accelerated at a truly enormous rate, far faster than the speed of light. This burst is called \"inflation\".</p><blockquote><p> There are many, perhaps infinitely many, universes appearing and growing all the time </p></blockquote><p>Inflationary theory explains why the Universe is relatively uniform everywhere we look. Inflation blew up the fireball to a cosmic scale before it had a chance to get too clumpy.</p><p>However, that primordial state would have been ruffled by tiny chance variations, which also got blown up by inflation. These fluctuations are now preserved in the cosmic microwave background radiation, the faint afterglow of the Big Bang. This radiation pervades the Universe, but it is not perfectly uniform.</p><p>Several satellite-based telescopes have mapped out these variations in fine detail, and compared them to those predicted by inflationary theory. The match is almost unbelievably good, suggesting that inflation really did happen.</p><p>{\"image\":{\"pid\":\"p03n5qs0\"}}</p><p>This suggests that we can understand how the Big Bang happened – in which case we can reasonably ask if it happened more than once.</p><p>The current view is that the Big Bang happened when a patch of ordinary space, containing no matter but filled with energy, appeared within a different kind of space called the \"false vacuum\". It then grew like an expanding bubble.</p><blockquote><p> Perhaps our Universe is simply one of a crowd </p></blockquote><p>But according to this theory, the false vacuum should also experience a kind of inflation, causing it to expand at fantastic speed. Meanwhile, other bubble universes of \"true vacuum\" can appear within it – and not just, like our Universe, 13.8 billion years ago, but constantly.</p><p>This scenario is called \"eternal inflation\". It suggests there are many, perhaps infinitely many, universes appearing and growing all the time. But we can never reach them, even if we travel at the speed of light forever, because they are receding too fast for us ever to catch up.</p><p>The UK Astronomer Royal Martin Rees suggests that the inflationary multiverse theory represents a \"fourth Copernican revolution\": the fourth time that we have been forced to downgrade our status in the heavens. After Copernicus suggested Earth was just one planet among others, we realized that our Sun is just one star in our galaxy, and that other stars might have planets. Then we discovered that our galaxy is just one among countless more in an expanding Universe. And now perhaps our Universe is simply one of a crowd.</p><p>{\"image\":{\"pid\":\"p03n5q6r\"}}</p><p>We do not yet know for sure if inflationary theory is true.</p><p>However, if eternal inflation does create a multiverse from an endless series of Big Bangs, it could help to resolve one of the biggest problems in modern physics.</p><blockquote><p> The fundamental constants of the laws of physics seem bizarrely fine-tuned to the values needed for life to exist </p></blockquote><p>Some physicists have long been searching for a \"<a href=\"http://www.bbc.com/earth/story/20150409-can-science-ever-explain-everything\">theory of everything</a>\": a set of basic laws, or perhaps just a single equation, from which all the other principles of physics can be derived. But they have found there are more alternatives to choose from than there are fundamental particles in the known universe.</p><p>Many physicists who delve into these waters believe that an idea called string theory is the best candidate for a \"final theory\". But the latest version offers a huge number of distinct solutions: 1 followed by 500 zeros. Each solution yields its own set of physical laws, and we have no obvious reason to prefer one over any other.</p><p>The inflationary multiverse relieves us of the need to choose at all. If parallel universes have been popping up in an inflating false vacuum for billions of years, each could have different physical laws, determined by one of these many solutions to string theory.</p><p>If that is true, it could help us explain a strange property of our own Universe.</p><p>{\"image\":{\"pid\":\"p03n5q47\"}}</p><p>The fundamental constants of the laws of physics seem bizarrely fine-tuned to the values needed for life to exist.</p><blockquote><p> Things have to be the way we find them: if they were not, we would not be here and the question would never arise </p></blockquote><p>For example, if the strength of the electromagnetic force were just a little different, atoms would not be stable. Just a 4% change would prevent all nuclear fusion in stars, the process that makes the carbon atoms our bodies are largely made of.</p><p>Similarly, there is a delicate balance between gravity, which pulls matter towards itself, and so-called dark energy, which does the opposite and makes the Universe expand ever faster. This is just what is needed to make stars possible while not collapsing the Universe on itself.</p><p>In this and several other ways, <a href=\"http://www.bbc.com/earth/story/20160229-the-place-where-you-can-walk-through-the-universe\">the Universe seems fine-tuned to host us</a>. This has made some people suspect the hand of God.</p><p>Yet an inflationary multiverse, in which all conceivable physical laws operate somewhere, offers an alternative explanation.</p><p>{\"image\":{\"pid\":\"p03n5pcw\"}}</p><p>In every universe set up in this life-friendly way, the argument goes, intelligent beings will be scratching their heads trying to understand their luck. In the far more numerous universes that are set up differently, there is no one to ask the question.</p><blockquote><p> If you don't want God, you'd better have a multiverse </p></blockquote><p>This is an example of the \"anthropic principle\", which says that things have to be the way we find them: if they were not, we would not be here and the question would never arise.</p><p>For many physicists and philosophers, this argument is a cheat: a way to evade rather than explain the fine-tuning problem.</p><p>How can we test these assertions, they ask? Surely it is defeatist to accept that there is no reason why the laws of nature are what they are, and simply say that in other universes they are different?</p><p>The trouble is, unless you have some other explanation for fine-tuning, someone will assert that God must have set things up this way. The astrophysicist Bernard Carr has put it bluntly: \"<a href=\"http://discovermagazine.com/2008/dec/10-sciences-alternative-to-an-intelligent-creator\">If you don't want God, you'd better have a multiverse</a>\".</p><p>{\"image\":{\"pid\":\"p03n5pl4\"}}</p><p><strong>Cosmic natural selection</strong></p><p>Another kind of multiverse avoids what some see as the slipperiness of this reasoning, offering a solution to the fine-tuning problem without invoking the anthropic principle.</p><blockquote><p> A \"mother\" universe can give birth to \"baby\" universes </p></blockquote><p>It was formulated by <a href=\"http://leesmolin.com/\">Lee Smolin</a> of the Perimeter Institute for Theoretical Physics in Waterloo, Canada. In 1992 <a href=\"http://dx.doi.org/10.1088/0264-9381/9/1/016\" target=\"_blank\">he proposed</a> that universes might reproduce and evolve rather like living things do.</p><p>On Earth, natural selection favours the emergence of \"useful\" traits such as fast running or opposable thumbs. In the multiverse, Smolin argues, there might be some pressure that favours universes like ours. He calls this \"cosmological natural selection\".</p><p>Smolin's idea is that a \"mother\" universe can give birth to \"baby\" universes, which form inside it. The mother universe can do this if it contains black holes.</p><p>{\"image\":{\"pid\":\"p03n5r7s\"}}</p><p>A <a href=\"http://www.bbc.com/earth/story/20150525-a-black-hole-would-clone-you\">black hole</a> forms when a huge star collapses under the pull of its own gravity, crushing all the atoms together until they reach infinite density.</p><blockquote><p> It is a neat idea, because our Universe then does not have to be the product of pure chance </p></blockquote><p>In the 1960s, Stephen Hawking and Roger Penrose pointed out that <a href=\"http://www.bbc.com/earth/story/20160107-these-are-the-discoveries-that-made-stephen-hawking-famous\">this collapse is like a mini-Big Bang in reverse</a>. This suggested to Smolin that a black hole could become a Big Bang, spawning an entire new universe within itself.</p><p>If that is so, then the new universe might have slightly different physical properties from the one that made the black hole. This is like the random genetic mutations that mean baby organisms are different from their parents.</p><p>If a baby universe has physical laws that permit the formation of atoms, stars and life, it will also inevitably contain black holes. That will mean it can have more baby universes of its own. Over time, universes like this will become more common than those without black holes, which cannot reproduce.</p><p>{\"image\":{\"pid\":\"p03n5r15\"}}</p><p>It is a neat idea, because our Universe then does not have to be the product of pure chance. If a fine-tuned universe arose at random, surrounded by many other universes that were not fine-tuned, cosmic natural selection would mean that fine-tuned universes subsequently became the norm.</p><blockquote><p> So far, there is no evidence that this is the case </p></blockquote><p>The details of the idea are a little woolly, but Smolin points out that it has one big advantage: we can test it.</p><p>For example, if Smolin is right we should expect our Universe to be especially suited to making black holes. This is a rather more demanding criterion than simply saying it should support the existence of atoms.</p><p>But so far, there is no evidence that this is the case – let alone proof that a black hole really can spawn an entirely new universe.</p><p>{\"image\":{\"pid\":\"p03n5qhf\"}}</p><p><strong>The brane multiverse</strong></p><p>When <a href=\"http://www.bbc.com/earth/story/20151118-watch-this-video-to-understand-the-biggest-idea-in-physics\">Albert Einstein's theory of general relativity</a> began to come to public attention in the 1920s, many people speculated about the \"fourth dimension\" that Einstein had allegedly invoked. What might be in there? A hidden universe, maybe?</p><blockquote><p> Perhaps the fifth dimension was curled up into an unimaginably small distance </p></blockquote><p>This was nonsense. Einstein was not proposing a new dimension. What he was saying was that time is a dimension, similar to the three dimensions of space. All four are woven into a single fabric called space-time, which matter distorts to produce gravity.</p><p>Even so, other physicists were already starting to speculate about genuinely new dimensions in space.</p><p>The first intimation of hidden dimensions began with the work of the theoretical physicist Theodor Kaluza. <a href=\"http://homepage.uibk.ac.at/~c705204/pdf/kaluza-1921.pdf\">In a 1921 paper</a> Kaluza showed that, by adding an extra dimension to the equations of Einstein's theory of general relativity, he could obtain an extra equation that seemed to predict the existence of light.</p><p>That looked promising. But where, then, was this extra dimension?</p><p>{\"image\":{\"pid\":\"p03n5q1d\"}}</p><p>The Swedish physicist Oskar Klein <a href=\"http://www.nature.com/nature/journal/v118/n2971/abs/118516a0.html\">offered an answer in 1926</a>. Perhaps the fifth dimension was curled up into an unimaginably small distance: about a billion-trillion-trillionth of a centimetre.</p><blockquote><p> In the modern version of string theory, known as M-theory, there are up to seven hidden dimensions </p></blockquote><p>The idea of a dimension being curled may seem strange, but it is actually a familiar phenomenon. A garden hose is a three-dimensional object, but from far enough away it looks like a one-dimensional line, because the other two dimensions are so small. Similarly, it takes so little time to cross Klein's extra dimension that we do not notice it.</p><p>Physicists have since taken Kaluza and Klein's ideas much further in string theory. This seeks to explain fundamental particles as the vibrations of even smaller entities called strings.</p><p>When string theory was developed in the 1980s, it turned out that it could only work if there were extra dimensions. In the modern version of string theory, known as M-theory, there are up to seven hidden dimensions.</p><p>{\"image\":{\"pid\":\"p03n5py2\"}}</p><p>What's more, these dimensions need not be compact after all. They can be extended regions called branes (short for \"membranes\"), which may be multi-dimensional.</p><blockquote><p> If branes collide, the results could be monumental </p></blockquote><p>A brane might be a perfectly adequate hiding place for an entire universe. M-theory postulates a multiverse of branes of various dimensions, coexisting rather like a stack of papers.</p><p>If this is true, there should be a new class of particles called Kaluza-Klein particles. In theory we could make them, perhaps in a particle accelerator like the Large Hadron Collider. They would have distinctive signatures, because some of their momentum is carried in the hidden dimensions.</p><p>These brane worlds should remain quite distinct and separate from each other, because forces like gravity do not pass between them. But if branes collide, the results could be monumental. Conceivably, such a collision could have triggered our own Big Bang.</p><p>{\"image\":{\"pid\":\"p03n5rfb\"}}</p><p>It has also been proposed that gravity, uniquely among the fundamental forces, might \"leak\" between branes. This leakage could explain why gravity is so weak compared to the other fundamental forces.</p><blockquote><p> If their idea is true, there is an awful lot of space out there for other universes </p></blockquote><p>As <a href=\"https://www.physics.harvard.edu/people/facpages/randall\">Lisa Randall</a> of Harvard University puts it: \"if gravity is spread out over large extra dimensions, its force would be diluted.\"</p><p>In 1999, Randall and her colleague Raman Sundrum suggested that the branes do not just carry gravity, <a href=\"https://dx.doi.org/10.1103/PhysRevLett.83.3370\">they produce it by curving space</a>. In effect this means that a brane \"concentrates\" gravity, so that it looks weak in a second brane nearby.</p><p>This could also explain why we could live on a brane with infinite extra dimensions without noticing them. If their idea is true, there is an awful lot of space out there for other universes.</p><p>{\"image\":{\"pid\":\"p03n5qfx\"}}</p><p><strong>The quantum multiverse</strong></p><p>The theory of quantum mechanics is one of the most successful in all of science. It explains the behaviour of very small objects, such as atoms and their constituent fundamental particles. It can predict all kinds of phenomena, from the shapes of molecules to the way light and matter interact, with phenomenal accuracy.</p><p>Quantum mechanics treats particles as if they are waves, and describes them with a mathematical expression called a wave function.</p><blockquote><p> When we make a measurement we only see one of those realities, but the others also exist </p></blockquote><p>Perhaps the strangest feature of a wave function is that it allows a quantum particle to exist in several states at once. This is called a superposition.</p><p>But superpositions are generally destroyed as soon as we measure the object in any way. An observation \"forces\" the object to \"choose\" one particular state.</p><p>This switch from a superposition to a single state, caused by measurement, is called \"collapse of the wave function\". The trouble is, it is not really described by quantum mechanics, so no one knows how or why it happens.</p><p><a href=\"http://www-tc.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf\">In his 1957 doctoral thesis</a>, the American physicist Hugh Everett suggested that we might stop fretting about the awkward nature of wave function collapse, and just do away with it.</p><p>{\"image\":{\"pid\":\"p03n5pn9\"}}</p><p>Everett suggested that objects do not switch from multiple states to a single state when they are measured or observed. Instead, all the possibilities encoded in the wave function are equally real. When we make a measurement we only see one of those realities, but the others also exist.</p><p>This is known as the \"many worlds interpretation\" of quantum mechanics.</p><blockquote><p> To avoid wave function collapse, you must make another universe </p></blockquote><p>Everett was not very specific about where these other states actually exist. But in the 1970s, the physicist <a href=\"http://www.utexas.edu/faculty/council/2006-2007/memorials/dewitt/dewitt.html\">Bryce DeWitt</a> argued that each alternative outcome must exist in a parallel reality: another world.</p><p>Suppose you conduct an experiment in which you measure the path of an electron. In this world it goes one way, but in another world it goes another way.</p><p>That requires a parallel apparatus for the electron to pass through. It also requires a parallel you to measure it. In fact you have to build an entire parallel universe around that one electron, identical in all respects except where the electron went.</p><p>In short, to avoid wave function collapse, you must make another universe.</p><p>{\"image\":{\"pid\":\"p03n5r9r\"}}</p><p>This picture really gets extravagant when you appreciate what a measurement is. In DeWitt's view, any interaction between two quantum entities, say <a href=\"http://www.bbc.com/earth/story/20150731-what-is-a-ray-of-light-made-of\">a photon of light</a> bouncing off an atom, can produce alternative outcomes and therefore parallel universes.</p><blockquote><p> The quantum multiverse must be in some sense real, because quantum theory demands it and quantum theory works </p></blockquote><p>As DeWitt put it, \"every quantum transition taking place on every star, in every galaxy, in every remote corner of the Universe is splitting our local world on earth into myriads of copies.\"</p><p>Not everyone sees Everett's many-worlds interpretation this way. Some say it is largely a mathematical convenience, and that we cannot say anything meaningful about the contents of those alternative universes.</p><p>But others take seriously the idea that there are countless other \"yous\", created every time a quantum measurement is made. The quantum multiverse must be in some sense real, they say, because quantum theory demands it and quantum theory works.</p><p>You either buy that argument or you do not. But if you accept it, you must also accept something rather unsettling.</p><p>{\"image\":{\"pid\":\"p03n6lxq\"}}</p><p>The other kinds of parallel universes, such as those created by eternal inflation, are truly \"other worlds\". They exist somewhere else in space and time, or in other dimensions. They might contain exact copies of you, but those copies are separate, like a body double living on another continent.</p><blockquote><p> Who are we to judge what is weird and what is not? </p></blockquote><p>In contrast, the other universes of the many-worlds interpretation do not exist in other dimensions or other regions of space. Instead, they are right here, superimposed on our Universe but invisible and inaccessible. The other selves they contain really are \"you\".</p><p>In fact, there is no meaningful \"you\" at all. \"You\" are becoming distinct beings an absurd number of times every second: just think of all the quantum events that happen as a single electrical signal travels along a single neuron in your brain. \"You\" vanish into the crowd.</p><p>In other words, an idea that started out as a mathematical convenience ends up implying that there is no such thing as individuality.</p><p>{\"image\":{\"pid\":\"p03n5q8z\"}}</p><p><strong>Testing the multiverse</strong></p><p>Given the strange implications of parallel universes, you might be forgiven some skepticism about whether they exist.</p><p>But who are we to judge what is weird and what is not? Scientific ideas stand or fall, not by how they \"feel\" to us, but by experimental testing.</p><p>And that is the problem. An alternative universe is separate from our own. By definition, it is beyond reach and out of sight. On the whole, multiverse theories cannot be tested by looking for those other worlds.</p><p>Yet even if other universes cannot be experienced directly, it might be possible to find evidence to support the reasoning behind them.</p><p>{\"image\":{\"pid\":\"p03n5r30\"}}</p><p>For example, we could find strong evidence for the inflationary theory of the Big Bang. That would strengthen, but not prove, the case for an inflationary multiverse.</p><blockquote><p> It's proven remarkably hard to write down a theory which produces exactly the universe we see and nothing more </p></blockquote><p>Some cosmologists have proposed that an inflationary multiverse might be more directly tested. A collision between our expanding bubble universe and another one should leave detectable traces in the cosmic microwave background – if we were close enough to see them.</p><p>Similarly, experiments envisaged for the <a href=\"http://www.bbc.co.uk/news/technology-35780444\">Large Hadron Collider</a> could search for evidence of the additional dimensions and particles implied by the braneworld theory.</p><p>Some argue that experimental verification is over-rated anyway. They say <a href=\"https://www.quantamagazine.org/20151216-physicists-and-philosophers-debate-the-boundaries-of-science/\">we can gauge the validity of a scientific idea by other means</a>, such as whether it rests on sound logic spun from premises that do have observational support.</p><p>Finally, we might make statistical predictions.</p><p>For example, we could use the inflationary multiverse theory to predict which values of the physical constants would be expected in most universes, and then see whether these are close to the ones we see – on the basis that there is no reason to expect us to be anywhere special in the multiverse.</p><p>{\"image\":{\"pid\":\"p03n5prt\"}}</p><p>At any rate, it does seem odd that the multiverse keeps cropping up wherever we look. \"It's proven remarkably hard to write down a theory which produces exactly the universe we see and nothing more,\" says physicist Max Tegmark.</p><blockquote><p> Multiverse theories cannot be tested by looking for those other worlds </p></blockquote><p>Even so, it is not clear that newspaper headlines will announce the discovery of another universe any time soon. Right now, these ideas lie on the border of physics and metaphysics.</p><p>In the absence of any evidence, then, here is a rough-and-ready – and frankly subjective – ranking of the probabilities of the various multiverses, the most likely first.</p><p><strong>The patchwork multiverse</strong> is hard to avoid – if our Universe really is infinite and uniform.</p><p><strong>The inflationary multiverse</strong> is rather likely if inflationary theory is true, and right now inflation is our best explanation for the Big Bang.</p><p><strong>Cosmic natural selection</strong> is an ingenious idea but involves speculative physics, and there are a lot of unanswered questions.</p><p><strong>Brane worlds</strong> are far more speculative, because they can only exist if all those extra dimensions do, and there is no direct evidence of that.</p><p><strong>The quantum multiverse</strong> is arguably the simplest interpretation of quantum theory, but it is also vaguely defined and leads to an incoherent view of selfhood.</p>","BusinessUnit":"worldwide","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[{"Content":{"AssetImage":[],"Campaign":null,"CollectionOverrides":null,"CollectionType":"column","Description":"The biggest questions about life, the universe and everything - and how to answer them","Name":"The Big Questions","Partner":null,"PrimaryVertical":"wwearth","Title":"The Big Questions","CreationDateTime":"2015-08-31T16:08:19.672121Z","Entity":"collection","Guid":"b223f348-1133-4674-aa14-89c671bab02b","Id":"wwearth/column/the-big-questions","ModifiedDateTime":"2015-09-03T15:02:18.264474Z","Project":"wwearth","Slug":"column/the-big-questions"},"Metadata":{"CreationDateTime":"2015-08-31T16:08:19.672121Z","Entity":"collection","Guid":"b223f348-1133-4674-aa14-89c671bab02b","Id":"wwearth/column/the-big-questions","ModifiedDateTime":"2015-09-03T15:02:18.264474Z","Project":"wwearth","Slug":"column/the-big-questions"},"Urn":"urn:pubstack:jative:collection:wwearth/column/the-big-questions","_id":"5db313aa200e03609818179a"}],"DisableAdverts":false,"DisplayDate":"2016-03-21T07:00:00Z","Geolocation":null,"HeadlineLong":"Why there might be many more universes besides our own","HeadlineShort":"Why parallel universes might be real","HideRelated":false,"Horizontal":null,"HyperHorizontal":null,"Intro":"The idea of parallel universes may seem bizarre, but physics has found all sorts of reasons why they should exist","IsSyndicated":true,"Latitude":"","Location":null,"Longitude":"","Option":[{"Content":{"Description":"Apple News Publish: Select to publish, remove to unpublish. (Do not just delete or unpublish the story)","Name":"publish-applenews-system-1"},"Metadata":{"CreationDateTime":"2016-02-05T14:32:31.186819Z","Entity":"option","Guid":"13f4bc85-ae27-4a34-9397-0e6ad3619619","Id":"option/publish-applenews-system-1","ModifiedDateTime":"2016-02-05T14:32:31.186819Z","Project":"","Slug":"publish-applenews-system-1"},"Urn":"urn:pubstack:jative:option:option/publish-applenews-system-1","_id":"5db3143b200e0360981d29ad"}],"Partner":null,"PrimaryVertical":"wwearth","Programme":null,"RelatedStory":[{"Content":{"AssetCustom":"","AssetIbroadcast":null,"AssetImage":[],"AssetImagePromo":null,"AssetInfographic":"","AssetInline":[],"AssetSelect":"","AssetVideoIb2":null,"AssetVideoMps":null,"Author":[],"BodyHtml":"<p>Don't panic, but our planet is doomed. It's just going to take a while. Roughly 6 billion years from now, <a href=\"http://www.bbc.com/earth/story/20150323-how-long-will-life-on-earth-last\">the Earth will probably be vaporized when the dying Sun expands into a red giant and engulfs our planet</a>.</p><p>But the Earth is just one planet in the solar system, the Sun is just one of hundreds of billions of stars in the galaxy, and there are hundreds of billions of galaxies in the observable universe. What's in store for all of that? How does the universe end?</p><p>The science is much less settled on how that will happen. We're not even sure if the universe will come to a firm, defined end, or just slowly tail off. Our best understanding of physics suggests there are several options for the universal apocalypse. It also offers some hints on how we might, just maybe, survive it.</p><p>{\"image\":{\"pid\":\"p02spg7d\"}}</p><p>Our first clue to the end of the universe comes from thermodynamics, the study of heat. Thermodynamics is the wild-eyed street preacher of physics, bearing a cardboard placard with a simple warning: \"THE HEAT DEATH IS COMING\".</p><blockquote><p> The heat death is far worse than being burnt to a crisp </p></blockquote><p>Despite the name, the heat death of the universe isn't a fiery inferno. Instead, it's the death of all differences in heat.</p><p>This may not sound scary, but the heat death is far worse than being burnt to a crisp. That's because nearly everything in everyday life requires some kind of temperature difference, either directly or indirectly.</p><p>For instance, your car runs because it's hotter inside its engine than outside. Your computer runs on electricity from the local power plant, which probably works by heating water and using that to power a turbine. And you run on food, which exists thanks to the enormous temperature difference between the Sun and the rest of the universe.</p><p>{\"image\":{\"pid\":\"p02spgc2\"}}</p><p>However, once the universe reaches heat death, everything everywhere will be the same temperature. That means nothing interesting will ever happen again.</p><blockquote><p> Heat death looked like the only possible way the universe could end </p></blockquote><p>Every star will die, nearly all matter will decay, and eventually all that will be left is a sparse soup of particles and radiation. Even the energy of that soup will be sapped away over time by the expansion of the universe, leaving everything just a fraction of a degree above absolute zero.</p><p>In this \"Big Freeze\", the universe ends up uniformly cold, dead and empty.</p><p>After the development of thermodynamics in the early 1800s, heat death looked like the only possible way the universe could end. But 100 years ago, Albert Einstein's theory of general relativity suggested that the universe had a far more dramatic fate.</p><p>{\"image\":{\"pid\":\"p02spf9f\"}}</p><p>General relativity says that matter and energy warp space and time. This relationship between space-time and matter-energy (stuff) — between the stage and the actors on it — extends to the entire universe. The stuff in the universe, according to Einstein, determines the ultimate fate of the universe itself.</p><blockquote><p> The universe began as something incredibly small, and then expanded incredibly quickly </p></blockquote><p>The theory predicted that the universe as a whole must either be expanding or contracting. It could not stay the same size. Einstein realized this in 1917, and was so reluctant to believe it that he fudged his own theory.</p><p>Then in 1929, the American astronomer Edwin Hubble found hard evidence that the universe was expanding. Einstein changed his mind, calling his previous insistence on a static universe the \"greatest blunder\" of his career.</p><p>If the universe is expanding, it must once have been much smaller than it is now. This realization led to the Big Bang theory: the idea that the universe began as something incredibly small, and then expanded incredibly quickly. We can see the \"afterglow\" of the Big Bang even today, in the cosmic microwave background radiation – a constant stream of radio waves, coming from all directions in the sky.</p><p>{\"image\":{\"pid\":\"p02spfrh\"}}</p><p>The fate of the universe, then, hinges on a very simple question: will the universe continue to expand, and how quickly?</p><blockquote><p> If there's too much stuff, the expansion of the universe will slow down and stop </p></blockquote><p>For a universe containing normal \"stuff\", such as matter and light, the answer to this question depends on how much stuff there is. More stuff means more gravity, which pulls everything back together and slows the expansion.</p><p>As long as the amount of stuff doesn't go over a critical threshold, the universe will continue to expand forever, and eventually suffer heat death, freezing out.</p><p>But if there's too much stuff, the expansion of the universe will slow down and stop. Then the universe will begin to contract. A contracting universe will shrink smaller and smaller, getting hotter and denser, eventually ending in a fabulously compact inferno, a sort of reverse Big Bang known as the Big Crunch.</p><p>{\"image\":{\"pid\":\"p02spf4l\"}}</p><p>For most of the 20th century, astrophysicists weren't sure which of these scenarios would play out. Would it be the Big Freeze or the Big Crunch? Ice or fire?</p><blockquote><p> Dark energy pulls the universe apart </p></blockquote><p>They tried to perform a cosmic census, adding up how much stuff there is in our universe. It turned out that we're strangely close to the critical threshold, leaving our fate uncertain.</p><p>That all changed at the end of the 20th century. In 1998, two competing teams of astrophysicists made an astonishing announcement: <a href=\"http://dx.doi.org/10.1086/300499\">the expansion of the universe</a> is <a href=\"http://dx.doi.org/10.1086/307221\">speeding up</a>.</p><p>Normal matter and energy can't make the universe behave this way. This was the first evidence of a fundamentally new kind of energy, dubbed \"dark energy\", which didn't behave like anything else in the cosmos.</p><p>Dark energy pulls the universe apart. We still don't understand what it is, but roughly 70% of the energy in the universe is dark energy, and that number is growing every day.</p><p>{\"image\":{\"pid\":\"p02spg64\"}}</p><p>The existence of dark energy means that the amount of stuff in the universe doesn't get to determine its ultimate fate.</p><p>Instead, dark energy controls the cosmos, accelerating the expansion of the universe for all time. This makes the Big Crunch much less likely.</p><p>But that doesn't mean that the Big Freeze is inevitable. There are other possibilities.</p><p>One of them originated, not in the study of the cosmos, but in the world of subatomic particles. This is perhaps the strangest fate for the universe. It sounds like something out of science fiction, and in a way, it is.</p><p>{\"image\":{\"pid\":\"p02spg23\"}}</p><p>In Kurt Vonnegut's classic sci-fi novel <em>Cat's Cradle</em>, ice-nine is a new form of water ice with a remarkable property: it freezes at 46 °C, not at 0 °C. When a crystal of ice-nine is dropped into a glass of water, all the water around it immediately patterns itself after the crystal, since it has lower energy than liquid water.</p><blockquote><p> There's nowhere for the ice to start forming </p></blockquote><p>The new crystals of ice-nine do the same thing to the water around them, and in the blink of an eye, the chain reaction turns all the water in the glass — or (spoiler alert!) all of Earth's oceans — into solid ice-nine.</p><p>The same thing can happen in real life with normal ice and normal water. If you put very pure water into a very clean glass, and cool it just below 0°C, the water will become supercooled: it stays liquid below its natural freezing point. There are no impurities in the water and no rough patches on the glass, so there's nowhere for the ice to start forming. But if you drop a crystal of ice into the glass, the water will freeze rapidly, just like ice-nine.</p><p>Ice-nine and supercooled water may not seem relevant to the fate of the universe. But something similar could happen to space itself.</p><p>{\"image\":{\"pid\":\"p02spfzd\"}}</p><p>Quantum physics dictates that even in a totally empty vacuum, there is a small amount of energy. But there might also be some other kind of vacuum, which holds less energy.</p><blockquote><p> The new vacuum will \"convert\" the old vacuum around it </p></blockquote><p>If that's true, then the entire universe is like a glass of supercooled water. It will only last until a \"bubble\" of lower-energy vacuum shows up.</p><p>Fortunately, there are no such bubbles that we're aware of. Unfortunately, quantum physics also dictates that if a lower-energy vacuum is possible, then a bubble of that vacuum will inevitably dart into existence somewhere in the universe.</p><p>When that happens, just like ice-nine, the new vacuum will \"convert\" the old vacuum around it. The bubble would expand at nearly the speed of light, so we'd never see it coming.</p><p>{\"image\":{\"pid\":\"p02spft2\"}}</p><p>Inside the bubble, things would be radically different, and not terribly hospitable.</p><blockquote><p> Humans, planets and even the stars themselves would be destroyed </p></blockquote><p>The properties of fundamental particles like electrons and quarks could be entirely different, radically rewriting the rules of chemistry and perhaps preventing atoms from forming.</p><p>Humans, planets and even the stars themselves would be destroyed in this Big Change. In a 1980 paper, Physicists Sidney Coleman and Frank de Luccia called it \"<a href=\"http://dx.doi.org/10.1103/PhysRevD.21.3305\">the ultimate ecological catastrophe</a>\".</p><p>Adding insult to injury, dark energy would probably behave differently after the Big Change. Rather than driving the universe to expand faster, dark energy might instead pull the universe in on itself, collapsing into a Big Crunch.</p><p>{\"image\":{\"pid\":\"p02spflj\"}}</p><p>There is a fourth possibility, and once again dark energy is at centre stage. This idea is very speculative and unlikely, but it can't yet be ruled out. Dark energy might be even more powerful than we thought, and might be enough to end the universe on its own, without any intervening Big Change, Freeze, or Crunch.</p><p>Dark energy has a peculiar property. As the universe expands, its density remains constant. That means more of it pops into existence over time, to keep pace with the increasing volume of the universe. This is unusual, <a href=\"http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/\">but doesn't break any laws of physics</a>.</p><p>However, it could get weirder. What if the density of dark energy increases as the universe expands? In other words, what if the amount of dark energy in the universe increases more quickly than the expansion of the universe itself?</p><p>This idea was put forward by <a href=\"http://www.dartmouth.edu/~caldwell/cosmology/Dartmouth_College_Physics_%26_Astronomy.html\">Robert Caldwell</a> of Dartmouth College in Hanover, New Hampshire. He calls it \"phantom dark energy\". It leads to a remarkably strange fate for the universe.</p><p>{\"image\":{\"pid\":\"p02spfp5\"}}</p><p>If phantom dark energy exists, then the dark side is our ultimate downfall, just like <em>Star Wars</em> warned us it would be.</p><blockquote><p> Atoms themselves would shatter, a fraction of a second before the universe itself ripped apart </p></blockquote><p>Right now, the density of dark energy is very low, far less than the density of matter here on Earth, or even the density of the Milky Way galaxy, which is much less dense than Earth. But as time goes on, the density of phantom dark energy would build up, and tear the universe apart.</p><p>In a 2003 paper, Caldwell and his colleagues outlined a scenario they called \"<a href=\"http://dx.doi.org/10.1103/PhysRevLett.91.071301\">cosmic doomsday</a>\". Once the phantom dark energy becomes more dense than a particular object, that object gets torn to shreds.</p><p>First, phantom dark energy would pull the Milky Way apart, sending its constituent stars flying. Then the solar system would be unbound, because the pull of dark energy would be stronger than the pull of the Sun on the Earth.</p><p>Finally, in a few frantic minutes the Earth would explode. Then atoms themselves would shatter, a fraction of a second before the universe itself ripped apart. Caldwell calls this the Big Rip.</p><p>{\"image\":{\"pid\":\"p02spfgn\"}}</p><p>The Big Rip is, by Caldwell's own admission, \"very outlandish\" – and not just because it sounds like something out of an over-the-top superhero comic.</p><blockquote><p> This is a remarkably grim portrait of the future </p></blockquote><p>Phantom dark energy flies in the face of some fairly basic ideas about the universe, like the assumption that matter and energy can't go faster than the speed of light. There are good reasons not to believe in it.</p><p>Based on our observations of the expansion of the universe, and particle physics experiments, it seems much more likely that the ultimate fate of our universe is a Big Freeze, possibly followed by a Big Change and a final Big Crunch.</p><p>But this is a remarkably grim portrait of the future — aeons of cold emptiness, finally terminated by a vacuum decay and a final implosion into nothingness. Is there any escape? Or are we doomed to book a table at <a href=\"http://hitchhikers.wikia.com/wiki/Milliways\">the Restaurant at the End of the Universe</a>?</p><p>{\"image\":{\"pid\":\"p02spftt\"}}</p><p>There's certainly no reason for us, individually, to worry about the end of the universe. All of these events are trillions of years into the future, with the possible exception of the Big Change, so they're not exactly an imminent problem.</p><p>Also, there's no reason to worry about humanity. If nothing else, genetic drift will have rendered our descendants unrecognizable long before then. But could intelligent feeling creatures of any kind, human or not, survive?</p><blockquote><p> If the universe is accelerating, that's really bad news </p></blockquote><p>Physicist <a href=\"http://www.sns.ias.edu/dyson\">Freeman Dyson</a> of the Institute for Advanced Studies in Princeton, New Jersey considered this question in <a href=\"http://dx.doi.org/10.1103/RevModPhys.51.447\">a classic paper published in 1979</a>. At the time, he concluded that life could modify itself to survive the Big Freeze, which he thought was less challenging than the inferno of the Big Crunch.</p><p>But these days, he's much less optimistic, thanks to the discovery of dark energy.</p><p>\"If the universe is accelerating, that's really bad news,\" says Dyson. Accelerating expansion means we'll eventually lose contact with all but a handful of galaxies, dramatically limiting the amount of energy available to us. \"It's a rather dismal situation in the long run.\"</p><p>The situation could still change. \"We really don't know whether the expansion is going to continue since we don't understand why it's accelerating,\" says Dyson. \"The optimistic view is that the acceleration will slow down as the universe gets bigger.\" If that happens, \"the future is much more promising.\"</p><p>But what if the expansion doesn't slow down, or if it becomes clear that the Big Change is coming? Some physicists have proposed a solution that is solidly in mad-scientist territory. To escape the end of the universe, we should build our own universe in a laboratory, and jump in.</p><p>{\"image\":{\"pid\":\"p02spf6g\"}}</p><p>One physicist who has worked on this idea is <a href=\"http://web.mit.edu/physics/people/faculty/guth_alan.html\">Alan Guth</a> of MIT in Cambridge, Massachusetts, who is known for his work on the very early universe.</p><blockquote><p> You would jump-start the creation of an entirely new universe </p></blockquote><p>\"I can't say that the laws of physics absolutely imply that it's possible,\" says Guth. \"If it is possible, it would require technology vastly beyond anything that we can foresee. It would require huge amounts of energy that one would need to be able to obtain and control.\"</p><p>The first step, according to Guth, would be creating an incredibly dense form of matter — so dense that it was on the verge of <a href=\"http://www.bbc.com/earth/story/20150525-a-black-hole-would-clone-you\">collapsing into a black hole</a>. By doing that in the right way, and then quickly clearing the matter out of the area, you might be able to force that region of space to start expanding rapidly.</p><p>In effect, you would jump-start the creation of an entirely new universe. As the space in the region expanded, the boundary would shrink, creating a bubble of warped space where the inside was bigger than the outside.</p><p>{\"image\":{\"pid\":\"p02spg13\"}}</p><p>That may sound familiar to <em>Doctor Who</em> fans, and according to Guth, the TARDIS is \"probably a very accurate analogy\" for the kind of warping of space he's talking about.</p><blockquote><p> We don't really know if it's possible or not </p></blockquote><p>Eventually, the outside would shrink to nothingness, and the new baby universe would pinch off from our own, spared from whatever fate our universe may meet.</p><p>It's far from certain that this scheme would actually work. \"I would have to say that it's unclear,\" says Guth. \"We don't really know if it's possible or not.\"</p><p>However, Guth also points out that there is another source of hope beyond the end of the universe – well, hope of a sort.</p><p>{\"image\":{\"pid\":\"p02spfdd\"}}</p><p>Guth was the first to propose that the very early universe expanded astonishingly fast for a tiny fraction of a second, an idea known as \"inflation\". Many cosmologists now believe inflation is the most promising approach for explaining the early universe, and Guth's plan for creating a new universe relies on recreating this rapid expansion.</p><blockquote><p> The multiverse as a whole is genuinely eternal </p></blockquote><p>Inflation has an intriguing consequence for the ultimate fate of the universe. The theory dictates that the universe we inhabit is just one small part of a multiverse, with an eternally inflating background continually spawning \"pocket universes\" like our own.</p><p>\"If that's the case, even if we're convinced that an individual pocket universe will ultimately die through refrigeration, the multiverse as a whole will go on living forever, with new life being created in each pocket universe as it's created,\" says Guth. \"In this picture, the multiverse as a whole is genuinely eternal, at least eternal into the future, even as individual pocket universes live and die.\"</p><p>In other words, Franz Kafka may have been right on the money when he said that there is \"plenty of hope, an infinite amount of hope—but not for us.\"</p><p>This is a bit of a bleak thought. If it upsets you, here is a picture of a cute kitten.</p><p>{\"image\":{\"pid\":\"p02spdw8\"}}</p>","BusinessUnit":"bbc.com","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[],"DisableAdverts":false,"DisplayDate":"2015-06-02T08:07:24Z","Geolocation":null,"HeadlineLong":"How will the universe end, and could anything survive?","HeadlineShort":"How will the universe end?","HideRelated":false,"Horizontal":null,"HyperHorizontal":null,"Intro":"Science has outlined four ways that our universe could meet its doom. They're called the Big Freeze, the Big Crunch, the Big Change and the Big Rip","IsSyndicated":true,"Latitude":"","Location":null,"Longitude":"","Option":null,"Partner":null,"PrimaryVertical":"wwearth","Programme":null,"RelatedStory":[],"RelatedTag":[],"StoryType":"image","SummaryLong":"Science has outlined four ways that our universe could meet its doom. They're called the Big Freeze, the Big Crunch, the Big Change and the Big Rip","SummaryShort":"The four possible fates of the cosmos","SuperSection":null,"Tag":[]},"Metadata":{"CreationDateTime":"2015-06-02T08:07:24Z","Entity":"story","Guid":"8174d313-9bdd-4e9d-b473-599e48e286db","Id":"wwearth/story/20150602-how-will-the-universe-end","ModifiedDateTime":"2015-09-03T09:11:39.152469Z","Project":"wwearth","Slug":"20150602-how-will-the-universe-end"},"Urn":"urn:pubstack:jative:story:wwearth/story/20150602-how-will-the-universe-end","_id":"5db5e358200e0360982b1579"},{"Content":{"AssetCustom":"","AssetIbroadcast":null,"AssetImage":[],"AssetImagePromo":null,"AssetInfographic":"","AssetInline":[],"AssetSelect":"","AssetVideoIb2":null,"AssetVideoMps":null,"Author":[],"BodyHtml":"<p>People have wrestled with the mystery of why the universe exists for thousands of years. Pretty much every ancient culture came up with its own creation story - most of them leaving the matter in the hands of the gods - and philosophers have written reams on the subject. But science has had little to say about this ultimate question.</p><p>However, in recent years a few physicists and cosmologists have started to tackle it. They point out that we now have an understanding of the history of the universe, and of the physical laws that describe how it works. That information, they say, should give us a clue about how and why the cosmos exists.</p><p>Their admittedly controversial answer is that the entire universe, from the fireball of the Big Bang to the star-studded cosmos we now inhabit, popped into existence from nothing at all. It had to happen, they say, because \"nothing\" is inherently unstable.</p><p>This idea may sound bizarre, or just another fanciful creation story. But the physicists argue that it follows naturally from science's two most powerful and successful theories: quantum mechanics and general relativity.</p><p>Here, then, is how everything could have come from nothing.</p><p>{\"image\":{\"pid\":\"p02b3t2n\"}}</p><p><strong>Particles from empty space</strong></p><p>First we have to take a look at the realm of quantum mechanics. This is the branch of physics that deals with very small things: atoms and even tinier particles. It is an immensely successful theory, and it underpins most modern electronic gadgets.</p><p>Quantum mechanics tells us that there is no such thing as empty space. Even the most perfect vacuum is actually filled by a roiling cloud of particles and antiparticles, which flare into existence and almost instantaneously fade back into nothingness.</p><p>These so-called virtual particles don't last long enough to be observed directly, but we know they exist by <a href=\"http://math.ucr.edu/home/baez/physics/Quantum/casimir.html\" target=\"_blank\">their effects</a>.</p><p>{\"image\":{\"pid\":\"p02b3tyd\"}}</p><p><strong>Space-time, from no space and no time</strong></p><p>From tiny things like atoms, to really big things like galaxies. Our best theory for describing such large-scale structures is general relativity, Albert Einstein's crowning achievement, which sets out how space, time and gravity work.</p><p>Relativity is very different from quantum mechanics, and so far nobody has been able to combine the two seamlessly. However, some theorists have been able to bring the two theories to bear on particular problems by using carefully chosen approximations. For instance, this approach was used by <a href=\"http://www.hawking.org.uk/\" target=\"_blank\">Stephen Hawking</a> at the University of Cambridge to describe black holes.</p><blockquote><p> In quantum physics, if something is not forbidden, it necessarily happens </p></blockquote><p>One thing they have found is that, when quantum theory is applied to space at the smallest possible scale, space itself becomes unstable. Rather than remaining perfectly smooth and continuous, space and time destabilize, churning and frothing into a foam of space-time bubbles.</p><p>In other words, little bubbles of space and time can form spontaneously. \"If space and time are quantized, they can fluctuate,\" says <a href=\"https://physics.asu.edu/people/faculty/lawrence-krauss\" target=\"_blank\">Lawrence Krauss</a> at Arizona State University in Tempe. \"So you can create virtual space-times just as you can create virtual particles.\"</p><p>What's more, if it's possible for these bubbles to form, you can guarantee that they will. \"In quantum physics, if something is not forbidden, it necessarily happens with some non-zero probability,\" says <a href=\"http://cosmos2.phy.tufts.edu/vilenkin.html\" target=\"_blank\">Alexander Vilenkin</a> of Tufts University in Boston, Massachusetts.</p><p>{\"image\":{\"pid\":\"p02b3tst\"}}</p><p><strong>A universe from a bubble</strong></p><p>So it's not just particles and antiparticles that can snap in and out of nothingness: bubbles of space-time can do the same. Still, it seems like a big leap from an infinitesimal space-time bubble to a massive universe that hosts 100 billion galaxies. Surely, even if a bubble formed, it would be doomed to disappear again in the blink of an eye?</p><blockquote><p> If all the galaxies are flying apart, they must once have been close together </p></blockquote><p>Actually, it is possible for the bubble to survive. But for that we need another trick: cosmic inflation.</p><p>Most physicists now think that the universe began with the Big Bang. At first all the matter and energy in the universe was crammed together in one unimaginably small dot, and this exploded. This follows from the discovery, in the early 20th century, that the universe is expanding. If all the galaxies are flying apart, they must once have been close together.</p><p>Inflation theory proposes that in the immediate aftermath of the Big Bang, the universe expanded much faster than it did later. This seemingly outlandish notion was put forward in the 1980s by <a href=\"http://web.mit.edu/physics/people/faculty/guth_alan.html\" target=\"_blank\">Alan Guth</a> at the Massachusetts Institute of Technology, and refined by <a href=\"http://web.stanford.edu/~alinde/\" target=\"_blank\">Andrei Linde</a>, now at Stanford University.</p><blockquote><p> As weird as it seems, inflation fits the facts </p></blockquote><p>The idea is that, a fraction of a second after the Big Bang, the quantum-sized bubble of space expanded stupendously fast. In an incredibly brief moment, it went from being smaller than the nucleus of an atom to the size of a grain of sand. When the expansion finally slowed, the force field that had powered it was transformed into the matter and energy that fill the universe today. Guth calls inflation \"the ultimate free lunch\".</p><p>As weird as it seems, inflation fits the facts rather well. In particular, it neatly explains why the cosmic microwave background, the faint remnant of radiation left over from the Big Bang, is almost perfectly uniform across the sky. If the universe had not expanded so rapidly, we would expect the radiation to be patchier than it is.</p><p>{\"image\":{\"pid\":\"p02b3vlw\"}}</p><p><strong>The universe is flat and why that's important</strong></p><p>Inflation also gave cosmologists the measuring tool they needed to determine the underlying geometry of the universe. It turns out this is also crucial for understanding how the cosmos came from nothing.</p><p>Einstein's theory of general relativity tells us that the space-time we live in could take three different forms. It could be as flat as a table top. It could curve back on itself like the surface of a sphere, in which case if you travel far enough in the same direction you would end up back where you started. Alternatively, space-time could curve outward like a saddle. So which is it?</p><p>There is a way to tell. You might remember from maths class that the three angles of a triangle add up to exactly 180 degrees. Actually your teachers left out a crucial point: this is only true on a flat surface. If you draw a triangle on the surface of a balloon, its three angles will add up to more than 180 degrees. Alternatively, if you draw a triangle on a surface that curves outward like a saddle, its angles will add up to less than 180 degrees.</p><p>So to find out if the universe is flat, we need to measure the angles of a really big triangle. That's where inflation comes in. It determined the average size of the warmer and cooler patches in the cosmic microwave background. Those patches were measured in 2003, and that gave astronomers a selection of triangles. As a result, we know that on the largest observable scale our universe is flat.</p><p>{\"image\":{\"pid\":\"p02b3w4b\"}}</p><p>It turns out that a flat universe is crucial. That's because only a flat universe is likely to have come from nothing.</p><p>Everything that exists, from stars and galaxies to the light we see them by, must have sprung from somewhere. We already know that particles spring into existence at the quantum level, so we might expect the universe to contain a few odds and ends. But it takes a huge amount of energy to make all those stars and planets.</p><blockquote><p> The energy of matter is exactly balanced by the energy of the gravity the mass creates </p></blockquote><p>Where did the universe get all this energy? Bizarrely, it may not have had to get any. That's because every object in the universe creates gravity, pulling other objects toward it. This balances the energy needed to create the matter in the first place.</p><p>It's a bit like an old-fashioned measuring scale. You can put a heavy weight on one side, so long as it is balanced by an equal weight on the other. In the case of the universe, the matter goes on one side of the scale, and has to be balanced by gravity.</p><p>Physicists have calculated that in a flat universe the energy of matter is exactly balanced by the energy of the gravity the mass creates. But this is only true in a flat universe. If the universe had been curved, the two sums would not cancel out.</p><p>{\"image\":{\"pid\":\"p02b3vrq\"}}</p><p><strong>Universe or multiverse?</strong></p><p>At this point, making a universe looks almost easy. Quantum mechanics tells us that \"nothing\" is inherently unstable, so the initial leap from nothing to something may have been inevitable. Then the resulting tiny bubble of space-time could have burgeoned into a massive, busy universe, thanks to inflation. As Krauss puts it, \"The laws of physics as we understand them make it eminently plausible that our universe arose from nothing - no space, no time, no particles, nothing that we now know of.\"</p><p>So why did it only happen once? If one space-time bubble popped into existence and inflated to form our universe, what kept other bubbles from doing the same?</p><blockquote><p> There could be a mind-boggling smorgasbord of universes </p></blockquote><p>Linde offers a simple but mind-bending answer. He thinks universes have always been springing into existence, and that this process will continue forever.</p><p>When a new universe stops inflating, says Linde, it is still surrounded by space that is continuing to inflate. That inflating space can spawn more universes, with yet more inflating space around them. So once inflation starts it should make an endless cascade of universes, which Linde calls eternal inflation. Our universe may be just one grain of sand on an endless beach.</p><p>Those universes might be profoundly different to ours. The universe next door might have five dimensions of space rather than the three – length, breadth and height – that ours does. Gravity might be ten times stronger or a thousand times weaker, or not exist at all. Matter might be built out of utterly different particles.</p><p>So there could be a mind-boggling smorgasbord of universes. Linde says eternal inflation is not just the ultimate free lunch: it is the only one at which all possible dishes are available.</p><p>As yet we don't have hard evidence that other universes exist. But either way, these ideas give a whole new meaning to the phrase \"Thanks for nothing\".</p><p>{\"image\":{\"pid\":\"p02b3wrr\"}}</p>","BusinessUnit":"bbc.com","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[],"DisableAdverts":false,"DisplayDate":"2014-11-06T10:18:39Z","Geolocation":null,"HeadlineLong":"Why is there something rather than nothing?","HeadlineShort":"Why does anything exist at all?","HideRelated":false,"Horizontal":null,"HyperHorizontal":null,"Intro":"Some physicists think they can explain why the universe first formed. If they are right, our entire cosmos may have sprung out of nothing at all","IsSyndicated":true,"Latitude":"","Location":null,"Longitude":"","Option":null,"Partner":null,"PrimaryVertical":"wwearth","Programme":null,"RelatedStory":[],"RelatedTag":[],"StoryType":"image","SummaryLong":"Some physicists think they can explain why the universe first formed. Our entire cosmos may have sprung out of nothing at all","SummaryShort":"Some physicists think they can explain why the universe first formed","SuperSection":null,"Tag":[]},"Metadata":{"CreationDateTime":"2014-11-06T10:18:39Z","Entity":"story","Guid":"8413a759-8f12-44e7-a4f0-e7792f69e51b","Id":"wwearth/story/20141106-why-does-anything-exist-at-all","ModifiedDateTime":"2015-09-03T09:11:55.512873Z","Project":"wwearth","Slug":"20141106-why-does-anything-exist-at-all"},"Urn":"urn:pubstack:jative:story:wwearth/story/20141106-why-does-anything-exist-at-all","_id":"5db86e55200e036098923f6b"},{"Content":{"AssetCustom":"","AssetIbroadcast":null,"AssetImage":[],"AssetImagePromo":null,"AssetInfographic":"","AssetInline":[],"AssetSelect":"","AssetVideoIb2":null,"AssetVideoMps":null,"Author":[],"BodyHtml":"<p>The recent film <em><a href=\"http://www.imdb.com/title/tt2980516/\">The Theory of Everything</a></em> tells the story of Stephen Hawking, who managed to become a world-famous physicist despite being confined to a wheelchair by a degenerative disease. It's mostly about his relationship with his ex-wife Jane, but it does find a bit of time to explain what Hawking has spent his career doing.</p><p>He certainly didn't lack ambition. Hawking has been one of many physicists trying to come up with a \"theory of everything\", a single theory that will explain everything about our universe. He was following in the footsteps of Albert Einstein, who tried and failed to devise such a theory.</p><p>Finding a theory of everything would be a staggering achievement, finally making sense of all the weird and wonderful things in our universe. For decades, confident physicists have said that one is just around the corner. So are we really on the verge of understanding everything?</p><p>{\"image\":{\"pid\":\"p02npnhp\"}}</p><p>On the face of it, a theory of everything sounds like a tall order. It would have to explain everything from the works of Shakespeare to the human brain and the forests and valleys of our natural world, says <a href=\"http://www.damtp.cam.ac.uk/user/jdb34/\">John Barrow</a> of the University of Cambridge in the UK. \"That's the question of the universe.\"</p><p>Nevertheless, Barrow thinks finding a theory of everything \"is quite conceivable\". That's because \"the laws of nature are rather few, they're simple and symmetrical and there are only four fundamental forces.\"</p><p>In a way we have to put aside the complexity of the world we live in. \"The outcomes of the laws - the things that we see around us - are infinitely more complicated,\" says Barrow. But the rules underlying it all may be simple.</p><p>{\"image\":{\"pid\":\"p02npnkd\"}}</p><p>In 1687, it seemed to many scientists that a theory of everything had been found.</p><blockquote><p> Newton was walking in a garden when he saw an apple fall from a tree </p></blockquote><p>The English physicist Isaac Newton published a book in which he explained how objects move, and set out how gravity works. The <em>Philosophiæ Naturalis Principia Mathematica</em> – that's \"Mathematical Principles of Natural Philosophy\" to you and me – presented the world as a beautiful, ordered place.</p><p>The story goes that, at the age of 23, Newton was walking in a garden when he saw an apple fall from a tree. At the time, physicists knew that the Earth somehow pulled objects down by the force of gravity. Newton would take this idea further.</p><p>According to John Conduitt, his assistant in later years, seeing the apple fall led Newton to the idea that the gravitational force \"<a href=\"https://books.google.co.uk/books?id=oxQ2i23IiMsC&pg=PT49#v=onepage&q&f=false\">was not limited to a certain distance from earth, but that this power must extend much further than was usually thought</a>\". According to Conduitt's account, Newton then asked: \"Why not as high as the Moon?\"</p><p>{\"image\":{\"pid\":\"p02npnlx\"}}</p><p>Inspired, Newton developed a law of gravity, which worked equally well for apples on Earth and planets orbiting the Sun.  All these objects, which seemed so different, turned out to obey the same laws.</p><p>In the same book, Newton set out three laws governing how objects move. Combined with the law of gravity, these laws explained how a ball moves when you throw it and why the Moon orbits the Earth.</p><p>\"People thought that he had explained everything there was to explain,\" says Barrow. \"His achievement was immense.\"</p><p>The problem was, Newton knew his work had holes.</p><p>{\"image\":{\"pid\":\"p02npnnn\"}}</p><p>For instance, gravity doesn't explain how small objects hold themselves together, as the force isn't strong enough.  Also, while Newton could describe what was happening, he couldn't explain how it worked. The theory was incomplete.</p><blockquote><p> Mercury wasn't playing ball </p></blockquote><p>But there was a bigger problem. While Newton's laws explained most of the common phenomena in the universe, in some cases objects broke his laws. These situations were rare, and generally involved extreme speeds or powerful gravity, but they were there.</p><p>One such circumstance was the orbit of Mercury, the closest planet to the Sun. As each planet orbits the Sun it also rotates. Newton's laws could be used to calculate how they should rotate, but Mercury wasn't playing ball. Equally strangely, its orbit was off-centre.</p><p>The evidence was clear. Newton's universal law of gravitation wasn't universal, and wasn't a law.</p><p>{\"image\":{\"pid\":\"p02npntn\"}}</p><p>Over two centuries later, Albert Einstein came to the rescue with his theory of general relativity.  Einstein's idea, which in 2015 celebrates its 100th anniversary, offered a much deeper understanding of gravity.</p><blockquote><p> Really heavy objects like planets, or really fast-moving ones, can distort space-time </p></blockquote><p>The core idea is that space and time, which seem like different things, are actually interwoven. Space has its three dimensions: length, breadth and height. Then there is a fourth dimension, which we call time. All four are linked in a kind of giant cosmic sheet. If you've ever heard a character in a science fiction movie mention \"the space-time continuum\", this is what they're talking about.</p><p>Einstein's big idea was that really heavy objects like planets, or really fast-moving ones, can distort space-time. It's a bit like the taut fabric of a trampoline: if you put a heavy weight on it, the fabric bows and curves. Any other objects will then roll down the sheet towards the object. This, according to Einstein, is why gravity pulls objects towards each other.</p><p>This is a deeply weird idea. But physicists are convinced that it is true. For one thing, it explains the strange orbit of Mercury.</p><p>{\"image\":{\"pid\":\"p02npp29\"}}</p><p>According to general relativity, the Sun's huge mass warps space and time around it.</p><p>As the closest planet to the sun, Mercury experiences much bigger distortions than any of the other planets.  The equations of general relativity describe how this warped space-time should affect Mercury's orbit, and predict the planet's position down to a tee.</p><p>But despite this success, general relativity isn't a theory of everything, any more than Newton's theories were. Just as Newton's theory didn't work for really massive objects, Einstein's didn't work on the very small.</p><p>Once you start looking at tiny things like atoms, matter starts to behave very oddly indeed.</p><p>{\"image\":{\"pid\":\"p02npp30\"}}</p><p>Up until the late 19th century, the atom was thought to be the smallest unit of matter. Coming from the Greek <em>atomos</em> meaning \"indivisible\", the atom by its very definition was not supposed to be able to be divided into smaller particles.</p><p>But in the 1870s, scientists found particles that were almost 2000 times lighter than atoms.</p><blockquote><p> Scientists have found ways to divide matter smaller and smaller </p></blockquote><p>By weighing light rays in a vacuum tube, they found extraordinarily light, negatively-charged particles. This was the first discovery of a subatomic particle: the electron.</p><p>In the next half-century scientists discovered that the atom had a nucleus hub, which the electrons buzzed around. This hub – which was by far the heaviest part of the atom – was made up of two types of subatomic particles: neutrons, which are neutrally charged and protons, which are positively charged.</p><p>But it didn't stop there. Since this time, scientists have found ways to divide matter smaller and smaller, continuing to redefine our notion of fundamental particles. By the 1960s, scientists had found dozens of elementary particles, drawing up a long list known as the particle zoo.</p><p>{\"image\":{\"pid\":\"p02npp42\"}}</p><p>As we understand it today, of the three components of an atom, electrons are the only fundamental particles. Neutrons and protons can be divided further into teeny, tiny particles called \"quarks\".</p><blockquote><p> Einstein never really believed in quantum theory </p></blockquote><p>These subatomic particles were governed by an entirely different set of laws than those governing big objects like trees or planets.  And these new laws – which were far less predictable - threw a spanner in the works.</p><p>In quantum physics, particles don't have defined locations: their whereabouts is a bit fuzzy.  All we can say is that each particle has a certain probability of being in each location. This means the world is a fundamentally uncertain place.</p><p>This may all seem very unfathomable and far-out. All we can say is, it's not just you that feels that way. The physicist Richard Feynman, an expert on the quantum, once said: \"<a href=\"http://bouman.chem.georgetown.edu/general/feynman.html\">I think I can safely say that no one understands quantum mechanics</a>.\"</p><p>Einstein was also disturbed by the fuzziness of quantum mechanics. \"Despite having instigated it, Einstein never really believed in quantum theory,\" says Barrow.</p><p>{\"image\":{\"pid\":\"p02npqt9\"}}</p><p>All the same, for their respective domains – the big and the small – both general relativity and quantum mechanics have proven, time and time again, to be tremendously accurate.  </p><p>Quantum physics has explained the structure and behaviour of atoms, including why some of them are radioactive. It also underlies all modern electronics. You could not read this article without it.</p><p>Meanwhile general relativity was used to predict the existence of black holes. These are stars so massive that they have collapsed in on themselves. Their gravitational attraction is so powerful that nothing – not even light – can escape from it.</p><p>{\"image\":{\"pid\":\"p02npp62\"}}   </p><p>But the issue is, the two theories are not compatible, so they can't both be right. General relativity says that objects' behaviours can be predicted exactly, whereas quantum mechanics says all you can know is the probability that they will do something.</p><p>That means there are some things physicists still can't describe. Black holes are a particular problem. They are massive so general relativity applies, but they are also small so quantum mechanics applies too.</p><p>Unless you're close to a black hole, this incompatibility doesn't affect your day-to-day life. But it has perplexed physicists for most of the last century. It's this incompatibility that has driven the quest for a theory of everything.</p><p>{\"image\":{\"pid\":\"p02nppcz\"}}</p><p>Einstein spent much of his life trying to find such a theory. Never a fan of the randomness of quantum mechanics, he wanted to create a theory that would bring together gravity and the rest of physics, with all the quantum weirdness as a secondary consequence.</p><blockquote><p> Einstein spent 30 years on a fruitless quest </p></blockquote><p>His major challenge was to make gravity work with electromagnetism. In the 1800s, physicists had worked out that electrically-charged particles could be attracted or repelled by each other. That's why some metals are attracted to magnets. This meant there were two kinds of force that objects could exert on each other: they could attract each other with their gravity, and either attract or repel with their electromagnetism.</p><p>Einstein wanted to bring the two forces together into a \"unified field theory\". To do this, he extended his space-time to five dimensions. As well as the three of space and one of time, he added a fifth dimension that was so small and curled up we couldn't see it.</p><p>This didn't work out, and Einstein spent 30 years on a fruitless quest. He died in 1955, his unified field theory still undiscovered. But in the following decade, the strongest contender for a theory of everything emerged: string theory.</p><p>{\"image\":{\"pid\":\"p02nppg8\"}}</p><p>The idea behind string theory is oddly simple. The basic ingredients of the world, such as electrons, are not actually particles at all. Instead they are little loops or \"strings\". It's just that these strings are so small, they seem to be mere points.</p><blockquote><p> All the different particles discovered in the 20th century are really the same kinds of strings </p></blockquote><p>Just like the strings on a guitar, these loops are under tension. That means they vibrate at different frequencies, depending on their size.</p><p>In turn, these oscillations determine what sort of \"particle\" each string appears to be. Vibrate a string one way and you get an electron. Vibrate it another way, and you get something else. All the different particles discovered in the 20th century are really the same kinds of strings, just vibrating in different ways.</p><p>It may not be immediately obvious why this is a good idea. But it seems to make sense of all the forces acting in nature: gravity and electromagnetism, plus two that were only discovered in the 20th century.</p><p>{\"image\":{\"pid\":\"p02nppln\"}}</p><p>The strong and weak nuclear forces are only active within the tiny nuclei of atoms, which is why it took so long for anyone to notice them. The strong force holds the nucleus together. The weak force normally does nothing, but if it gets strong enough it breaks the nucleus apart: this is why some atoms are radioactive.</p><blockquote><p> For the first time, general relativity and quantum mechanics had found common ground </p></blockquote><p>Any theory of everything would have to explain all four. Fortunately, the two nuclear forces and electromagnetism are all covered by quantum mechanics. Each is carried by a specialized particle. But there's no particle to carry the force of gravity.</p><p>Some physicists think there is. They call this particle the \"graviton\". Gravitons would have to have no mass, spin in a particular way, and travel at the speed of light. Unfortunately, nobody has ever managed to find one.</p><p>This is where string theory comes in. It describes a string that looks exactly like a graviton: it spins in the right way, is massless and travels at the speed of light. For the first time, general relativity and quantum mechanics had found common ground.</p><p>As a result, in the mid-1980s physicists became hugely excited about string theory. \"In 1985 we realised string theory solved a lot of the problems people had struggled with for the last 50 years,\" says Barrow. But it also has a host of problems.</p><p>{\"image\":{\"pid\":\"p02nppsv\"}}</p><p>For starters, \"we don't really understand what string theory is in full detail,\" according to <a href=\"https://www.maths.ox.ac.uk/people/philip.candelas\">Philip Candelas</a> of the University of Oxford in the UK. \"We don't have a good way to describe it.\"</p><p>It also makes some predictions that seem outright bizarre. While Einstein's unified field theory relied on a single hidden extra dimension, the earliest forms of string theory called for a total of 26 dimensions. These had to be there to make the mathematics consistent with what we already know about the universe.</p><p>More advanced versions, known as \"superstring theories\", get by with just 10 dimensions. But even that is a far cry from the three dimensions we see on Earth.</p><p>\"The way we reconcile this is by saying that only three expanded in our world and became large,\" says Barrow. \"The others are there but remain fantastically small.\"</p><p>{\"image\":{\"pid\":\"p02nppz5\"}}</p><p>Because of these and other problems, many physicists are unconvinced by string theory. Some have instead studied another theory: loop quantum gravity.</p><blockquote><p> Loop quantum gravity proposes that space-time is actually divided into small chunks </p></blockquote><p>This isn't an attempt at an overarching theory that incorporates particle physics. Instead, loop quantum gravity just sets out to find a quantum theory of gravity. It's more limited than string theory – but it's also not as unwieldy.</p><p>Loop quantum gravity proposes that space-time is actually divided into small chunks. When you zoom out it appears to be a smooth sheet, but when you zoom in, it is a bunch of dots connected by lines or loops. These small fibres, which are woven together, offer an explanation for gravity.</p><p>This idea is just as boggling as string theory, and it has the same problem: there's no hard experimental evidence. </p><p>{\"image\":{\"pid\":\"p02npqr8\"}}</p><p>Why do these theories keep stumbling? One possibility is that we simply don't know enough yet. If there are major phenomena that we've never even seen, we are trying to understand the big picture while missing half the pieces.</p><p>\"It's very tempting to think we've discovered everything,\" says Barrow. \"But it would be very suspicious if in the year 2015 we could make all the observations necessary to have a theory of everything.  Why should it be us?\"</p><blockquote><p> For all its problems, string theory still looks promising </p></blockquote><p>There's also a more immediate problem. The theories are really difficult to test, largely because the maths is so fiendish. Candelas has struggled for years to find a way to test string theory, so far without success.</p><p>\"The main obstacle to the advancement of string theory is there's not enough maths known to advance the study of physics,\" says Barrow. \"It's such an early stage and there's so much to explore.\"</p><p>For all its problems, string theory still looks promising. \"For many years people have been trying to unify gravity with the rest of physics,\" says Candelas. \"We had theories that explained electromagnetism and the other forces well, but not gravity. With string theory we put them together.\"</p><p>The real problem is that a theory of everything may simply be impossible to identify.</p><p>{\"image\":{\"pid\":\"p02npq6n\"}}</p><p>When string theory became popular in the 1980s, there were actually five different versions of it. \"People began to worry,\" says Barrow. \"If there's a theory of everything, why are there five of them?\"</p><p>Over the next decade, physicists discovered that these theories could be transformed into each other. They were different ways of looking at the same thing.</p><blockquote><p> M-theory doesn't offer a single theory of everything </p></blockquote><p>The end result was M-theory, put forward in 1995. This is a deeper version of string theory, incorporating all the earlier versions. That looks good: at least we're back to a single theory. M-theory also only needs 11 dimensions, which is at least better than 26.</p><p>But M-theory doesn't offer a single theory of everything. It offers billions upon billions of them. In total, M-theory gives us 10 to the power of 500 theories, all of them logically consistent and capable of describing a universe.</p><p>That looks worse than useless, but many physicists now think it points to a deeper truth.</p><p>{\"image\":{\"pid\":\"p02npqdb\"}}</p><p>The simplest conclusion is that our universe is one of many, each of them described by one of the trillions of versions of M-theory. This huge collection of universes is called the \"multiverse\".</p><p>At the beginning of time, the multiverse was like \"a great foam of bubbles, all slightly different shapes and sizes,\" says Barrow. Each bubble then expanded into its own universe.</p><p>\"We're in just one of those bubbles,\" says Barrow. As the bubbles expand, other bubbles can arise inside them, each one a new universe. \"It's making the geography of the universe really complicated.\"</p><p>{\"image\":{\"pid\":\"p02npqhq\"}}</p><p>Within each bubble universe, the same physical laws will apply. That's why everything in our universe seems to behave the same.</p><blockquote><p> There are trillions of other universes, each one unique </p></blockquote><p>But the rules will be different in other universes. \"The laws we see in our universe are just like bylaws,\" says Barrow. \"They govern our bit, but not all of the universes.\"</p><p>This leads us to a strange conclusion. If string theory really is the best way to combine general relativity and quantum mechanics, then it both is and isn't a theory of everything.</p><p>On the one hand, string theory may give us a perfect description of our own universe. But it also seems to lead, inescapably, to the idea that there are trillions of other universes, each one unique.</p><p>\"The big change in thinking is we don't expect there to be a unique theory of everything,\" says Barrow. \"There are so many possible theories they're almost filling every possibility of thinking.\"</p><p>{\"image\":{\"pid\":\"p02npqnf\"}}</p>","BusinessUnit":"bbc.com","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[],"DisableAdverts":false,"DisplayDate":"2015-04-08T15:50:28Z","Geolocation":null,"HeadlineLong":"Will we ever have a theory of everything?","HeadlineShort":"Quest for a theory of everything","HideRelated":false,"Horizontal":null,"HyperHorizontal":null,"Intro":"Physicists want to find a single theory that describes the entire universe, but to do so they must solve some of the hardest problems in 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should exist","SummaryShort":"Our Universe could be one of 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cosmic microwave background (Credit: NASA / WMAP Science Team)","SynopsisMedium":"The cosmic microwave background (Credit: NASA / WMAP Science Team)","SynopsisShort":"The cosmic microwave background (Credit: NASA / WMAP Science Team)","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2b/3v/p02b3vlw.jpg","Title":"ilc_9yr_moll4096-cmb-wmap-crop.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02b3vlw","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02b3vlw","_id":"5db86e50200e036098923af1"},{"Content":{"Copyright":"NASA, ESA, and The Hubble Heritage Team (AURA/STScI)","FileSizeBytes":813210,"MimeType":"image/jpeg","SourceHeight":2041,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2b/3w/p02b3w4b.jpg","SourceWidth":3628,"SynopsisLong":"It may not look flat... (Credit: NASA, ESA, and The Hubble Heritage Team (AURA/STScI))","SynopsisMedium":"It may not look flat... (Credit: NASA, ESA, and The Hubble Heritage Team (AURA/STScI))","SynopsisShort":"It may not look flat... (Credit: NASA, ESA, and The Hubble Heritage Team (AURA/STScI))","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2b/3w/p02b3w4b.jpg","Title":"3195127602_d82e5e45f1_o crop.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02b3w4b","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02b3w4b","_id":"5db86e51200e036098923bbf"},{"Content":{"Copyright":"Da Sal, CC by 2.0","FileSizeBytes":502215,"MimeType":"image/jpeg","SourceHeight":2227,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2b/3v/p02b3vrq.jpg","SourceWidth":3959,"SynopsisLong":"Matter on one side, gravity on the other (Credit: Da Sal, CC by 2.0)","SynopsisMedium":"Matter on one side, gravity on the other (Credit: Da Sal, CC by 2.0)","SynopsisShort":"Matter on one side, gravity on the other (Credit: Da Sal, CC by 2.0)","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2b/3v/p02b3vrq.jpg","Title":"11646155493_0074ded011_o scales crop.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02b3vrq","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02b3vrq","_id":"5db86e51200e036098923c36"},{"Content":{"Copyright":"NASA, ESA, J. Hester, A. Loll (ASU)","FileSizeBytes":1334942,"MimeType":"image/jpeg","SourceHeight":2174,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2b/3w/p02b3wrr.jpg","SourceWidth":3864,"SynopsisLong":"(Credit: NASA, ESA, J. Hester, A. Loll (ASU))","SynopsisMedium":"(Credit: NASA, ESA, J. Hester, A. Loll (ASU))","SynopsisShort":"(Credit: NASA, ESA, J. Hester, A. Loll (ASU))","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2b/3w/p02b3wrr.jpg","Title":"430453main_crabmosaic_hst_big_full crop.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02b3wrr","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02b3wrr","_id":"5db86e54200e036098923e4e"},{"Content":{"Copyright":"NASA, ESA, M. Postman (STScI), CLASH Team, Hubble Heritage Team (STScI/AURA)","FileSizeBytes":377597,"MimeType":"image/jpeg","SourceHeight":1442,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2b/3t/p02b3t2n.jpg","SourceWidth":2564,"SynopsisLong":"(Credit: NASA, ESA, M. Postman (STScI), CLASH Team, Hubble Heritage Team (STScI/AURA))","SynopsisMedium":"(Credit: NASA, ESA, M. Postman (STScI), CLASH Team, Hubble Heritage Team (STScI/AURA))","SynopsisShort":"(Credit: NASA, ESA, M. Postman (STScI), CLASH Team, Hubble Heritage Team (STScI/AURA))","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2b/3t/p02b3t2n.jpg","Title":"hs-2011-25-b-full_ cluster crop.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02b3t2n","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02b3t2n","_id":"5db86e55200e036098923f3f"},{"Content":{"Copyright":"NASA, ESA, and the Hubble SM4 ERO Team","FileSizeBytes":2429072,"MimeType":"image/jpeg","SourceHeight":3375,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2b/3t/p02b3tyd.jpg","SourceWidth":6000,"SynopsisLong":"The Stephan's Quintet group of galaxies (Credit: NASA, ESA, and the Hubble SM4 ERO Team)","SynopsisMedium":"The Stephan's Quintet group of galaxies (Credit: NASA, ESA, and the Hubble SM4 ERO Team)","SynopsisShort":"The Stephan's Quintet group of galaxies (Credit: NASA, ESA, and the Hubble SM4 ERO Team)","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2b/3t/p02b3tyd.jpg","Title":"hs-2009-25-x-hires crop.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02b3tyd","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02b3tyd","_id":"5db86e53200e036098923e3a"},{"Content":{"Copyright":"amira_a, CC by 2.0","FileSizeBytes":436823,"MimeType":"image/jpeg","SourceHeight":2079,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2b/3t/p02b3tst.jpg","SourceWidth":3696,"SynopsisLong":"Maybe it all began with bubbles (Credit: amira_a, CC by 2.0)","SynopsisMedium":"Maybe it all began with bubbles (Credit: amira_a, CC by 2.0)","SynopsisShort":"Maybe it all began with bubbles (Credit: amira_a, CC by 2.0)","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2b/3t/p02b3tst.jpg","Title":"7924094800_21562a78d9_o bubbles crop.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02b3tst","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02b3tst","_id":"5db86e54200e036098923ec2"}],"AssetSelect":"","AssetVideoIb2":null,"AssetVideoMps":null,"Author":[{"Content":{"AssetImage":null,"Description":"<p>Robert Adler is a science writer based in northern California and southern Mexico</p>","Email":"","Links":null,"Name":"Robert Adler","PrimaryVertical":"wwearth"},"Metadata":{"CreationDateTime":"2014-11-05T15:38:55Z","Entity":"author","Guid":"94a0f313-5c88-4c4c-92ec-69ecb711ee50","Id":"wwearth/author/robert-adler","ModifiedDateTime":"2015-09-03T09:12:53.281408Z","Project":"wwearth","Slug":"robert-adler"},"Urn":"urn:pubstack:jative:author:wwearth/author/robert-adler","_id":"5db313a7200e0360981800bc"}],"BodyHtml":"<p>People have wrestled with the mystery of why the universe exists for thousands of years. Pretty much every ancient culture came up with its own creation story - most of them leaving the matter in the hands of the gods - and philosophers have written reams on the subject. But science has had little to say about this ultimate question.</p><p>However, in recent years a few physicists and cosmologists have started to tackle it. They point out that we now have an understanding of the history of the universe, and of the physical laws that describe how it works. That information, they say, should give us a clue about how and why the cosmos exists.</p><p>Their admittedly controversial answer is that the entire universe, from the fireball of the Big Bang to the star-studded cosmos we now inhabit, popped into existence from nothing at all. It had to happen, they say, because \"nothing\" is inherently unstable.</p><p>This idea may sound bizarre, or just another fanciful creation story. But the physicists argue that it follows naturally from science's two most powerful and successful theories: quantum mechanics and general relativity.</p><p>Here, then, is how everything could have come from nothing.</p><p>{\"image\":{\"pid\":\"p02b3t2n\"}}</p><p><strong>Particles from empty space</strong></p><p>First we have to take a look at the realm of quantum mechanics. This is the branch of physics that deals with very small things: atoms and even tinier particles. It is an immensely successful theory, and it underpins most modern electronic gadgets.</p><p>Quantum mechanics tells us that there is no such thing as empty space. Even the most perfect vacuum is actually filled by a roiling cloud of particles and antiparticles, which flare into existence and almost instantaneously fade back into nothingness.</p><p>These so-called virtual particles don't last long enough to be observed directly, but we know they exist by <a href=\"http://math.ucr.edu/home/baez/physics/Quantum/casimir.html\" target=\"_blank\">their effects</a>.</p><p>{\"image\":{\"pid\":\"p02b3tyd\"}}</p><p><strong>Space-time, from no space and no time</strong></p><p>From tiny things like atoms, to really big things like galaxies. Our best theory for describing such large-scale structures is general relativity, Albert Einstein's crowning achievement, which sets out how space, time and gravity work.</p><p>Relativity is very different from quantum mechanics, and so far nobody has been able to combine the two seamlessly. However, some theorists have been able to bring the two theories to bear on particular problems by using carefully chosen approximations. For instance, this approach was used by <a href=\"http://www.hawking.org.uk/\" target=\"_blank\">Stephen Hawking</a> at the University of Cambridge to describe black holes.</p><blockquote><p> In quantum physics, if something is not forbidden, it necessarily happens </p></blockquote><p>One thing they have found is that, when quantum theory is applied to space at the smallest possible scale, space itself becomes unstable. Rather than remaining perfectly smooth and continuous, space and time destabilize, churning and frothing into a foam of space-time bubbles.</p><p>In other words, little bubbles of space and time can form spontaneously. \"If space and time are quantized, they can fluctuate,\" says <a href=\"https://physics.asu.edu/people/faculty/lawrence-krauss\" target=\"_blank\">Lawrence Krauss</a> at Arizona State University in Tempe. \"So you can create virtual space-times just as you can create virtual particles.\"</p><p>What's more, if it's possible for these bubbles to form, you can guarantee that they will. \"In quantum physics, if something is not forbidden, it necessarily happens with some non-zero probability,\" says <a href=\"http://cosmos2.phy.tufts.edu/vilenkin.html\" target=\"_blank\">Alexander Vilenkin</a> of Tufts University in Boston, Massachusetts.</p><p>{\"image\":{\"pid\":\"p02b3tst\"}}</p><p><strong>A universe from a bubble</strong></p><p>So it's not just particles and antiparticles that can snap in and out of nothingness: bubbles of space-time can do the same. Still, it seems like a big leap from an infinitesimal space-time bubble to a massive universe that hosts 100 billion galaxies. Surely, even if a bubble formed, it would be doomed to disappear again in the blink of an eye?</p><blockquote><p> If all the galaxies are flying apart, they must once have been close together </p></blockquote><p>Actually, it is possible for the bubble to survive. But for that we need another trick: cosmic inflation.</p><p>Most physicists now think that the universe began with the Big Bang. At first all the matter and energy in the universe was crammed together in one unimaginably small dot, and this exploded. This follows from the discovery, in the early 20th century, that the universe is expanding. If all the galaxies are flying apart, they must once have been close together.</p><p>Inflation theory proposes that in the immediate aftermath of the Big Bang, the universe expanded much faster than it did later. This seemingly outlandish notion was put forward in the 1980s by <a href=\"http://web.mit.edu/physics/people/faculty/guth_alan.html\" target=\"_blank\">Alan Guth</a> at the Massachusetts Institute of Technology, and refined by <a href=\"http://web.stanford.edu/~alinde/\" target=\"_blank\">Andrei Linde</a>, now at Stanford University.</p><blockquote><p> As weird as it seems, inflation fits the facts </p></blockquote><p>The idea is that, a fraction of a second after the Big Bang, the quantum-sized bubble of space expanded stupendously fast. In an incredibly brief moment, it went from being smaller than the nucleus of an atom to the size of a grain of sand. When the expansion finally slowed, the force field that had powered it was transformed into the matter and energy that fill the universe today. Guth calls inflation \"the ultimate free lunch\".</p><p>As weird as it seems, inflation fits the facts rather well. In particular, it neatly explains why the cosmic microwave background, the faint remnant of radiation left over from the Big Bang, is almost perfectly uniform across the sky. If the universe had not expanded so rapidly, we would expect the radiation to be patchier than it is.</p><p>{\"image\":{\"pid\":\"p02b3vlw\"}}</p><p><strong>The universe is flat and why that's important</strong></p><p>Inflation also gave cosmologists the measuring tool they needed to determine the underlying geometry of the universe. It turns out this is also crucial for understanding how the cosmos came from nothing.</p><p>Einstein's theory of general relativity tells us that the space-time we live in could take three different forms. It could be as flat as a table top. It could curve back on itself like the surface of a sphere, in which case if you travel far enough in the same direction you would end up back where you started. Alternatively, space-time could curve outward like a saddle. So which is it?</p><p>There is a way to tell. You might remember from maths class that the three angles of a triangle add up to exactly 180 degrees. Actually your teachers left out a crucial point: this is only true on a flat surface. If you draw a triangle on the surface of a balloon, its three angles will add up to more than 180 degrees. Alternatively, if you draw a triangle on a surface that curves outward like a saddle, its angles will add up to less than 180 degrees.</p><p>So to find out if the universe is flat, we need to measure the angles of a really big triangle. That's where inflation comes in. It determined the average size of the warmer and cooler patches in the cosmic microwave background. Those patches were measured in 2003, and that gave astronomers a selection of triangles. As a result, we know that on the largest observable scale our universe is flat.</p><p>{\"image\":{\"pid\":\"p02b3w4b\"}}</p><p>It turns out that a flat universe is crucial. That's because only a flat universe is likely to have come from nothing.</p><p>Everything that exists, from stars and galaxies to the light we see them by, must have sprung from somewhere. We already know that particles spring into existence at the quantum level, so we might expect the universe to contain a few odds and ends. But it takes a huge amount of energy to make all those stars and planets.</p><blockquote><p> The energy of matter is exactly balanced by the energy of the gravity the mass creates </p></blockquote><p>Where did the universe get all this energy? Bizarrely, it may not have had to get any. That's because every object in the universe creates gravity, pulling other objects toward it. This balances the energy needed to create the matter in the first place.</p><p>It's a bit like an old-fashioned measuring scale. You can put a heavy weight on one side, so long as it is balanced by an equal weight on the other. In the case of the universe, the matter goes on one side of the scale, and has to be balanced by gravity.</p><p>Physicists have calculated that in a flat universe the energy of matter is exactly balanced by the energy of the gravity the mass creates. But this is only true in a flat universe. If the universe had been curved, the two sums would not cancel out.</p><p>{\"image\":{\"pid\":\"p02b3vrq\"}}</p><p><strong>Universe or multiverse?</strong></p><p>At this point, making a universe looks almost easy. Quantum mechanics tells us that \"nothing\" is inherently unstable, so the initial leap from nothing to something may have been inevitable. Then the resulting tiny bubble of space-time could have burgeoned into a massive, busy universe, thanks to inflation. As Krauss puts it, \"The laws of physics as we understand them make it eminently plausible that our universe arose from nothing - no space, no time, no particles, nothing that we now know of.\"</p><p>So why did it only happen once? If one space-time bubble popped into existence and inflated to form our universe, what kept other bubbles from doing the same?</p><blockquote><p> There could be a mind-boggling smorgasbord of universes </p></blockquote><p>Linde offers a simple but mind-bending answer. He thinks universes have always been springing into existence, and that this process will continue forever.</p><p>When a new universe stops inflating, says Linde, it is still surrounded by space that is continuing to inflate. That inflating space can spawn more universes, with yet more inflating space around them. So once inflation starts it should make an endless cascade of universes, which Linde calls eternal inflation. Our universe may be just one grain of sand on an endless beach.</p><p>Those universes might be profoundly different to ours. The universe next door might have five dimensions of space rather than the three – length, breadth and height – that ours does. Gravity might be ten times stronger or a thousand times weaker, or not exist at all. Matter might be built out of utterly different particles.</p><p>So there could be a mind-boggling smorgasbord of universes. Linde says eternal inflation is not just the ultimate free lunch: it is the only one at which all possible dishes are available.</p><p>As yet we don't have hard evidence that other universes exist. But either way, these ideas give a whole new meaning to the phrase \"Thanks for nothing\".</p><p>{\"image\":{\"pid\":\"p02b3wrr\"}}</p>","BusinessUnit":"bbc.com","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[{"Content":{"AssetImage":[],"Campaign":null,"CollectionOverrides":null,"CollectionType":"column","Description":"The biggest questions about life, the universe and everything - and how to answer them","Name":"The Big Questions","Partner":null,"PrimaryVertical":"wwearth","Title":"The Big Questions","CreationDateTime":"2015-08-31T16:08:19.672121Z","Entity":"collection","Guid":"b223f348-1133-4674-aa14-89c671bab02b","Id":"wwearth/column/the-big-questions","ModifiedDateTime":"2015-09-03T15:02:18.264474Z","Project":"wwearth","Slug":"column/the-big-questions"},"Metadata":{"CreationDateTime":"2015-08-31T16:08:19.672121Z","Entity":"collection","Guid":"b223f348-1133-4674-aa14-89c671bab02b","Id":"wwearth/column/the-big-questions","ModifiedDateTime":"2015-09-03T15:02:18.264474Z","Project":"wwearth","Slug":"column/the-big-questions"},"Urn":"urn:pubstack:jative:collection:wwearth/column/the-big-questions","_id":"5db313aa200e03609818179a"}],"DisableAdverts":false,"DisplayDate":"2014-11-06T10:18:39Z","Geolocation":null,"HeadlineLong":"Why is there something rather than nothing?","HeadlineShort":"Why does anything exist at all?","HideRelated":false,"Horizontal":null,"HyperHorizontal":null,"Intro":"Some physicists think they can explain why the universe first formed. If they are right, our entire cosmos may have sprung out of nothing at all","IsSyndicated":true,"Latitude":"","Location":null,"Longitude":"","Option":null,"Partner":null,"PrimaryVertical":"wwearth","Programme":null,"RelatedStory":[{"Content":{"AssetCustom":"","AssetIbroadcast":null,"AssetImage":[],"AssetImagePromo":null,"AssetInfographic":"","AssetInline":[],"AssetSelect":"","AssetVideoIb2":null,"AssetVideoMps":null,"Author":[],"BodyHtml":"<p>A tiny nearby galaxy seems to have formed in a merger, when two even smaller galaxies collided. It is the smallest such \"compound\" galaxy ever found.</p><p>The finding hints that the universe's first ever galaxies must have been really rather small. What's more, the tiny merged galaxy offers clues to an even deeper mystery: the nature of dark matter, the invisible stuff that seems to make up most of the material in the universe.</p><p>There is lots of evidence that large galaxies grow through repeated mergers – witness the fiery smashup between the two Antennae galaxies (pictured above), which boast a combined mass of billions of suns. But that evidence dries up for galaxies weighing less than about a billion suns. \"They are difficult to observe because the objects are very faint,\" says <a href=\"http://users.camk.edu.pl/lokas/\" target=\"_blank\">Ewa Lokas</a> of the Nicolaus Copernicus Astronomical Center in Warsaw, Poland.</p><p>Earlier this year, though, a team led by <a href=\"http://dark.nbi.ku.dk/people/nicola_amorisco/\" target=\"_blank\">Nicola Amorisco</a> at the University of Copenhagen in Denmark was able to tease out <a href=\"http://dx.doi.org/10.1038/nature12995\" target=\"_blank\">signs of a past merger</a> in a nearby galaxy whose stars weigh a total of just 10 million suns.</p><p>{\"image\":{\"pid\":\"p029nhpq\"}}</p><p>The little galaxy is one of three dozen or so that orbit the Milky Way's nearest large neighbour, <a href=\"http://www.bbc.co.uk/science/space/universe/key_places/andromeda_galaxy\" target=\"_blank\">Andromeda</a> (above). Known as Andromeda II, it has been on astronomers' radar since 2012.</p><p>That's when another group noticed that <a href=\"http://dx.doi.org/10.1088/0004-637X/758/2/124\" target=\"_blank\">its stars did not move in the expected way</a>. \"It's rotating like a cigar, around its longest axis,\" says Amorisco. \"That is very strange.\"</p><p><strong>A galaxy cobbled together</strong></p><p>So his team combed through the earlier observations again and found what appeared to be a stream of stars all moving in sync within Andromeda II. \"This is essentially what you would find if you had a smaller galaxy destructing within a larger one,\" he says.</p><blockquote><p> Signs of mergers can disappear very quickly </p></blockquote><p>Amorisco thinks the merger happened when two small galaxies fell into orbit around the much larger Andromeda galaxy.</p><p>The merger probably took place within the past 3 billion years, since over longer periods the gravity of the galaxy's other inhabitants should scramble such coherent stellar flows. \"It was a very lucky finding – signs of mergers can disappear very quickly,\" Amorisco says.</p><p>Lokas and her colleagues have now come up with a scenario that can explain the details of the merger. They ran computer simulations and found that a head-on collision between two small disc galaxies can produce a merged galaxy that rotates like a cigar, where the cigar points along the axis of the collision (see images below). \"We are able to reproduce this structure along the long axis,\" says Lokas. Her <a href=\"http://dx.doi.org/10.1093/mnrasl/slu128\" target=\"_blank\">study</a> will be printed next month in <em>Monthly Notices Letters of the Royal Astronomical Society</em>.</p><p>{\"image\":{\"pid\":\"p029njs3\"}}</p><p>{\"image\":{\"pid\":\"p029njzp\"}}</p><p>{\"image\":{\"pid\":\"p029nk1f\"}}</p><p>The merger explanation for Andromeda II's stellar motions is \"pretty convincing\", says <a href=\"http://www.nottingham.ac.uk/physics/people/christopher.conselice\" target=\"_blank\">Christopher Conselice</a> of the University of Nottingham in the UK, who was not involved in either study.</p><blockquote><p> The first galaxies are thought to have formed within nests of dark matter </p></blockquote><p>Conselice says there is a hidden benefit from finding out the size of the smallest galaxies to form directly from collapsing gas, rather than from mergers. It would shed light on dark matter.</p><p>The first galaxies are thought to have formed within nests made of dark matter particles, whose speeds should have affected the size of the resulting galaxies. If dark matter is fast-moving, \"the first things to form would be very large\", says Conselice. Sluggish, or \"cold\", dark matter particles, the favoured explanation, would instead have seeded small galaxies.</p><p>The timing of the mergers that followed should also be set by dark matter, says Amorisco. Heavier dark matter particles should have quickly drawn in gas to produce the first galaxies, leading to more mergers earlier on in the universe's history. Lighter dark matter particles would have taken longer to form galaxies, leading to fewer – and later – mergers.</p><p>Andromeda II is the only galaxy in its size range known to have merged, so it is too early to pin down a likely mass for the dark matter this way, Amorisco says. But his team is looking for evidence of mergers in other nearby dwarf galaxies.</p><p>{\"image\":{\"pid\":\"p029nk7t\"}}</p><p>Our own Milky Way is orbited by 20-odd dwarf galaxies, as well as <a href=\"http://news.bbc.co.uk/2/hi/science/nature/3142582.stm\" target=\"_blank\">the entrails of galaxies its gravity has already torn apart</a>. Worryingly, its merging days are not all behind it.</p><p>In <a href=\"http://science.nasa.gov/science-news/science-at-nasa/2012/31may_andromeda/\" target=\"_blank\">about four billion years</a>, <a href=\"http://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html\" target=\"_blank\">the Milky Way will slam into Andromeda</a>, likely tossing our solar system out of its current seat relatively close to the galactic centre.</p><p>But Amorisco says we have nothing to fear from the coming crash. \"The sun will destroy us much earlier than Andromeda,\" he says cheerfully.</p>","BusinessUnit":"worldwide","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[],"DisableAdverts":false,"DisplayDate":"2014-10-30T10:51:52Z","Geolocation":null,"HeadlineLong":"Little galaxy was born of two colliding galaxies","HeadlineShort":"Tiny galaxy born in stellar crash","HideRelated":false,"Horizontal":null,"HyperHorizontal":null,"Intro":"A nearby dwarf galaxy was formed when two even smaller galaxies collided. The finding could tell us about the mysterious dark matter that fills the universe","IsSyndicated":true,"Latitude":"","Location":null,"Longitude":"","Option":null,"Partner":null,"PrimaryVertical":"wwearth","Programme":null,"RelatedStory":[],"RelatedTag":[],"StoryType":"image","SummaryLong":"A nearby dwarf galaxy was formed when two even smaller galaxies collided. The finding could tell us about the mysterious dark matter that fills the universe","SummaryShort":"A nearby dwarf galaxy was formed when two even smaller galaxies collided","SuperSection":null,"Tag":[]},"Metadata":{"CreationDateTime":"2014-10-30T10:51:52Z","Entity":"story","Guid":"fd7ecfa8-f00a-4268-85dd-0ce850b61847","Id":"wwearth/story/20141030-dwarf-galaxy-formed-in-collision","ModifiedDateTime":"2015-09-03T09:18:47.303257Z","Project":"wwearth","Slug":"20141030-dwarf-galaxy-formed-in-collision"},"Urn":"urn:pubstack:jative:story:wwearth/story/20141030-dwarf-galaxy-formed-in-collision","_id":"5db4c301200e036098265707"},{"Content":{"AssetCustom":"","AssetIbroadcast":null,"AssetImage":[],"AssetImagePromo":null,"AssetInfographic":"","AssetInline":[],"AssetSelect":"","AssetVideoIb2":null,"AssetVideoMps":null,"Author":[],"BodyHtml":"<p>Space is cold. Very cold. In fact, empty space, far from any star or other hot object, is about -270 degrees C.</p><p>While downright frigid—a temperature low enough to freeze hydrogen on Earth—that's still about 2.7 degrees above absolute zero, the lowest possible temperature. The source of those couple of degrees is primordial: the leftover glow of the big bang that gave birth to our universe.</p><p>The entire cosmos is bathed in this radiation, called the cosmic microwave background. As a result, it's hard to avoid this bit of heat, meaning that in most of the cosmos, -270 degrees is as cold as it gets.</p><p>But not everywhere.</p><p>{\"image\":{\"pid\":\"p0262lrs\"}}</p><p>5,000 light years away in Centaurus, a large constellation in the southern sky, is the Boomerang Nebula, a cloud of gas being expelled from a dying star.</p><blockquote><p> This is how stars die </p></blockquote><p>This cloud is one of the most bizarre and mysterious objects in the universe. Here, within the gas streaming outwards, astronomers have found that the temperature drops as low as half a degree above absolute zero.</p><p>It is, as far as anyone knows, the coldest place in the universe.</p><p>It may also prove to be quite important. Because this most frigid place, and objects like it, albeit a tad warmer—may help astronomers unravel a host of cosmic conundrums, from the violent yet spectacular deaths of stars and the formation of galaxies to cosmic explosions and the origin of life itself.</p><p><strong>Death of stars, birth of life</strong></p><p>In many respects the Boomerang Nebula is unremarkable. All stars have to die some day. When smaller stars end, those up to about eight times as massive as our own sun, they produce a similar display of gas and dust.</p><p>{\"image\":{\"pid\":\"p0262m9h\"}}</p><p>During this transformation, each dying low-mass star will cool and swell, becoming what's called a red giant. In a few billion years, when our own sun exhausts its nuclear fuel, it will similarly cool and grow, until it engulfs Mercury, Venus, and possibly even Earth.</p><p>The temperature of the star's outer layers drops low enough such that molecules start clumping together, condensing into dust particles. Starlight radiating from below smacks into these particles and ejects them outward. The particles drag the star's outer gas layers along, creating vast clouds like the ones seen in the Boomerang.</p><blockquote><p> In every which way we look at this object, it's extreme </p></blockquote><p>Ultraviolet radiation from the dying star heats the gas, making it glow. Eventually, the radiation strips away the electrons from the atoms that make up the clouds. Once this ionisation process is complete, what’s left is called a planetary nebula, which is a misnomer of a name, originating when astronomers a century ago mistook these bright objects for planets. Meanwhile, the dying star collapses into its final stage: a hot and dense object called a white dwarf. Our sun will collapse into a white dwarf the size of Earth.</p><p>{\"image\":{\"pid\":\"p0262mg3\"}}</p><p>\"This is how stars die,\" says Sun Kwok, an astronomer at the University of Hong Kong. \"They are born; they have a long life—billions of years of life. And they die very suddenly over a very short period of time.”</p><p>But that also means that objects such as the Boomerang Nebula are incredibly useful; as by studying them astronomers can solve the mysteries of stellar death. \"We're interested in how they die, why they die,\" Kwok says. \"The good thing is that before they die, they put up a huge spectacular show—like fireworks.\"</p><p>And the deaths of stars play a crucial role in the birth of life. Astronomers have long known that many elements such as carbon, oxygen, and even iron are fused inside the cores of stars. When the stars die, those elements are distributed across the galaxy. And when very massive stars die—those more than about eight times the mass of the sun—they explode instead of creating planetary nebulae, creating even heavier elements that become the building blocks for rocks, planets, and even life.</p><p>{\"image\":{\"pid\":\"p0262mlf\"}}</p><p>In the last decade, Kwok says, he and his colleagues are learning that even planetary nebulae may be contributing to life by producing complex organic compounds. Some of these compounds may have made their way to our solar system as the planets were forming. And, they may have been key ingredients for the origin of life on Earth.</p><p><strong>A special nebula</strong></p><p>The Boomerang Nebula, however, is special.</p><p>For a start, the planetary nebula phase of a star's lifecycle lasts only a few tens of thousands of years. The Boomerang is not yet a full-fledged planetary nebula, since its central star hasn't ionised its surroundings. So it's a pre-planetary nebula, a transition stage that lasts only about a thousand years—a mere blink in cosmic time, and one that we are lucky to witness.</p><p>{\"image\":{\"pid\":\"p0262nh4\"}}</p><p>Pre-planetary nebulae are important to astronomers such as Kwok because they provide a glimpse for how stars transform from a swollen red giant to a complex and dazzling planetary nebula. Although a dying star is round, a planetary nebula is not. It often has a bipolar shape, with lobes expanding out from two ends. As the Hubble Space Telescope has revealed in dramatic fashion, from our point of view on Earth, these nebulae sometimes appear to have intricate structures of interlocking rings and arcs.</p><blockquote><p> I don't think there's any theoretical explanation yet as to how this object is what it is right now </p></blockquote><p>The metamorphosis of a round star into a planetary nebula is akin to a caterpillar turning into a butterfly, says Kwok, who did pioneering work on pre-planetary nebulae in the 1990s. Looking at the Boomerang, he says, is like peering into a cocoon just before a butterfly emerges.</p><p>But none of that explains why the Boomerang is so cold?</p><p>{\"image\":{\"pid\":\"p0262nwf\"}}</p><p>The Boomerang Nebula got its name because it appeared to have a curved shape like a boomerang. In 1995, Raghvendra Sahai, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, California, and Lars-Åke Nyman, now at the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile, took a closer look with a telescope in Chile, observing in millimetre wavelengths that revealed clouds of gas molecules. They found that the Boomerang wasn't a boomerang, but a round cloud expanding at a prodigious rate.</p><p>\"In every which way we look at this object, it's extreme,\" Sahai says.</p><p>He and Nyman discovered that the gas was gushing out at 164 km/s, almost 4,000 times faster than the average high-speed train and ten times faster than the typical speeds seen in similar objects. Such high speeds meant that for the last 1,500 years, the central star was losing mass at a rate of one-thousandths of a sun every year, ten times faster than what's been measured in similar stars that are ejecting gas.</p><p>{\"image\":{\"pid\":\"p0262ny0\"}}</p><p>This speed is why the Boomerang is so cold, Sahai explains.</p><p>Gas gets cold as it expands, which you can feel if you place your hand over a tyre nozzle as air is being let out. And if the gas expands as fast as it does in the Boomerang, it can get really cold. The nebula also contains a lot of gas, which makes it difficult for the ambient heat from the cosmic microwave background to seep in, helping the gas remain at a low temperature. With the exception of the artificial conditions created in certain laboratories on Earth, there's no known colder place in the universe.</p><p>The cold wasn't a complete surprise, however.</p><p>Sahai previously hypothesised that if certain conditions were just right, and if the central star were ejecting gas fast enough, the temperature could drop below the cosmic microwave background. Still, it was just a theoretical possibility. When he started analysing that Boomerang data nearly 20 years ago, however, he realised that his prediction was coming true. \"My hair stood on end,\" he recalls. \"That was one of the most exciting parts of my career.\"</p><p>{\"image\":{\"pid\":\"p0262p37\"}}</p><p>Still, exactly how the central star ejects gas so fast remains a mystery, Sahai says.</p><p>According to conventional theory, it's the radiation from the star that's pushing out all that material. But, the star inside the Boomerang is nowhere bright enough to produce the radiation needed to cause gas to be ejected at 164 km/s. \"I don't think there's any theoretical explanation yet as to how this object is what it is right now,\" Sahai says.</p><p>Bizarre indeed.</p><p><strong>Boomerang revisited</strong></p><p>The Boomerang has also perplexed scientists in other ways. In the nearly two decades since the Boomerang was discovered to be the coldest region in the universe, Sahai and his colleagues have continued to explore the extreme object, slowly peeling back layers of complexity and mystery.</p><p>And one of the first puzzles to solve was its shape.</p><p>{\"image\":{\"pid\":\"p0262pm6\"}}</p><p>Sahai and Nyman's observations in submillimetre wavelengths revealed that the Boomerang consisted of a round, expanding molecular cloud. But what did it look like in visible light? In 1998, astronomers pointed the Hubble Space Telescope at the Boomerang to find out. The nebula didn't look round nor did it look like a boomerang. Instead, it boasted an hourglass figure.</p><p>Astronomers didn't know why the Boomerang looked so different in visible light compared to submillimetre, and the problem wasn't solved until last year, when Sahai and his colleagues described their latest observations using the new ALMA telescope in Chile, which allowed them make the most detailed observations of the nebula yet.</p><p>The researchers discovered that the Boomerang has a complex structure consisting of three parts. First, there is the large, round, expanding molecular gas cloud—the same cloud that was observed earlier. But zooming in, the astronomers found a denser, doughnut-shaped cloud of dust surrounding the central star.</p><p>{\"image\":{\"pid\":\"p0262prp\"}}</p><p>This dusty doughnut, the astronomers realised, acts like a mask, blocking the starlight emanating from the star's equator. Because light can only escape from the two poles, it illuminates the surrounding gas like two flashlights pointing in opposite directions. So the two lobes seen in the Hubble images are the beams of those flashlights shining through the gas—just as how you can see the beams of a car's headlights on a foggy night.</p><blockquote><p> These are not just beautiful objects. They hold many secrets </p></blockquote><p>The new ALMA observations showed why the Boomerang could appear both round and hourglass-shaped. But zooming in further, the astronomers found yet another structure: a hollow cylindrical nebula surrounding the central star. Sahai suspects that the cylindrical walls were formed by powerful jets of hydrogen or helium gas blasting from the star's poles, carving out a tunnel in the ambient gas.</p><p><strong>Jetting forward</strong></p><p>Where do these jets come from?</p><p>It turns out that jets are a common phenomenon in the universe, shooting out from many kinds of stars and even enormous black holes billions of times more massive than the sun. Although the details are unknown, they happen when a disc of gas and dust spirals into the star or black hole. The falling matter carries energy, which is released via narrow jets shooting out in opposite directions.</p><p>{\"image\":{\"pid\":\"p0262px3\"}}</p><p>In planetary nebulae, these jets are made of gas. But in the supermassive black holes that reside at the centre of galaxies, they're likely charged particles blasting out at extreme speeds. These black-hole powered jets are so powerful that they can blow bubbles in the hot gas that permeates the space between the galaxies in a galaxy cluster. The way these jets inject heat and gas into their environments influences how galaxies form and evolve.</p><p>Even though the bubbles blown by these jets are up to a million times bigger and even though the gas, at tens of million of degrees, is far from cold, the general process is the same as what happens in systems such as the Boomerang, says astronomer Noam Soker of Technion University in Israel. So by studying the jets in the Boomerang and other planetary nebulae, astronomers can learn about galaxies and the supermassive black hole at their centres.</p><p>Jets are also thought to be involved in the strange explosions known as gamma-ray bursts, which are some of the most powerful cosmic phenomena observed, Soker says. He also thinks they may help drive supernovae—the explosive deaths of very massive stars.</p><p>{\"image\":{\"pid\":\"p0262pys\"}}</p><p>\"This is a highly controversial subject,\" he notes, as most astronomers think supernovae are propelled by an eruption of energetic particles called neutrinos. Still, current theories aren't satisfactory and the ubiquity of jets makes them a plausible mechanism, he says.</p><p>As for the Boomerang, there's still much to learn about what Sahai calls one of his favourite objects in the universe.</p><p>He and his colleagues plan to study it further with ALMA later this year. Their earlier observations showed that in the inner regions, the gas moves at a mere 35 km/s. With more detailed data, they hope to map exactly how fast different regions of the expanding gas cloud are moving. They also want to better understand the dusty doughnut at the centre.</p><p>The Boomerang is bizarre because it's a frigid place. But for astronomers, the nebula and its brethren are more than that. \"These are not just beautiful objects,\" Soker says. \"They hold many secrets.\"</p><p>{\"image\":{\"pid\":\"p0262q7r\"}}</p>","BusinessUnit":"bbc.com","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[],"DisableAdverts":false,"DisplayDate":"2014-09-05T04:35:44Z","Geolocation":null,"HeadlineLong":"Why the coldest place in the universe is so special","HeadlineShort":"The coldest place in the universe","HideRelated":false,"Horizontal":null,"HyperHorizontal":null,"Intro":"The Boomerang Nebula, a strange and extremely frigid object 5,000 light years away, gradually gives up its secrets","IsSyndicated":true,"Latitude":"","Location":null,"Longitude":"","Option":null,"Partner":null,"PrimaryVertical":"wwearth","Programme":null,"RelatedStory":null,"RelatedTag":null,"StoryType":"image","SummaryLong":"A strange and extremely frigid object 5,000 light years away gradually gives up its secrets","SummaryShort":"A strange object 5,000 light years away gradually gives up its secrets ","SuperSection":null,"Tag":[]},"Metadata":{"CreationDateTime":"2014-09-05T04:35:44Z","Entity":"story","Guid":"fbf639fb-ef73-4602-93f7-15dbd89304fc","Id":"wwearth/story/20140916-the-coldest-place-in-the-universe","ModifiedDateTime":"2015-09-03T09:18:47.303257Z","Project":"wwearth","Slug":"20140916-the-coldest-place-in-the-universe"},"Urn":"urn:pubstack:jative:story:wwearth/story/20140916-the-coldest-place-in-the-universe","_id":"5db880f9200e036098a51b5a"}],"RelatedTag":[{"Content":{"AssetImage":null,"Description":"","LinkUrl":"","Name":"Physics"},"Metadata":{"CreationDateTime":"2014-11-05T16:17:17Z","Entity":"tag","Guid":"6e9f0e97-66ce-4f69-bed7-82ed4ab69aa1","Id":"tag/physics","ModifiedDateTime":"2015-09-03T09:10:14.283862Z","Project":"","Slug":"physics"},"Urn":"urn:pubstack:jative:tag:tag/physics","_id":"5db314e9200e0360981f0527"}],"StoryType":"image","SummaryLong":"Some physicists think they can explain why the universe first formed. Our entire cosmos may have sprung out of nothing at all","SummaryShort":"Some physicists think they can explain why the universe first formed","SuperSection":null,"Tag":[{"Content":{"AssetImage":null,"Description":"","LinkUrl":"","Name":"Universe","CreationDateTime":"2014-09-02T14:03:15Z","Entity":"tag","Guid":"46dd8487-4d26-4943-bbc3-f43ee6c0a529","Id":"tag/universe","ModifiedDateTime":"2015-09-03T09:07:52.535982Z","Project":"","Slug":"universe"},"Metadata":{"CreationDateTime":"2014-09-02T14:03:15Z","Entity":"tag","Guid":"46dd8487-4d26-4943-bbc3-f43ee6c0a529","Id":"tag/universe","ModifiedDateTime":"2015-09-03T09:07:52.535982Z","Project":"","Slug":"universe"},"Urn":"urn:pubstack:jative:tag:tag/universe","_id":"5db314e9200e0360981f0574"}],"CreationDateTime":"2014-11-06T10:18:39Z","Entity":"story","Guid":"8413a759-8f12-44e7-a4f0-e7792f69e51b","Id":"wwearth/story/20141106-why-does-anything-exist-at-all","ModifiedDateTime":"2015-09-03T09:11:55.512873Z","Project":"wwearth","Slug":"20141106-why-does-anything-exist-at-all"},"Metadata":{"CreationDateTime":"2014-11-06T10:18:39Z","Entity":"story","Guid":"8413a759-8f12-44e7-a4f0-e7792f69e51b","Id":"wwearth/story/20141106-why-does-anything-exist-at-all","ModifiedDateTime":"2015-09-03T09:11:55.512873Z","Project":"wwearth","Slug":"20141106-why-does-anything-exist-at-all"},"Urn":"urn:pubstack:jative:story:wwearth/story/20141106-why-does-anything-exist-at-all","_id":"5db86e55200e036098923f6b"},{"Content":{"AssetCustom":"","AssetIbroadcast":null,"AssetImage":[{"Content":{"Copyright":"Henning Dalhoff/SPL","FileSizeBytes":1827965,"MimeType":"image/jpeg","SourceHeight":2781,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2r/z7/p02rz7np.jpg","SourceWidth":4944,"SynopsisLong":"Black holes are distortions in the fabric of space (Credit: Henning Dalhoff/SPL)","SynopsisMedium":"Black holes are distortions in the fabric of space (Credit: Henning Dalhoff/SPL)","SynopsisShort":"Black holes are distortions in the fabric of space (Credit: Henning Dalhoff/SPL)","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2r/z7/p02rz7np.jpg","Title":"Black_hole_artwork-Henning_Dalhoff-SPL.jpg","CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02rz7np","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02rz7np","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02rz7np","_id":"5db5658f200e0360982903d6"}],"AssetImagePromo":null,"AssetInfographic":"","AssetInline":[{"Content":{"Copyright":"Richard Kail/SPL","FileSizeBytes":800056,"MimeType":"image/jpeg","SourceHeight":2199,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2r/z7/p02rz7mc.jpg","SourceWidth":3909,"SynopsisLong":"The event horizon is not a solid barrier (Credit: Richard Kail/SPL)","SynopsisMedium":"The event horizon is not a solid barrier (Credit: Richard Kail/SPL)","SynopsisShort":"The event horizon is not a solid barrier (Credit: Richard Kail/SPL)","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2r/z7/p02rz7mc.jpg","Title":"event_horizon-Richard_Kail-SPL.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02rz7mc","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02rz7mc","_id":"5db92107200e03609834cce9"},{"Content":{"Copyright":"Richard Kail/SPL","FileSizeBytes":403229,"MimeType":"image/jpeg","SourceHeight":1978,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2r/z7/p02rz7nb.jpg","SourceWidth":3516,"SynopsisLong":"\"Hawking radiation\" flows out of the event horizon (Credit: Richard Kail/SPL)","SynopsisMedium":"\"Hawking radiation\" flows out of the event horizon (Credit: Richard Kail/SPL)","SynopsisShort":"\"Hawking radiation\" flows out of the event horizon (Credit: Richard Kail/SPL)","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2r/z7/p02rz7nb.jpg","Title":"Hawking_radiation-Richard_Kail-SPL.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02rz7nb","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02rz7nb","_id":"5db8c05f200e036098e2a835"},{"Content":{"Copyright":"NASA/CXC/M. 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(Credit: Victor de Schwanberg/SPL)","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2r/z7/p02rz7l0.jpg","Title":"C0196918-Quantum_entanglement,_conceptual_artwork-Victor_de_Schwanberg-SPL.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02rz7l0","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02rz7l0","_id":"5db8c065200e036098e2affa"},{"Content":{"Copyright":"Ute Kraus, CC by 2.5","FileSizeBytes":873763,"MimeType":"image/jpeg","SourceHeight":1440,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2r/zf/p02rzf13.jpg","SourceWidth":2560,"SynopsisLong":"Black holes distort passing light rays, causing \"lensing\" (Credit: Ute Kraus, CC by 2.5)","SynopsisMedium":"Black holes distort passing light rays, causing \"lensing\" (Credit: Ute Kraus, CC by 2.5)","SynopsisShort":"Black holes distort passing light rays, causing \"lensing\" (Credit: Ute Kraus, CC by 2.5)","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2r/zf/p02rzf13.jpg","Title":"Black_Hole_Milkyway-Ute_Kraus_CCby25.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02rzf13","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02rzf13","_id":"5db8c061200e036098e2ab09"},{"Content":{"Copyright":"ESO/WFI/MPIfR/APEX/A. Weiss/NASA/CXC/CfA/R. Kraft","FileSizeBytes":286129,"MimeType":"image/jpeg","SourceHeight":1502,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2r/zf/p02rzf5l.jpg","SourceWidth":2669,"SynopsisLong":"Centaurus A has a black hole (Credit: ESO/WFI/MPIfR/APEX/A. Weiss/NASA/CXC/CfA/R. Kraft)","SynopsisMedium":"Centaurus A has a black hole (Credit: ESO/WFI/MPIfR/APEX/A. Weiss/NASA/CXC/CfA/R. Kraft)","SynopsisShort":"Centaurus A has a black hole (Credit: ESO/WFI/MPIfR/APEX/A. Weiss/NASA/CXC/CfA/R. Kraft)","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2r/zf/p02rzf5l.jpg","Title":"Black_Hole_Outflows_From_Centaurus_A-ESO-WFI-MPIfR-ESO-APEX-A_Weiss-NASA-CXC-CfA-R_Kraft.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02rzf5l","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02rzf5l","_id":"5db8c05e200e036098e2a7f1"},{"Content":{"Copyright":"Julian Baum/SPL","FileSizeBytes":1597852,"MimeType":"image/jpeg","SourceHeight":2276,"SourceUrl":"https://web.archive.org/web/20191030084519/http://deltaorigin.bbc.co.uk/images/live/p0/2r/z7/p02rz7kc.jpg","SourceWidth":4047,"SynopsisLong":"Heavy objects warp the fabric of space itself (Credit: Julian Baum/SPL)","SynopsisMedium":"Heavy objects warp the fabric of space itself (Credit: Julian Baum/SPL)","SynopsisShort":"Heavy objects warp the fabric of space itself (Credit: Julian Baum/SPL)","TemplateUrl":"https://web.archive.org/web/20191030084519/https://ichef.bbci.co.uk/wwfeatures/$recipe/images/live/p0/2r/z7/p02rz7kc.jpg","Title":"warped_space-Julian_Baum-SPL.jpg"},"Metadata":{"CreationDateTime":"0001-01-01T00:00:00Z","Entity":"","Guid":"","Id":"p02rz7kc","ModifiedDateTime":"0001-01-01T00:00:00Z","Project":"","Slug":""},"Urn":"urn:pubstack:jative:image:p02rz7kc","_id":"5db91f17200e0360983353dd"}],"AssetSelect":"","AssetVideoIb2":null,"AssetVideoMps":null,"Author":[{"Content":{"AssetImage":null,"Description":"","Email":"","Links":null,"Name":"Amanda Gefter","PrimaryVertical":"wwearth"},"Metadata":{"CreationDateTime":"2015-05-22T12:04:25Z","Entity":"author","Guid":"8fecaa00-f311-48d4-bd5d-b366e2215e39","Id":"wwearth/author/amanda-gefter","ModifiedDateTime":"2015-09-03T09:12:35.415216Z","Project":"wwearth","Slug":"amanda-gefter"},"Urn":"urn:pubstack:jative:author:wwearth/author/amanda-gefter","_id":"5db313a8200e0360981801bf"}],"BodyHtml":"<p><em>This was the most-read story on <strong>BBC Earth</strong> in 2015. Here is another chance to read it.</em></p><p>It could happen to anyone. Maybe you're out trying to find a new habitable planet for the human race, or maybe you're just on a long walk and you slip. Whatever the circumstances, at some point we all find ourselves confronted with the age-old question: what happens when you fall into a black hole?</p><p>You might expect to get crushed, or maybe torn to pieces. But the reality is stranger than that.</p><p>The instant you entered the black hole, reality would split in two. In one, you would be instantly incinerated, and in the other you would plunge on into the black hole utterly unharmed.</p><p>{\"image\":{\"pid\":\"p02rz7kc\"}}</p><p>A black hole is a place where the laws of physics as we know them break down. Einstein taught us that gravity warps space itself, causing it to curve. So given a dense enough object, space-time can become so warped that it twists in on itself, burrowing a hole through the very fabric of reality.</p><p>A massive star that has run out of fuel can produce the kind of extreme density needed to create such a mangled bit of world. As it buckles under its own weight and collapses inward, space-time caves in with it. The gravitational field becomes so strong that not even light can escape, rendering the region where the star used to be profoundly dark: a black hole.</p><blockquote><p> As you go deeper into the black hole, space becomes ever more curvy </p></blockquote><p>The outermost boundary of the hole is its event horizon, the point at which the gravitational force precisely counteracts the light's efforts to escape it. Go closer than this, and there's no escape.</p><p>The event horizon is ablaze with energy. Quantum effects at the edge create streams of hot particles that radiate back out into the universe. This is called Hawking radiation, after the physicist Stephen Hawking, who predicted it. Given enough time, the black hole will radiate away its mass, and vanish.</p><p>As you go deeper into the black hole, space becomes ever more curvy until, at the centre, it becomes infinitely curved. This is the singularity. Space and time cease to be meaningful ideas, and the laws of physics as we know them — all of which require space and time — no longer apply.</p><p>What happens here, no one knows. Another universe? Oblivion? <a href=\"http://www.imdb.com/title/tt0816692/\">The back of a bookcase</a>? It's a mystery.</p><p>{\"image\":{\"pid\":\"p02rz7lq\"}}</p><p>So what happens if you accidentally fall into one of these cosmic aberrations? Let's start by asking your space companion — we'll call her Anne — who watches in horror as you plunge toward the black hole, while she remains safely outside. From where she's floating, things are about to get weird.</p><p>As you accelerate toward the event horizon, Anne sees you stretch and contort, as if she were viewing you through a giant magnifying glass. What's more, the closer you get to the horizon the more you appear to move in slow motion.</p><blockquote><p> Before you ever cross over into the black hole's darkness, you're reduced to ash </p></blockquote><p>You can't shout to her, as there's no air in space, but you might try flashing her a Morse message with the light on your iPhone (there's an app for that). However, your words reach her ever more slowly, the light waves stretching to increasingly lower and redder frequencies: \"Alright, a l r i g h t,   a   l    r     i…\"</p><p>When you reach the horizon, Anne sees you freeze, like someone has hit the pause button. You remain plastered there, motionless, stretched across the surface of the horizon as a growing heat begins to engulf you.</p><p>According to Anne, you are slowly obliterated by the stretching of space, the stopping of time and the fires of Hawking radiation. Before you ever cross over into the black hole's darkness, you're reduced to ash.</p><p>But before we plan your funeral, let's forget about Anne and view this gruesome scene from your point of view. Now, something even stranger happens: nothing.</p><p>{\"image\":{\"pid\":\"p02rz7jf\"}}</p><p>You sail straight into nature's most ominous destination without so much as a bump or a jiggle – and certainly no stretching, slowing or scalding radiation. That's because you're in freefall, and therefore you feel no gravity: something Einstein called his \"happiest thought\".</p><blockquote><p> In a big enough black hole, you could live out the rest of your life pretty normally </p></blockquote><p>After all, the event horizon is not like a brick wall floating in space. It's an artefact of perspective. An observer who remains outside the black hole can't see through it, but that's not your problem. As far as you're concerned there is no horizon.</p><p>Sure, if the black hole were smaller you'd have a problem. The force of gravity would be much stronger at your feet than at your head, stretching you out like a piece of spaghetti. But lucky for you this is a big one, millions of times more massive than our Sun, so the forces that might spaghettify you are feeble enough to be ignored.</p><p>In fact, in a big enough black hole, you could live out the rest of your life pretty normally before dying at the singularity.</p><p>{\"image\":{\"pid\":\"p02rz7mc\"}}</p><p>How normal could it really be, you might wonder, given that you're being sucked toward a rupture in the space-time continuum, pulled along against your will, unable to head back the other way?</p><blockquote><p> You can't turn around and escape the black hole </p></blockquote><p>But when you think about it, we all know that feeling, not from our experience with space but with time. <a href=\"http://www.bbc.com/earth/story/20150309-why-does-time-only-run-forwards\">Time only goes forwards, never backwards, and it pulls us along against our will</a>, preventing us from turning around.</p><p>This isn't just an analogy. Black holes warp space and time to such an extreme that inside the black hole's horizon, space and time actually swap roles. In a sense, it really is time that pulls you in toward the singularity. You can't turn around and escape the black hole, any more than you can turn around and travel back to the past.</p><p>At this point you might want to stop and ask yourself a pressing question: What the hell is wrong with Anne? If you're chilling inside the black hole, surrounded by nothing weirder than empty space, why is she insisting that you've been burned to a crisp by radiation outside the horizon? Is she hallucinating?</p><p>{\"image\":{\"pid\":\"p02rz7nb\"}}</p><p>Actually, Anne is being perfectly reasonable. From her point of view, you really have been burned to a crisp at the horizon. It's not an illusion. She could even collect your ashes and send them back to your loved ones.</p><p>In fact, the laws of nature require that you remain outside the black hole as seen from Anne's perspective. That's because quantum physics demands that information can never be lost. Every bit of information that accounts for your existence has to stay on the outside of the horizon, lest Anne's laws of physics be broken.</p><blockquote><p> You have to be in two places, but there can only be one copy of you </p></blockquote><p>On the other hand, the laws of physics also require that you sail through the horizon without encountering hot particles or anything out of the ordinary. Otherwise you'd be in violation of Einstein's happiest thought, and his theory of general relativity.</p><p>So the laws of physics require that you be both outside the black hole in a pile of ashes and inside the black hole alive and well. Last but not least, there's a third law of physics that says information can't be cloned. You have to be in two places, but there can only be one copy of you.</p><p>Somehow, the laws of physics point us towards a conclusion that seems rather nonsensical. Physicists call this infuriating conundrum the black hole information paradox. Luckily, in the 1990s they found a way to resolve it.</p><p>{\"image\":{\"pid\":\"p02rz7hw\"}}</p><p><a href=\"http://theoreticalminimum.com/biography\">Leonard Susskind</a> realized that there is no paradox, because no one person ever sees your clone. Anne only sees one copy of you. You only see one copy of you. You and Anne can never compare notes. And there's no third observer who can see both inside and outside a black hole simultaneously. So, no laws of physics are broken.</p><blockquote><p> Reality depends on whom you ask </p></blockquote><p>Unless, that is, you demand to know which story is really true. Are you really dead or are you really alive?</p><p>The great secret that black holes have revealed to us is that there is no really<em>. </em>Reality depends on whom you ask. There is Anne's reality and there is your reality. End of story.</p><p>Well, almost. In the summer of 2012, the physicists Ahmed Almheiri, Donald Marolf, Joe Polchinski and James Sully, collectively known as AMPS, <a href=\"http://dx.doi.org/10.1007/JHEP02(2013)062\">devised a thought experiment</a> that threatened to upend everything we thought we knew about black holes.</p><p>{\"image\":{\"pid\":\"p02rz7hl\"}}</p><p>They realized that Susskind's solution hinged on the fact that any disagreement between you and Anne is mediated by the event horizon. It didn't matter if Anne saw the unlucky version of you scattered amongst the Hawking radiation, because the horizon prevented her from seeing the other version of you floating along inside the black hole.</p><blockquote><p> Anne might sneak a peek behind the horizon </p></blockquote><p>But what if there was a way for her to find out what was on the other side of the horizon, without actually crossing it?</p><p>Ordinary relativity would say that's a no-no, but quantum mechanics makes the rules a little fuzzier. Anne might sneak a peek behind the horizon, using a little trick that Einstein called \"spooky action-at-a-distance\".</p><p>This happens when two sets of particles that are separated in space are mysteriously \"entangled\". They are part of a single, indivisible whole, so that the information needed to describe them can't be found in either set alone, but in the spooky links between them.</p><p>{\"image\":{\"pid\":\"p02rz7l0\"}}</p><p>The AMPS idea went something like this. Let's say Anne grabs hold of a bit of information near the horizon — call it A.</p><blockquote><p> Each bit of information can only be entangled once </p></blockquote><p>If her story is right, and you are a goner, scrambled amongst the Hawking radiation outside the black hole, then A must be entangled with another bit of information, B, which is also part of the hot cloud of radiation.</p><p>On the other hand, if your story is the true one, and you're alive and well on the other side of the event horizon, then A must be entangled with a different bit of information, C, which is somewhere inside the black hole.</p><p>Here's the kicker: each bit of information can only be entangled once. That means A can only be entangled with B or with C, not with both.</p><p>{\"image\":{\"pid\":\"p02rzdx6\"}}</p><p>So Anne takes her bit, A, and puts it through her handy entanglement-decoding machine, which spits out an answer: either B or C.</p><blockquote><p> Do you glide right through and live a normal life? </p></blockquote><p>If the answer turns out to be C, then your story wins, but the laws of quantum mechanics are broken. If A is entangled with C, which is deep inside the black hole, then that piece of information is lost to Anne forever. That breaks the quantum law that information can never be lost.</p><p>That leaves B. If Anne's decoding machine finds that A is entangled with B, then Anne wins, and general relativity loses. If A is entangled with B, then Anne's story is the one true story, which means you really were burned to a crisp. Instead of sailing straight through the horizon, as relativity says you should, you hit a burning firewall.</p><p>So we're back where we started: what happens when you fall into a black hole? Do you glide right through and live a normal life, thanks to a reality that's strangely observer-dependent? Or do you approach the black hole's horizon only to collide with a deadly firewall?</p><p>{\"image\":{\"pid\":\"p02rzf13\"}}</p><p>No one knows the answer, and it's become one of the most contentious questions in fundamental physics.</p><blockquote><p> It would take Anne an extraordinarily long time to decode the entanglement </p></blockquote><p>Physicists have spent more than a century <a href=\"http://www.bbc.com/earth/story/20150409-can-science-ever-explain-everything\">trying to reconcile general relativity with quantum mechanics</a>, knowing that eventually one or the other was going to have to give. The solution to the firewall paradox should tell us which, and point the way to an even deeper theory of the universe.</p><p>One clue might lie in Anne's decoding machine. Figuring out which other bit of information A is entangled with is an extraordinarily complicated problem. So physicists <a href=\"https://scholar.google.co.uk/citations?user=lz4HDRwAAAAJ&hl=en\">Daniel Harlow</a> of Princeton University in New Jersey and <a href=\"http://web.stanford.edu/~phayden/\">Patrick Hayden</a>, now at Stanford University in California, wondered how long it would take.</p><p>In 2013 they calculated that, even given the fastest computer that the laws of physics would allow, it would take Anne an extraordinarily long time to decode the entanglement. By the time she had an answer, <a href=\"http://dx.doi.org/10.1007/JHEP06(2013)085\">the black hole would have long evaporated</a>, disappearing from the universe and taking with it the threat of a deadly firewall.</p><p>{\"image\":{\"pid\":\"p02rzf5l\"}}</p><p>If that's the case, the sheer complexity of the problem could prevent Anne from ever figuring out which story is the real one. That would leave both stories simultaneously true, reality intriguingly observer-dependent, all the laws of physics intact, and no one in danger of running into an inexplicable wall of fire.</p><blockquote><p> If the true nature of reality lies hidden somewhere, the best place to look is a black hole </p></blockquote><p>It also gives physicists something new to think about: the tantalizing connections between complex calculations (like the one Anne apparently can't do) and space-time. This may open the door to something deeper still.</p><p>That's the thing about black holes. They're not just annoying obstacles for space travellers. They're also theoretical laboratories that take the subtlest quirks in the laws of physics, then amplify them to such proportions that they can't be ignored.</p><p>If the true nature of reality lies hidden somewhere, the best place to look is a black hole. It's probably best to look from the outside, though: at least until they figure out this whole firewall thing. Or send Anne in. It's her turn already.</p>","BusinessUnit":"bbc.com","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[{"Content":{"AssetImage":[],"Campaign":null,"CollectionOverrides":null,"CollectionType":"column","Description":"The biggest questions about life, the universe and everything - and how to answer them","Name":"The Big Questions","Partner":null,"PrimaryVertical":"wwearth","Title":"The Big Questions","CreationDateTime":"2015-08-31T16:08:19.672121Z","Entity":"collection","Guid":"b223f348-1133-4674-aa14-89c671bab02b","Id":"wwearth/column/the-big-questions","ModifiedDateTime":"2015-09-03T15:02:18.264474Z","Project":"wwearth","Slug":"column/the-big-questions"},"Metadata":{"CreationDateTime":"2015-08-31T16:08:19.672121Z","Entity":"collection","Guid":"b223f348-1133-4674-aa14-89c671bab02b","Id":"wwearth/column/the-big-questions","ModifiedDateTime":"2015-09-03T15:02:18.264474Z","Project":"wwearth","Slug":"column/the-big-questions"},"Urn":"urn:pubstack:jative:collection:wwearth/column/the-big-questions","_id":"5db313aa200e03609818179a"},{"Content":{"AssetImage":null,"Campaign":null,"CollectionOverrides":null,"CollectionType":"list","Description":"Stories are published through Apple News.","Name":"applenews","Partner":null,"PrimaryVertical":"wwearth","Title":"Apple News"},"Metadata":{"CreationDateTime":"2015-09-08T09:36:25.05565Z","Entity":"collection","Guid":"215de30c-f020-421e-8b8f-44bae51f5380","Id":"wwearth/list/applenews","ModifiedDateTime":"2015-09-08T09:36:25.05565Z","Project":"wwearth","Slug":"list/applenews"},"Urn":"urn:pubstack:jative:collection:wwearth/list/applenews","_id":"5db313aa200e03609818193c"}],"DisableAdverts":false,"DisplayDate":"2015-05-25T04:56:18Z","Geolocation":null,"HeadlineLong":"The strange fate of a person falling into a black hole","HeadlineShort":"What's it like in a black hole?","HideRelated":false,"Horizontal":null,"HyperHorizontal":null,"Intro":"If you fell into a black hole, you might expect to die instantly. But in fact your fate would be far stranger than that","IsSyndicated":true,"Latitude":"","Location":null,"Longitude":"","Option":null,"Partner":null,"PrimaryVertical":"wwearth","Programme":null,"RelatedStory":[{"Content":{"AssetCustom":"","AssetIbroadcast":null,"AssetImage":[],"AssetImagePromo":null,"AssetInfographic":"","AssetInline":[],"AssetSelect":"","AssetVideoIb2":null,"AssetVideoMps":null,"Author":[],"BodyHtml":"<p>There's egg on your face, literally. You tried to juggle some eggs, it all went wrong, and now you've got to shower and change your clothes.</p><p>Wouldn't it be faster to just un-break the egg? Breaking it only took a few seconds, so why not do that again, but in reverse? Just reassemble the shell and throw the yolk and the white back inside. You'd have a clean face, clean clothes, and no yolk in your hair, like nothing ever happened.</p><blockquote><p> Why don't things happen in reverse all the time? </p></blockquote><p>Sounds ridiculous — but why? Why, exactly, is it impossible to un-break an egg?</p><p>It isn't. There's no fundamental law of nature that prevents us from un-breaking eggs. In fact, physics says that any event in our day-to-day lives could happen in reverse, at any time. So why can't we un-break eggs, or un-burn matches, or even un-sprain an ankle? Why don't things happen in reverse all the time? Why does the future look different from the past at all?</p><p>It sounds like a simple question. But to answer it, we've got to go back to the birth of the universe, down to the atomic realm, and out to the frontiers of physics.</p><p>{\"image\":{\"pid\":\"p02l8lm2\"}}</p><p>Like many stories about physics, this one starts with Isaac Newton. In 1666, an outbreak of bubonic plague forced him to leave the University of Cambridge, and move back in with his mother in the Lincolnshire countryside. Bored and isolated, Newton threw himself into the study of physics.</p><blockquote><p> You might mix up east and west, but you would not mix up yesterday and tomorrow </p></blockquote><p>He came up with three laws of motion, including the famous maxim that every action has an equal and opposite reaction. He also devised an explanation of how gravity works.</p><p>Newton's laws are astonishingly successful at describing the world. They explain why apples fall from trees and why the Earth orbits the Sun. But they have an odd feature: they work just as well backwards as forwards. If an egg can break, then Newton's laws say it can un-break.</p><p>This is obviously wrong, but nearly every theory that physicists have discovered since Newton has the same problem. The laws of physics simply don't care whether time runs forwards or backwards, any more than they care about whether you're left-handed or right-handed.</p><p>But we certainly do. In our experience, time has an arrow, always pointing into the future. \"You might mix up east and west, but you would not mix up yesterday and tomorrow,\" says <a href=\"http://preposterousuniverse.com/\" target=\"_blank\">Sean Carroll</a>, a physicist at the California Institute of Technology in Pasadena. \"But the fundamental laws of physics don't distinguish between past and future.\"</p><p>{\"image\":{\"pid\":\"p02l8ky8\"}}</p><p>The first person to seriously tackle this problem was an Austrian physicist named Ludwig Boltzmann, who lived in the late 19th century. At this time, many ideas that are now known to be true were still up for debate. In particular, physicists were not convinced – as they are today - that everything is made up of tiny particles called atoms. The idea of atoms, according to many physicists, was simply impossible to test.</p><blockquote><p> He was ostracised by the physics community for his ideas </p></blockquote><p>Boltzmann was convinced that atoms really did exist. So he set out to use this idea to explain all sorts of everyday stuff, such as the glow of a fire, how our lungs work, and why blowing on tea cools it down. He thought he could make sense of all these things using the concept of atoms.</p><p>A few physicists were impressed with Boltzmann's work, but most dismissed it. Before long he was ostracised by the physics community for his ideas.</p><p>He got into particularly hot water because of his ideas about the nature of heat. This may not sound like it has much to do with the nature of time, but Boltzmann would show that the two things were closely linked.</p><p>{\"image\":{\"pid\":\"p02l8klt\"}}</p><p>At the time, physicists had come up with a theory called thermodynamics, which describes how heat behaves. For instance, thermodynamics describes how a refrigerator can keep food cold on a hot day.</p><p>Boltzmann's opponents thought that heat couldn't be explained in terms of anything else. They said that heat was just heat.</p><p>Boltzmann set out to prove them wrong. He thought heat was caused by the random motion of atoms, and that all of thermodynamics could be explained in those terms. He was absolutely right, but he would spend the rest of his life struggling to convince others.</p><p>{\"image\":{\"pid\":\"p02l8l11\"}}</p><p>Boltzmann started by trying to explain something strange: \"entropy\". According to thermodynamics, every object in the world has a certain amount of entropy associated with it, and whenever anything happens to it, the amount of entropy increases. For instance, if you put ice cubes into a glass of water and let them melt, the entropy inside the glass goes up.</p><p>Rising entropy is unlike anything else in physics: a process that has to go in one direction. But nobody knew why entropy always increased.</p><p>Once again, Boltzmann's colleagues argued that it wasn't possible to explain why entropy always went up. It just did. And again, Boltzmann was unsatisfied, and went searching for a deeper meaning. The result was a radical new understanding of entropy — a discovery so important that he had it engraved on his tombstone.</p><p>{\"image\":{\"pid\":\"p02ldd0t\"}}</p><p>Boltzmann found that entropy measured the number of ways atoms, and the energy they carry, can be arranged. When entropy increases, it's because the atoms are getting more jumbled up.</p><p>According to Boltzmann, this is why ice melts in water. When water is liquid, there are far more ways for the water molecules to arrange themselves, and far more ways for the heat energy to be shared among those molecules, than when the water is solid. There are simply so many ways for the ice to melt, and relatively few ways for it to stay solid, that it's overwhelmingly likely the ice will eventually melt.</p><p>{\"image\":{\"pid\":\"p02llgf9\"}}</p><p>Similarly, if you put a drop of cream into your coffee, the cream will spread throughout the entire cup, because that's a state of higher entropy. There are more ways to arrange the bits of cream throughout your coffee than there are for the cream to remain in one small region.</p><p>Entropy, according to Boltzmann, is about what's probable. Objects with low entropy are tidy, and therefore unlikely to exist. High-entropy objects are untidy, which makes them likely to exist. Entropy always increases, because it's much easier for things to be untidy.</p><p>That may sound a bit depressing, at least if you like your home to be well-organised. But Boltzmann's ideas about entropy do have an upside: they seem to explain the arrow of time.</p><p>{\"image\":{\"pid\":\"p02l8l5y\"}}</p><p>Boltzmann's take on entropy explains why it always increases. That in turn suggests why we always experience time moving forwards. If the universe as a whole moves from low entropy to high entropy, then we should never see events go in reverse.</p><blockquote><p> The future looks different from the past simply because entropy increases </p></blockquote><p>We won't see eggs un-break, because there are lots of ways to arrange the pieces of an egg, and nearly all of them lead to a broken egg rather than an intact one. Similarly, ice won't un-melt, matches won't un-burn, and ankles won't un-sprain.</p><p>Boltzmann's definition of entropy even explains why we can remember the past but not the future. Imagine the opposite: that you have a memory of an event, then the event happens, and then the memory disappears. The odds of that happening to your brain are very low.</p><p>According to Boltzmann, the future looks different from the past simply because entropy increases. But his pesky opponents pointed out a flaw in his reasoning.</p><p>{\"image\":{\"pid\":\"p02l8kvj\"}}</p><p>Boltzmann said that entropy increases as you go into the future, because of the probabilities that govern the behaviour of small objects like atoms. But those small objects are themselves obeying the fundamental laws of physics, which don't draw a distinction between the past and the future.</p><blockquote><p> Why is there an arrow of time at all? </p></blockquote><p>So Boltzmann's argument can be turned on its head. If you can argue that entropy should increase as you go into the future, you can also argue that entropy should increase as you go into the past.</p><p>Boltzmann thought that, because broken eggs are more likely than intact ones, it was reasonable to expect intact eggs to turn into broken ones. But there's another interpretation. Intact eggs are unlikely and rare, so eggs must spend most of their time broken, very occasionally leaping together to become intact for a moment before breaking again.</p><p>In short, you can use Boltzmann's ideas about entropy to argue that the future and the past should look similar. That's not what we see, so we're back to square one. Why is there an arrow of time at all?</p><p>{\"image\":{\"pid\":\"p02l8ksd\"}}</p><p>Boltzmann suggested several solutions to this problem. The one that worked best came to be known as the past hypothesis. It's very simple: at some point in the distant past, the universe was in a low-entropy state.</p><p>If that's true, then the flaw in Boltzmann's reasoning disappears. The future and the past look very different, because the past has much lower entropy than the future. So eggs break, but they don't un-break.</p><blockquote><p> Within a decade, physicists accepted his ideas </p></blockquote><p>This is neat, but it raises a whole new question: why is the past hypothesis true? Low entropy is unlikely, so why was the entropy of the universe in such a remarkable state sometime in the distant past?</p><p>Boltzmann never managed to crack that one. A manic-depressive whose ideas had been rejected by much of the physics community, he felt sure that his life's work would be forgotten. On a family holiday near Trieste in 1906, Ludwig Boltzmann hanged himself.</p><p>His suicide was particularly tragic since, within a decade, physicists accepted his ideas about atoms. What's more, in the decades that followed, new discoveries suggested that there might be an explanation for the past hypothesis after all.</p><p>{\"image\":{\"pid\":\"p02l8kx3\"}}</p><p>In the twentieth century, our picture of the universe changed radically. We discovered that it had a beginning.</p><blockquote><p> The universe began as an infinitely tiny speck, which exploded </p></blockquote><p>In Boltzmann's time, most physicists believed that the universe was eternal – it had always existed. But in the 1920s, astronomers discovered that galaxies are flying apart. The universe, they realised, is expanding. That means everything was once close together.</p><p>Over the next few decades, physicists came to agree that the universe began as an incredibly hot, dense speck. This quickly expanded and cooled, forming everything that now exists. This fast expansion from a tiny hot universe is called the Big Bang.</p><p>This seemed to support the past hypothesis. \"People said 'okay, the trick is clearly that the early universe had low entropy,'\" says Carroll. \"But why [entropy] was ever low in the first place, 14 billion years ago near the Big Bang, is something we don't know the answer to.\"</p><p>{\"image\":{\"pid\":\"p02l8lfk\"}}</p><p>It's fair to say that an enormous cosmic explosion doesn't sound like something with low entropy. After all, explosions are messy. There are plenty of ways of rearranging the matter and energy in the early universe so that it is still hot, tiny, and expanding. But as it turns out, entropy is a little different when there's so much matter around.</p><p>Imagine a vast empty region of space, in the middle of which is a cloud of gas with the mass of the Sun. Gravity pulls the gas together, so the gas will get clumpy and ultimately collapse into a star. How is this possible, if entropy always increases? There are more ways to arrange the gas when it's wispy and scattered.</p><p><strong>The importance of being clumpy</strong></p><p>The answer is that gravity affects entropy, in a way that physicists still don't fully understand. With truly massive objects, being clumpy is higher entropy than being dense and uniform. So a universe with galaxies, stars and planets actually has a higher entropy than a universe filled with hot, dense gas.</p><p>This means we have a new problem. The sort of universe that emerged immediately after the Big Bang, one that is hot and dense, is low-entropy and therefore unlikely. \"It's not what you would randomly expect out of a bag of universes,\" says Carroll.</p><p>So how did our universe start in such an unlikely state? It's not even clear what kind of answer to that question would be a satisfying one. \"What would count as a scientific explanation of the initial state [of the universe]?\" asks <a href=\"http://philosophy.fas.nyu.edu/object/timmaudlin.html\" target=\"_blank\">Tim Maudlin</a>, a philosopher of physics at New York University.</p><p>{\"image\":{\"pid\":\"p02l8kmp\"}}</p><p>One idea is that there was something before the Big Bang. Could that account for the low entropy of the early universe?</p><p>Carroll and one of his former students proposed a model in which \"baby\" universes are constantly popping into existence, calving off from their parent universe and expanding to become universes like our own. These baby universes could start out with low entropy, but the entropy of the \"multiverse\" as a whole would always be high.</p><blockquote><p> Our best theories of physics can't actually handle the Big Bang </p></blockquote><p>If that's true, the early universe only looks like it has low entropy because we can't see the bigger picture. The same would be true for the arrow of time. \"That kind of idea implies that the far past of our big-picture universe looks the same as the far future,\" says Carroll.</p><p>But there's no wide agreement on Carroll's explanation of the past hypothesis, or any other explanation. \"There are proposals, but nothing is even promising, much less settled,\" says Carroll.</p><p>Part of the trouble is that our best theories of physics can't actually handle the Big Bang. Without a way to describe what happened at the universe's birth, we can't explain why it had low entropy.</p><p>{\"image\":{\"pid\":\"p02l8ktz\"}}</p><p>Modern physics relies on two major theories. Quantum mechanics explains the behaviour of small things like atoms, while general relativity describes heavy things like stars. But the two can't be made to combine.</p><blockquote><p> Nobody has managed to come up with a theory of everything </p></blockquote><p>So if something is both very small and very heavy, like the universe during the Big Bang, physicists get a bit stuck. To describe the early universe, they need to combine the two theories into a \"theory of everything\".</p><p>This ultimate theory will be the key to understanding the arrow of time. \"Finding that theory will ultimately let us know how nature builds space and builds time,\" says <a href=\"http://www.roe.ac.uk/ifa/people/cortes.html\" target=\"_blank\">Marina Cortês</a>, a physicist at the University of Edinburgh in the UK.</p><p>Unfortunately, despite decades of trying, nobody has managed to come up with a theory of everything. But there are some candidates.</p><p>{\"image\":{\"pid\":\"p02l8kj0\"}}</p><p>The most promising theory of everything is string theory, which says that all subatomic particles are actually made of tiny strings. String theory also says that space has extra dimensions, beyond the familiar three, that are curled up to microscopic size, and that we live in a kind of multiverse where the laws of physics are different in different universes.</p><blockquote><p> String theory might not help explain the arrow of time </p></blockquote><p>This all sounds quite outlandish. Nevertheless, most particle physicists see string theory as our best hope for a theory of everything.</p><p>But that doesn't help us explain why time moves forwards. Like almost every other fundamental physical theory, the equations of string theory don't draw a strong distinction between the past and the future.</p><p>String theory, if it turns out to be correct, might not help explain the arrow of time. So Cortês is trying to come up with something better.</p><p>{\"image\":{\"pid\":\"p02l8lbt\"}}</p><p>Working with <a href=\"http://leesmolin.com/\" target=\"_blank\">Lee Smolin</a> of the Perimeter Institute in Waterloo, Canada, Cortês has been working on alternatives to string theory that incorporate the arrow of time at a fundamental level.</p><blockquote><p> Time isn't really an illusion </p></blockquote><p>Cortês and Smolin suggest that the universe is made up of a series of entirely unique events, never repeating itself. Each set of events can only influence events in the next set, so the arrow of time is built in. \"We are hoping that if we can use these types of equations to do cosmology, we can then arrive at the problem of the initial conditions [of the universe] and find they're not as special,\" says Cortês.</p><p>This is completely unlike Boltzmann's explanation, in which the arrow of time emerges as a kind of accident from the laws of probability. \"Time isn't really an illusion,\" says Cortês. \"It exists and it's really moving forward.\"</p><p>But most physicists don't see a problem with Boltzmann's explanation. \"Boltzmann pointed the correct direction to the solution here, a long time ago,\" says <a href=\"http://philosophy.columbia.edu/directories/faculty/david-z-albert\" target=\"_blank\">David Albert</a>, a philosopher of physics at Columbia University in New York. \"There's a real hope that if you dig carefully enough, the whole story is in what Boltzmann said.\"</p><p>Carroll agrees. \"If you have that low-entropy Big Bang, then we're done,\" he says. \"We can explain all the differences between the past and the future.\"</p><p>{\"image\":{\"pid\":\"p02l8kq3\"}}</p><p>One way or another, to explain the arrow of time we need to explain that low-entropy state at the beginning of the universe. That will take a theory of everything, be it string theory, Cortês and Smolin's causal sets, or something else. But people have been searching for a theory of everything for 90 years. How do we find one? And how do we know we have the right one once we've got it?</p><blockquote><p> Our best hope lies with the largest machine in human history </p></blockquote><p>We could test it using something very small and very dense. But we can't go back in time to the Big Bang, and regardless of what <a href=\"http://www.imdb.com/title/tt0816692/\" target=\"_blank\">a recent blockbuster movie</a> suggested, we also can't dive into a black hole and send information back about it. So what can we do, if we really want to explain why eggs don't un-break?</p><p>For now, our best hope lies with the largest machine in human history. The <a href=\"http://home.web.cern.ch/topics/large-hadron-collider\" target=\"_blank\">Large Hadron Collider</a> (LHC) is a particle accelerator that runs in a 27km circle under the border of France and Switzerland. It smashes protons together at nearly the speed of light. The phenomenal energy of these collisions creates new particles.</p><p>The LHC has been closed for repairs for the last two years, but in the spring of 2015 <a href=\"http://home.web.cern.ch/about/updates/2015/02/cerns-two-year-shutdown-drawing-close\" target=\"_blank\">it will turn back on</a> — and for the first time, it will be operating at full power. At half-strength in 2012, it found the long-sought-after Higgs boson, the particle that gives all the others mass. That discovery <a href=\"http://www.bbc.co.uk/news/science-environment-24436781\" target=\"_blank\">led to a Nobel Prize</a>, but <a href=\"http://www.bbc.com/news/science-environment-31162725\" target=\"_blank\">the LHC could now top it</a>. With any luck, the LHC will catch a glimpse of new and unexpected fundamental particles that will point the way to a theory of everything.</p><p>It will take several years for the LHC to collect the necessary data, and for that data to be processed and interpreted. But once it's in, we may finally understand why you can't get that stupid egg off your face.</p>","BusinessUnit":"bbc.com","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[],"DisableAdverts":false,"DisplayDate":"2015-03-09T09:04:24Z","Geolocation":null,"HeadlineLong":"Why does time always run forwards and never backwards?","HeadlineShort":"Why does time only run forwards?","HideRelated":false,"Horizontal":null,"HyperHorizontal":null,"Intro":"Why is it you can break an egg, but not make the pieces spring back together again? To find out, we have to go back to the birth of the universe","IsSyndicated":true,"Latitude":"","Location":null,"Longitude":"","Option":null,"Partner":null,"PrimaryVertical":"wwearth","Programme":null,"RelatedStory":[],"RelatedTag":[],"StoryType":"image","SummaryLong":"Why is it you can break an egg, but not make the pieces spring back together again? To find out, we have to go back to the birth of the universe","SummaryShort":"It's one of the biggest questions in science","SuperSection":null,"Tag":[]},"Metadata":{"CreationDateTime":"2015-03-09T09:04:24Z","Entity":"story","Guid":"a2236f96-c6db-46e3-8a82-502eb3788a89","Id":"wwearth/story/20150309-why-does-time-only-run-forwards","ModifiedDateTime":"2015-09-03T09:14:03.055822Z","Project":"wwearth","Slug":"20150309-why-does-time-only-run-forwards"},"Urn":"urn:pubstack:jative:story:wwearth/story/20150309-why-does-time-only-run-forwards","_id":"5db4b400200e036098261a40"},{"Content":{"AssetCustom":"","AssetIbroadcast":null,"AssetImage":[],"AssetImagePromo":null,"AssetInfographic":"","AssetInline":[],"AssetSelect":"","AssetVideoIb2":null,"AssetVideoMps":null,"Author":[],"BodyHtml":"<p>The recent film <em><a href=\"http://www.imdb.com/title/tt2980516/\">The Theory of Everything</a></em> tells the story of Stephen Hawking, who managed to become a world-famous physicist despite being confined to a wheelchair by a degenerative disease. It's mostly about his relationship with his ex-wife Jane, but it does find a bit of time to explain what Hawking has spent his career doing.</p><p>He certainly didn't lack ambition. Hawking has been one of many physicists trying to come up with a \"theory of everything\", a single theory that will explain everything about our universe. He was following in the footsteps of Albert Einstein, who tried and failed to devise such a theory.</p><p>Finding a theory of everything would be a staggering achievement, finally making sense of all the weird and wonderful things in our universe. For decades, confident physicists have said that one is just around the corner. So are we really on the verge of understanding everything?</p><p>{\"image\":{\"pid\":\"p02npnhp\"}}</p><p>On the face of it, a theory of everything sounds like a tall order. It would have to explain everything from the works of Shakespeare to the human brain and the forests and valleys of our natural world, says <a href=\"http://www.damtp.cam.ac.uk/user/jdb34/\">John Barrow</a> of the University of Cambridge in the UK. \"That's the question of the universe.\"</p><p>Nevertheless, Barrow thinks finding a theory of everything \"is quite conceivable\". That's because \"the laws of nature are rather few, they're simple and symmetrical and there are only four fundamental forces.\"</p><p>In a way we have to put aside the complexity of the world we live in. \"The outcomes of the laws - the things that we see around us - are infinitely more complicated,\" says Barrow. But the rules underlying it all may be simple.</p><p>{\"image\":{\"pid\":\"p02npnkd\"}}</p><p>In 1687, it seemed to many scientists that a theory of everything had been found.</p><blockquote><p> Newton was walking in a garden when he saw an apple fall from a tree </p></blockquote><p>The English physicist Isaac Newton published a book in which he explained how objects move, and set out how gravity works. The <em>Philosophiæ Naturalis Principia Mathematica</em> – that's \"Mathematical Principles of Natural Philosophy\" to you and me – presented the world as a beautiful, ordered place.</p><p>The story goes that, at the age of 23, Newton was walking in a garden when he saw an apple fall from a tree. At the time, physicists knew that the Earth somehow pulled objects down by the force of gravity. Newton would take this idea further.</p><p>According to John Conduitt, his assistant in later years, seeing the apple fall led Newton to the idea that the gravitational force \"<a href=\"https://books.google.co.uk/books?id=oxQ2i23IiMsC&pg=PT49#v=onepage&q&f=false\">was not limited to a certain distance from earth, but that this power must extend much further than was usually thought</a>\". According to Conduitt's account, Newton then asked: \"Why not as high as the Moon?\"</p><p>{\"image\":{\"pid\":\"p02npnlx\"}}</p><p>Inspired, Newton developed a law of gravity, which worked equally well for apples on Earth and planets orbiting the Sun.  All these objects, which seemed so different, turned out to obey the same laws.</p><p>In the same book, Newton set out three laws governing how objects move. Combined with the law of gravity, these laws explained how a ball moves when you throw it and why the Moon orbits the Earth.</p><p>\"People thought that he had explained everything there was to explain,\" says Barrow. \"His achievement was immense.\"</p><p>The problem was, Newton knew his work had holes.</p><p>{\"image\":{\"pid\":\"p02npnnn\"}}</p><p>For instance, gravity doesn't explain how small objects hold themselves together, as the force isn't strong enough.  Also, while Newton could describe what was happening, he couldn't explain how it worked. The theory was incomplete.</p><blockquote><p> Mercury wasn't playing ball </p></blockquote><p>But there was a bigger problem. While Newton's laws explained most of the common phenomena in the universe, in some cases objects broke his laws. These situations were rare, and generally involved extreme speeds or powerful gravity, but they were there.</p><p>One such circumstance was the orbit of Mercury, the closest planet to the Sun. As each planet orbits the Sun it also rotates. Newton's laws could be used to calculate how they should rotate, but Mercury wasn't playing ball. Equally strangely, its orbit was off-centre.</p><p>The evidence was clear. Newton's universal law of gravitation wasn't universal, and wasn't a law.</p><p>{\"image\":{\"pid\":\"p02npntn\"}}</p><p>Over two centuries later, Albert Einstein came to the rescue with his theory of general relativity.  Einstein's idea, which in 2015 celebrates its 100th anniversary, offered a much deeper understanding of gravity.</p><blockquote><p> Really heavy objects like planets, or really fast-moving ones, can distort space-time </p></blockquote><p>The core idea is that space and time, which seem like different things, are actually interwoven. Space has its three dimensions: length, breadth and height. Then there is a fourth dimension, which we call time. All four are linked in a kind of giant cosmic sheet. If you've ever heard a character in a science fiction movie mention \"the space-time continuum\", this is what they're talking about.</p><p>Einstein's big idea was that really heavy objects like planets, or really fast-moving ones, can distort space-time. It's a bit like the taut fabric of a trampoline: if you put a heavy weight on it, the fabric bows and curves. Any other objects will then roll down the sheet towards the object. This, according to Einstein, is why gravity pulls objects towards each other.</p><p>This is a deeply weird idea. But physicists are convinced that it is true. For one thing, it explains the strange orbit of Mercury.</p><p>{\"image\":{\"pid\":\"p02npp29\"}}</p><p>According to general relativity, the Sun's huge mass warps space and time around it.</p><p>As the closest planet to the sun, Mercury experiences much bigger distortions than any of the other planets.  The equations of general relativity describe how this warped space-time should affect Mercury's orbit, and predict the planet's position down to a tee.</p><p>But despite this success, general relativity isn't a theory of everything, any more than Newton's theories were. Just as Newton's theory didn't work for really massive objects, Einstein's didn't work on the very small.</p><p>Once you start looking at tiny things like atoms, matter starts to behave very oddly indeed.</p><p>{\"image\":{\"pid\":\"p02npp30\"}}</p><p>Up until the late 19th century, the atom was thought to be the smallest unit of matter. Coming from the Greek <em>atomos</em> meaning \"indivisible\", the atom by its very definition was not supposed to be able to be divided into smaller particles.</p><p>But in the 1870s, scientists found particles that were almost 2000 times lighter than atoms.</p><blockquote><p> Scientists have found ways to divide matter smaller and smaller </p></blockquote><p>By weighing light rays in a vacuum tube, they found extraordinarily light, negatively-charged particles. This was the first discovery of a subatomic particle: the electron.</p><p>In the next half-century scientists discovered that the atom had a nucleus hub, which the electrons buzzed around. This hub – which was by far the heaviest part of the atom – was made up of two types of subatomic particles: neutrons, which are neutrally charged and protons, which are positively charged.</p><p>But it didn't stop there. Since this time, scientists have found ways to divide matter smaller and smaller, continuing to redefine our notion of fundamental particles. By the 1960s, scientists had found dozens of elementary particles, drawing up a long list known as the particle zoo.</p><p>{\"image\":{\"pid\":\"p02npp42\"}}</p><p>As we understand it today, of the three components of an atom, electrons are the only fundamental particles. Neutrons and protons can be divided further into teeny, tiny particles called \"quarks\".</p><blockquote><p> Einstein never really believed in quantum theory </p></blockquote><p>These subatomic particles were governed by an entirely different set of laws than those governing big objects like trees or planets.  And these new laws – which were far less predictable - threw a spanner in the works.</p><p>In quantum physics, particles don't have defined locations: their whereabouts is a bit fuzzy.  All we can say is that each particle has a certain probability of being in each location. This means the world is a fundamentally uncertain place.</p><p>This may all seem very unfathomable and far-out. All we can say is, it's not just you that feels that way. The physicist Richard Feynman, an expert on the quantum, once said: \"<a href=\"http://bouman.chem.georgetown.edu/general/feynman.html\">I think I can safely say that no one understands quantum mechanics</a>.\"</p><p>Einstein was also disturbed by the fuzziness of quantum mechanics. \"Despite having instigated it, Einstein never really believed in quantum theory,\" says Barrow.</p><p>{\"image\":{\"pid\":\"p02npqt9\"}}</p><p>All the same, for their respective domains – the big and the small – both general relativity and quantum mechanics have proven, time and time again, to be tremendously accurate.  </p><p>Quantum physics has explained the structure and behaviour of atoms, including why some of them are radioactive. It also underlies all modern electronics. You could not read this article without it.</p><p>Meanwhile general relativity was used to predict the existence of black holes. These are stars so massive that they have collapsed in on themselves. Their gravitational attraction is so powerful that nothing – not even light – can escape from it.</p><p>{\"image\":{\"pid\":\"p02npp62\"}}   </p><p>But the issue is, the two theories are not compatible, so they can't both be right. General relativity says that objects' behaviours can be predicted exactly, whereas quantum mechanics says all you can know is the probability that they will do something.</p><p>That means there are some things physicists still can't describe. Black holes are a particular problem. They are massive so general relativity applies, but they are also small so quantum mechanics applies too.</p><p>Unless you're close to a black hole, this incompatibility doesn't affect your day-to-day life. But it has perplexed physicists for most of the last century. It's this incompatibility that has driven the quest for a theory of everything.</p><p>{\"image\":{\"pid\":\"p02nppcz\"}}</p><p>Einstein spent much of his life trying to find such a theory. Never a fan of the randomness of quantum mechanics, he wanted to create a theory that would bring together gravity and the rest of physics, with all the quantum weirdness as a secondary consequence.</p><blockquote><p> Einstein spent 30 years on a fruitless quest </p></blockquote><p>His major challenge was to make gravity work with electromagnetism. In the 1800s, physicists had worked out that electrically-charged particles could be attracted or repelled by each other. That's why some metals are attracted to magnets. This meant there were two kinds of force that objects could exert on each other: they could attract each other with their gravity, and either attract or repel with their electromagnetism.</p><p>Einstein wanted to bring the two forces together into a \"unified field theory\". To do this, he extended his space-time to five dimensions. As well as the three of space and one of time, he added a fifth dimension that was so small and curled up we couldn't see it.</p><p>This didn't work out, and Einstein spent 30 years on a fruitless quest. He died in 1955, his unified field theory still undiscovered. But in the following decade, the strongest contender for a theory of everything emerged: string theory.</p><p>{\"image\":{\"pid\":\"p02nppg8\"}}</p><p>The idea behind string theory is oddly simple. The basic ingredients of the world, such as electrons, are not actually particles at all. Instead they are little loops or \"strings\". It's just that these strings are so small, they seem to be mere points.</p><blockquote><p> All the different particles discovered in the 20th century are really the same kinds of strings </p></blockquote><p>Just like the strings on a guitar, these loops are under tension. That means they vibrate at different frequencies, depending on their size.</p><p>In turn, these oscillations determine what sort of \"particle\" each string appears to be. Vibrate a string one way and you get an electron. Vibrate it another way, and you get something else. All the different particles discovered in the 20th century are really the same kinds of strings, just vibrating in different ways.</p><p>It may not be immediately obvious why this is a good idea. But it seems to make sense of all the forces acting in nature: gravity and electromagnetism, plus two that were only discovered in the 20th century.</p><p>{\"image\":{\"pid\":\"p02nppln\"}}</p><p>The strong and weak nuclear forces are only active within the tiny nuclei of atoms, which is why it took so long for anyone to notice them. The strong force holds the nucleus together. The weak force normally does nothing, but if it gets strong enough it breaks the nucleus apart: this is why some atoms are radioactive.</p><blockquote><p> For the first time, general relativity and quantum mechanics had found common ground </p></blockquote><p>Any theory of everything would have to explain all four. Fortunately, the two nuclear forces and electromagnetism are all covered by quantum mechanics. Each is carried by a specialized particle. But there's no particle to carry the force of gravity.</p><p>Some physicists think there is. They call this particle the \"graviton\". Gravitons would have to have no mass, spin in a particular way, and travel at the speed of light. Unfortunately, nobody has ever managed to find one.</p><p>This is where string theory comes in. It describes a string that looks exactly like a graviton: it spins in the right way, is massless and travels at the speed of light. For the first time, general relativity and quantum mechanics had found common ground.</p><p>As a result, in the mid-1980s physicists became hugely excited about string theory. \"In 1985 we realised string theory solved a lot of the problems people had struggled with for the last 50 years,\" says Barrow. But it also has a host of problems.</p><p>{\"image\":{\"pid\":\"p02nppsv\"}}</p><p>For starters, \"we don't really understand what string theory is in full detail,\" according to <a href=\"https://www.maths.ox.ac.uk/people/philip.candelas\">Philip Candelas</a> of the University of Oxford in the UK. \"We don't have a good way to describe it.\"</p><p>It also makes some predictions that seem outright bizarre. While Einstein's unified field theory relied on a single hidden extra dimension, the earliest forms of string theory called for a total of 26 dimensions. These had to be there to make the mathematics consistent with what we already know about the universe.</p><p>More advanced versions, known as \"superstring theories\", get by with just 10 dimensions. But even that is a far cry from the three dimensions we see on Earth.</p><p>\"The way we reconcile this is by saying that only three expanded in our world and became large,\" says Barrow. \"The others are there but remain fantastically small.\"</p><p>{\"image\":{\"pid\":\"p02nppz5\"}}</p><p>Because of these and other problems, many physicists are unconvinced by string theory. Some have instead studied another theory: loop quantum gravity.</p><blockquote><p> Loop quantum gravity proposes that space-time is actually divided into small chunks </p></blockquote><p>This isn't an attempt at an overarching theory that incorporates particle physics. Instead, loop quantum gravity just sets out to find a quantum theory of gravity. It's more limited than string theory – but it's also not as unwieldy.</p><p>Loop quantum gravity proposes that space-time is actually divided into small chunks. When you zoom out it appears to be a smooth sheet, but when you zoom in, it is a bunch of dots connected by lines or loops. These small fibres, which are woven together, offer an explanation for gravity.</p><p>This idea is just as boggling as string theory, and it has the same problem: there's no hard experimental evidence. </p><p>{\"image\":{\"pid\":\"p02npqr8\"}}</p><p>Why do these theories keep stumbling? One possibility is that we simply don't know enough yet. If there are major phenomena that we've never even seen, we are trying to understand the big picture while missing half the pieces.</p><p>\"It's very tempting to think we've discovered everything,\" says Barrow. \"But it would be very suspicious if in the year 2015 we could make all the observations necessary to have a theory of everything.  Why should it be us?\"</p><blockquote><p> For all its problems, string theory still looks promising </p></blockquote><p>There's also a more immediate problem. The theories are really difficult to test, largely because the maths is so fiendish. Candelas has struggled for years to find a way to test string theory, so far without success.</p><p>\"The main obstacle to the advancement of string theory is there's not enough maths known to advance the study of physics,\" says Barrow. \"It's such an early stage and there's so much to explore.\"</p><p>For all its problems, string theory still looks promising. \"For many years people have been trying to unify gravity with the rest of physics,\" says Candelas. \"We had theories that explained electromagnetism and the other forces well, but not gravity. With string theory we put them together.\"</p><p>The real problem is that a theory of everything may simply be impossible to identify.</p><p>{\"image\":{\"pid\":\"p02npq6n\"}}</p><p>When string theory became popular in the 1980s, there were actually five different versions of it. \"People began to worry,\" says Barrow. \"If there's a theory of everything, why are there five of them?\"</p><p>Over the next decade, physicists discovered that these theories could be transformed into each other. They were different ways of looking at the same thing.</p><blockquote><p> M-theory doesn't offer a single theory of everything </p></blockquote><p>The end result was M-theory, put forward in 1995. This is a deeper version of string theory, incorporating all the earlier versions. That looks good: at least we're back to a single theory. M-theory also only needs 11 dimensions, which is at least better than 26.</p><p>But M-theory doesn't offer a single theory of everything. It offers billions upon billions of them. In total, M-theory gives us 10 to the power of 500 theories, all of them logically consistent and capable of describing a universe.</p><p>That looks worse than useless, but many physicists now think it points to a deeper truth.</p><p>{\"image\":{\"pid\":\"p02npqdb\"}}</p><p>The simplest conclusion is that our universe is one of many, each of them described by one of the trillions of versions of M-theory. This huge collection of universes is called the \"multiverse\".</p><p>At the beginning of time, the multiverse was like \"a great foam of bubbles, all slightly different shapes and sizes,\" says Barrow. Each bubble then expanded into its own universe.</p><p>\"We're in just one of those bubbles,\" says Barrow. As the bubbles expand, other bubbles can arise inside them, each one a new universe. \"It's making the geography of the universe really complicated.\"</p><p>{\"image\":{\"pid\":\"p02npqhq\"}}</p><p>Within each bubble universe, the same physical laws will apply. That's why everything in our universe seems to behave the same.</p><blockquote><p> There are trillions of other universes, each one unique </p></blockquote><p>But the rules will be different in other universes. \"The laws we see in our universe are just like bylaws,\" says Barrow. \"They govern our bit, but not all of the universes.\"</p><p>This leads us to a strange conclusion. If string theory really is the best way to combine general relativity and quantum mechanics, then it both is and isn't a theory of everything.</p><p>On the one hand, string theory may give us a perfect description of our own universe. But it also seems to lead, inescapably, to the idea that there are trillions of other universes, each one unique.</p><p>\"The big change in thinking is we don't expect there to be a unique theory of everything,\" says Barrow. \"There are so many possible theories they're almost filling every possibility of thinking.\"</p><p>{\"image\":{\"pid\":\"p02npqnf\"}}</p>","BusinessUnit":"bbc.com","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[],"DisableAdverts":false,"DisplayDate":"2015-04-08T15:50:28Z","Geolocation":null,"HeadlineLong":"Will we ever have a theory of everything?","HeadlineShort":"Quest for a theory of everything","HideRelated":false,"Horizontal":null,"HyperHorizontal":null,"Intro":"Physicists want to find a single theory that describes the entire universe, but to do so they must solve some of the hardest problems in science","IsSyndicated":true,"Latitude":"","Location":null,"Longitude":"","Option":null,"Partner":null,"PrimaryVertical":"wwearth","Programme":null,"RelatedStory":[],"RelatedTag":[],"StoryType":"image","SummaryLong":"Physicists want to find a single theory that describes the entire universe, but to do so they must solve some of the hardest problems in science","SummaryShort":"Physicists want to explain the entire universe","SuperSection":null,"Tag":[]},"Metadata":{"CreationDateTime":"2015-04-08T15:50:28Z","Entity":"story","Guid":"fd2f5b45-205e-403c-88c6-09a282571e5a","Id":"wwearth/story/20150409-can-science-ever-explain-everything","ModifiedDateTime":"2015-09-03T09:18:47.303257Z","Project":"wwearth","Slug":"20150409-can-science-ever-explain-everything"},"Urn":"urn:pubstack:jative:story:wwearth/story/20150409-can-science-ever-explain-everything","_id":"5db7a92a200e036098329f6a"},{"Content":{"AssetCustom":"","AssetIbroadcast":null,"AssetImage":[],"AssetImagePromo":null,"AssetInfographic":"","AssetInline":[],"AssetSelect":"","AssetVideoIb2":null,"AssetVideoMps":null,"Author":[],"BodyHtml":"<p>People have wrestled with the mystery of why the universe exists for thousands of years. Pretty much every ancient culture came up with its own creation story - most of them leaving the matter in the hands of the gods - and philosophers have written reams on the subject. But science has had little to say about this ultimate question.</p><p>However, in recent years a few physicists and cosmologists have started to tackle it. They point out that we now have an understanding of the history of the universe, and of the physical laws that describe how it works. That information, they say, should give us a clue about how and why the cosmos exists.</p><p>Their admittedly controversial answer is that the entire universe, from the fireball of the Big Bang to the star-studded cosmos we now inhabit, popped into existence from nothing at all. It had to happen, they say, because \"nothing\" is inherently unstable.</p><p>This idea may sound bizarre, or just another fanciful creation story. But the physicists argue that it follows naturally from science's two most powerful and successful theories: quantum mechanics and general relativity.</p><p>Here, then, is how everything could have come from nothing.</p><p>{\"image\":{\"pid\":\"p02b3t2n\"}}</p><p><strong>Particles from empty space</strong></p><p>First we have to take a look at the realm of quantum mechanics. This is the branch of physics that deals with very small things: atoms and even tinier particles. It is an immensely successful theory, and it underpins most modern electronic gadgets.</p><p>Quantum mechanics tells us that there is no such thing as empty space. Even the most perfect vacuum is actually filled by a roiling cloud of particles and antiparticles, which flare into existence and almost instantaneously fade back into nothingness.</p><p>These so-called virtual particles don't last long enough to be observed directly, but we know they exist by <a href=\"http://math.ucr.edu/home/baez/physics/Quantum/casimir.html\" target=\"_blank\">their effects</a>.</p><p>{\"image\":{\"pid\":\"p02b3tyd\"}}</p><p><strong>Space-time, from no space and no time</strong></p><p>From tiny things like atoms, to really big things like galaxies. Our best theory for describing such large-scale structures is general relativity, Albert Einstein's crowning achievement, which sets out how space, time and gravity work.</p><p>Relativity is very different from quantum mechanics, and so far nobody has been able to combine the two seamlessly. However, some theorists have been able to bring the two theories to bear on particular problems by using carefully chosen approximations. For instance, this approach was used by <a href=\"http://www.hawking.org.uk/\" target=\"_blank\">Stephen Hawking</a> at the University of Cambridge to describe black holes.</p><blockquote><p> In quantum physics, if something is not forbidden, it necessarily happens </p></blockquote><p>One thing they have found is that, when quantum theory is applied to space at the smallest possible scale, space itself becomes unstable. Rather than remaining perfectly smooth and continuous, space and time destabilize, churning and frothing into a foam of space-time bubbles.</p><p>In other words, little bubbles of space and time can form spontaneously. \"If space and time are quantized, they can fluctuate,\" says <a href=\"https://physics.asu.edu/people/faculty/lawrence-krauss\" target=\"_blank\">Lawrence Krauss</a> at Arizona State University in Tempe. \"So you can create virtual space-times just as you can create virtual particles.\"</p><p>What's more, if it's possible for these bubbles to form, you can guarantee that they will. \"In quantum physics, if something is not forbidden, it necessarily happens with some non-zero probability,\" says <a href=\"http://cosmos2.phy.tufts.edu/vilenkin.html\" target=\"_blank\">Alexander Vilenkin</a> of Tufts University in Boston, Massachusetts.</p><p>{\"image\":{\"pid\":\"p02b3tst\"}}</p><p><strong>A universe from a bubble</strong></p><p>So it's not just particles and antiparticles that can snap in and out of nothingness: bubbles of space-time can do the same. Still, it seems like a big leap from an infinitesimal space-time bubble to a massive universe that hosts 100 billion galaxies. Surely, even if a bubble formed, it would be doomed to disappear again in the blink of an eye?</p><blockquote><p> If all the galaxies are flying apart, they must once have been close together </p></blockquote><p>Actually, it is possible for the bubble to survive. But for that we need another trick: cosmic inflation.</p><p>Most physicists now think that the universe began with the Big Bang. At first all the matter and energy in the universe was crammed together in one unimaginably small dot, and this exploded. This follows from the discovery, in the early 20th century, that the universe is expanding. If all the galaxies are flying apart, they must once have been close together.</p><p>Inflation theory proposes that in the immediate aftermath of the Big Bang, the universe expanded much faster than it did later. This seemingly outlandish notion was put forward in the 1980s by <a href=\"http://web.mit.edu/physics/people/faculty/guth_alan.html\" target=\"_blank\">Alan Guth</a> at the Massachusetts Institute of Technology, and refined by <a href=\"http://web.stanford.edu/~alinde/\" target=\"_blank\">Andrei Linde</a>, now at Stanford University.</p><blockquote><p> As weird as it seems, inflation fits the facts </p></blockquote><p>The idea is that, a fraction of a second after the Big Bang, the quantum-sized bubble of space expanded stupendously fast. In an incredibly brief moment, it went from being smaller than the nucleus of an atom to the size of a grain of sand. When the expansion finally slowed, the force field that had powered it was transformed into the matter and energy that fill the universe today. Guth calls inflation \"the ultimate free lunch\".</p><p>As weird as it seems, inflation fits the facts rather well. In particular, it neatly explains why the cosmic microwave background, the faint remnant of radiation left over from the Big Bang, is almost perfectly uniform across the sky. If the universe had not expanded so rapidly, we would expect the radiation to be patchier than it is.</p><p>{\"image\":{\"pid\":\"p02b3vlw\"}}</p><p><strong>The universe is flat and why that's important</strong></p><p>Inflation also gave cosmologists the measuring tool they needed to determine the underlying geometry of the universe. It turns out this is also crucial for understanding how the cosmos came from nothing.</p><p>Einstein's theory of general relativity tells us that the space-time we live in could take three different forms. It could be as flat as a table top. It could curve back on itself like the surface of a sphere, in which case if you travel far enough in the same direction you would end up back where you started. Alternatively, space-time could curve outward like a saddle. So which is it?</p><p>There is a way to tell. You might remember from maths class that the three angles of a triangle add up to exactly 180 degrees. Actually your teachers left out a crucial point: this is only true on a flat surface. If you draw a triangle on the surface of a balloon, its three angles will add up to more than 180 degrees. Alternatively, if you draw a triangle on a surface that curves outward like a saddle, its angles will add up to less than 180 degrees.</p><p>So to find out if the universe is flat, we need to measure the angles of a really big triangle. That's where inflation comes in. It determined the average size of the warmer and cooler patches in the cosmic microwave background. Those patches were measured in 2003, and that gave astronomers a selection of triangles. As a result, we know that on the largest observable scale our universe is flat.</p><p>{\"image\":{\"pid\":\"p02b3w4b\"}}</p><p>It turns out that a flat universe is crucial. That's because only a flat universe is likely to have come from nothing.</p><p>Everything that exists, from stars and galaxies to the light we see them by, must have sprung from somewhere. We already know that particles spring into existence at the quantum level, so we might expect the universe to contain a few odds and ends. But it takes a huge amount of energy to make all those stars and planets.</p><blockquote><p> The energy of matter is exactly balanced by the energy of the gravity the mass creates </p></blockquote><p>Where did the universe get all this energy? Bizarrely, it may not have had to get any. That's because every object in the universe creates gravity, pulling other objects toward it. This balances the energy needed to create the matter in the first place.</p><p>It's a bit like an old-fashioned measuring scale. You can put a heavy weight on one side, so long as it is balanced by an equal weight on the other. In the case of the universe, the matter goes on one side of the scale, and has to be balanced by gravity.</p><p>Physicists have calculated that in a flat universe the energy of matter is exactly balanced by the energy of the gravity the mass creates. But this is only true in a flat universe. If the universe had been curved, the two sums would not cancel out.</p><p>{\"image\":{\"pid\":\"p02b3vrq\"}}</p><p><strong>Universe or multiverse?</strong></p><p>At this point, making a universe looks almost easy. Quantum mechanics tells us that \"nothing\" is inherently unstable, so the initial leap from nothing to something may have been inevitable. Then the resulting tiny bubble of space-time could have burgeoned into a massive, busy universe, thanks to inflation. As Krauss puts it, \"The laws of physics as we understand them make it eminently plausible that our universe arose from nothing - no space, no time, no particles, nothing that we now know of.\"</p><p>So why did it only happen once? If one space-time bubble popped into existence and inflated to form our universe, what kept other bubbles from doing the same?</p><blockquote><p> There could be a mind-boggling smorgasbord of universes </p></blockquote><p>Linde offers a simple but mind-bending answer. He thinks universes have always been springing into existence, and that this process will continue forever.</p><p>When a new universe stops inflating, says Linde, it is still surrounded by space that is continuing to inflate. That inflating space can spawn more universes, with yet more inflating space around them. So once inflation starts it should make an endless cascade of universes, which Linde calls eternal inflation. Our universe may be just one grain of sand on an endless beach.</p><p>Those universes might be profoundly different to ours. The universe next door might have five dimensions of space rather than the three – length, breadth and height – that ours does. Gravity might be ten times stronger or a thousand times weaker, or not exist at all. Matter might be built out of utterly different particles.</p><p>So there could be a mind-boggling smorgasbord of universes. Linde says eternal inflation is not just the ultimate free lunch: it is the only one at which all possible dishes are available.</p><p>As yet we don't have hard evidence that other universes exist. But either way, these ideas give a whole new meaning to the phrase \"Thanks for nothing\".</p><p>{\"image\":{\"pid\":\"p02b3wrr\"}}</p>","BusinessUnit":"bbc.com","CalloutBody":"","CalloutPosition":"","CalloutSubtitle":"","CalloutTitle":"","Campaign":null,"Collection":[],"DisableAdverts":false,"DisplayDate":"2014-11-06T10:18:39Z","Geolocation":null,"HeadlineLong":"Why is there something rather than nothing?","HeadlineShort":"Why does anything exist at all?","HideRelated":false,"Horizontal":null,"HyperHorizontal":null,"Intro":"Some physicists think they can explain why the universe first formed. If they are right, our entire cosmos may have sprung out of nothing at all","IsSyndicated":true,"Latitude":"","Location":null,"Longitude":"","Option":null,"Partner":null,"PrimaryVertical":"wwearth","Programme":null,"RelatedStory":[],"RelatedTag":[],"StoryType":"image","SummaryLong":"Some physicists think they can explain why the universe first formed. 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But in fact your fate would be far stranger than that","SummaryShort":"Baffling things happen when you fall in","SuperSection":null,"Tag":[{"Content":{"AssetImage":null,"Description":"","LinkUrl":"","Name":"Black 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href="/web/20191030084519/http://www.bbc.com/earth/tags/universe" title="View Universe"> Universe</a> </span> </li> </ul> <h1 class="primary-heading" role="heading">Why there could be many identical copies of you</h1> </div> </div> <div class="introduction-wrapper"> <p class="introduction">We ask one of the hardest questions in the Universe, is there another you out there and if so, can we find our cosmic twin?</p> </div> <div id="share-tools-top"> <ul class="share share--mobile small share--article"> <li class="share-item"> <div class="share-item-lining"> <a class="share-button facebook" href="https://web.archive.org/web/20191030084519/https://www.facebook.com/sharer/sharer.php?u=http%3A%2F%2Fwww.bbc.com%2Fearth%2Fstory%2F20161021-why-there-could-be-many-copies-of-you%3Focid%3Dww.social.link.facebook&t=Why%20there%20could%20be%20many%20identical%20copies%20of%20you" data-share-destination="facebook" data-tracking-id="facebook" target="_blank"> <span class="share-icon-wrapper 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BBC:Why%20there%20could%20be%20many%20identical%20copies%20of%20you&body=http%3A%2F%2Fwww.bbc.com%2Fearth%2Fstory%2F20161021-why-there-could-be-many-copies-of-you%3Focid%3Dww.social.link.email" data-share-destination="email" data-tracking-id="email" target="_blank"> <span class="share-icon-wrapper share-icon-wrapper--social"> <span class="icon gelicon--mail"></span> </span> <span class="share-item-text">Share by Email</span> </a> </div> </li></ul> </div> </div>   <div class="mid-unit"> </div>   <div class="bottom-unit"> <div class="byline"> <span class="byline-heading"></span> <div class="source-attribution-wrapper"> <div class="source-attribution-detail"> <ul class="seperated-list source-attribution"> <li class="seperated-list-item source-attribution-author"><span class="index-body">By Melissa Hogenboom and Pierangelo Pirak</span></li> </ul> <span class="publication-date index-body">24 October 2016</span> </div> </div> </div> <div class="body-content"> <p><em>This story is nominated for a Webby Award for Best Film & Video. <a href="https://web.archive.org/web/20191030084519/https://vote.webbyawards.com/PublicVoting#/2017/film-video/general-film/science-education" target="_blank">Vote here</a>.<br/></em></p><p><em>BBC Earth is also nominated for a Webby, for Best Science Website. <a href="https://web.archive.org/web/20191030084519/https://vote.webbyawards.com/PublicVoting#/2017/websites/general-website/science" target="_blank">Vote here</a>.</em></p><p>In theory this is not the only Universe that might exist, and in many others, identical copies of us can be found.</p><p>The question is, how do we get there? </p><p>BBC Earth's Melissa Hogenboom goes on the hunt for her cosmic twin.</p><p><em>Melissa Hogenboom is BBC Earth's feature writer. 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