Can We Quantum Tunnel Through A Wall?

<!DOCTYPE html> <html lang="en-US"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/> <meta name="viewport" content="user-scalable=yes, initial-scale=1.0, width=device-width"/> <meta name="robots" content="max-image-preview:large"> <meta http-equiv="Content-Security-Policy" content="block-all-mixed-content" /> <link rel="canonical" href="https://www.worldatlas.com/science/can-we-quantum-tunnel-through-a-wall.html"/> <meta name="description" content="Quantum mechanics defies intuition with phenomena like uncertainty, wavefunctions, and quantum tunneling, enabling particles to cross forbidden barriers."> <link rel="icon" href="/nwa_assets/img/site/favicon.png" type="image/x-icon"> <link rel="preload" href="https://www.worldatlas.com/nwa_assets/fonts/fontello/fontello.woff2?25012021" as="font" type="font/woff2" crossorigin> <title>Can We Quantum Tunnel Through A Wall? - WorldAtlas</title> <meta property="og:title" content="Can We Quantum Tunnel Through A Wall?"/> <meta property="og:type" content="article"/> <meta property="og:image" content="https://www.worldatlas.com/r/w1200-q80/upload/9f/0c/84/quantum-tunneling-01-01.png"/> <meta property="og:image:width:" content="1200"/> <meta property="og:image:height:" content="625"/> <meta property="og:url" content="https://www.worldatlas.com/science/can-we-quantum-tunnel-through-a-wall.html"/> <meta property="og:locale" content="en_US"/> <meta property="og:description" content="Quantum mechanics defies intuition with phenomena like uncertainty, wavefunctions, and quantum tunneling, enabling particles to cross forbidden barriers."/> <meta property="og:site_name" content="WorldAtlas"/> <meta property="og:article:published_time" content="2024-11-24T08:14:00-05:00"/> <meta property="og:article:section" content="WorldAtlas Originals"/> <meta property="fb:app_id" content="1534891833401557"/> <link rel="preload" as="style" href="/build/editorial-blog.5f19975b.css"> <link href="/build/editorial-blog.5f19975b.css" rel="stylesheet">  <script async src="https://www.googletagmanager.com/gtag/js?id=G-EKNV4JVDSB"></script> <script> window.dataLayer = window.dataLayer || []; function gtag(){dataLayer.push(arguments);} gtag('js', new Date()); gtag('config', 'G-EKNV4JVDSB', { 'topic_id': '36246', 'writer_id': '987', 'publisher_id': '574', 'editor_id': '574', 'published_date': '2024-11-24', 'published_time': '08' }); </script> <script> window.sentinelData = window.sentinelData || []; function sentinelTrack(){sentinelData.push(arguments)}; sentinelTrack('initTimestamp', Date.now()); window.addEventListener('DOMContentLoaded', () => { sentinelTrack('footerTimestamp', Date.now()); }); sentinelSetup = { accountName: 'reuniontech', dimensions: { topic_id: '36246', writer_id: '987', publisher_id: '574' , editor_id: '574', published_date: '2024-11-24', published_time: '08' } }; sentinelTrack('config', sentinelSetup); </script> <script async src="https://cdn.sentinelbi.com/sentinel_analytics.js"></script> <script src="/build/runtime.7576e7bf.js" type="text/javascript" defer></script> <script src="/build/editorial-blog.505c4490.js" type="text/javascript" defer></script> <script type="application/ld+json">{"@context":"https://schema.org","@type":"Article","mainEntityOfPage":{"@type":"WebPage","@id":"https://www.worldatlas.com/science/can-we-quantum-tunnel-through-a-wall.html"},"headline":"Can We Quantum Tunnel Through A Wall?","image":{"@type":"ImageObject","url":"https://www.worldatlas.com/r/w2560-q80/upload/9f/0c/84/quantum-tunneling-01-01.png","width":2560,"height":1332},"datePublished":"2024-11-24T08:14:00-05:00","articleSection":"Science","description":"Quantum mechanics defies intuition with phenomena like uncertainty, wavefunctions, and quantum tunneling, enabling particles to cross forbidden barriers.","author":{"@type":"Person","name":"Shadi Ali Ahmad","@id":"https://www.worldatlas.com/contributor/shadi-ali-ahmad","sameAs":"https://www.worldatlas.com/contributor/shadi-ali-ahmad"},"publisher":{"@type":"Organization","@id":"https://www.worldatlas.com/","name":"WorldAtlas","sameAs":["https://www.facebook.com/worldatlas","https://twitter.com/worldatlas","https://www.pinterest.ca/worldatlas/","https://www.linkedin.com/company/world-atlas"],"logo":{"@type":"ImageObject","url":"https://worldatlas.com/nwa_assets/img/site/wa_2_logo_text_primary_amp.png","height":60,"width":326,"caption":"WorldAtlas Logo"}},"hasPart":{"@type":"WebPageElement","value":"Quantum mechanics defies all intuition about our perceptible classical universe. The simple act of measuring a particle's position and then its momentum doesn't yield the same result as measuring the momentum first and then the position. This is a reflection of Heisenberg's uncertainty principle. The more we try to sharpen our knowledge about the particle's position, the muddier our understanding of its momentum gets.\r\nWhat\u2019s even stranger is that in the quantum world, particles are not confined to a single spot; their position is described by a wavefunction encoding the probability of finding it at some position. This wavefunction does not end abruptly anywhere in space; it decays exponentially, never reaching zero. If the wavefunction \"leaks\" through a barrier (or a wall), there's a non-zero chance of it appearing on the other side. This gives rise to the phenomenon of quantum tunneling, which is when a quantum system can cross barriers that are forbidden classically. In this article, we demystify quantum tunneling in simple terms and give reassurance to those wondering if they might instantaneously tunnel through a wall.\r\nThe Quantum Wavefunction\r\n\r\nThe wavefunction of a particle with spin-1/2\r\n\r\nQuantum theory differs fundamentally from classical theory in many ways. The most basic way is that quantum mechanics is a theory of probability amplitudes, while classical mechanics is one of probabilities. Amplitudes are essentially square roots of probabilities, but weirdly, they can take on imaginary values. In classical theory, we can ask what the probability of observing the particle in one position or another is. The total probability of finding the particle in either position is simply given by the sum of the individual probabilities. Life is good.\r\nIn quantum theory, the probability of finding the particle in either position is the square of the sum of the amplitudes. When we square two terms, we get mixed terms that signal quantum interference. This is radically different from classical theory, in which those mixed terms do not exist.\r\nTo describe a system in quantum mechanics, we simply put all the probability amplitudes for finding it somewhere in what we call the wavefunction. This is in opposition to classical theory where the system can be described by a probability distribution. One can think of the wavefunction as a probability amplitude distribution.\r\nBarriers, Potentials, and Walls\r\n\r\nA particle in 1-D trapped in an infinite box\r\n\r\nThe quantum description of a free particle is simple. Depending on its momentum, there will exist some probability of finding the particle in some position dictated by how heavy it is and how fast it is moving. The wavefunction which is sharply peaked at some position (momentum) is not valid which is a reflection of uncertainty.\r\nTo make life more interesting, we can imagine placing the particle in a well. Now, there will exist different regimes for the description. The particle can have:\r\n\r\nEnergy less than that of the well (or barrier), in which case it is seemingly trapped and cannot escape.\r\nEnergy greater than that of the well, in which case it does not even register the barrier and behaves as if it were freely propagating.\r\nEnergy equal to that of the well, in which case it can go either way.\r\n\r\nAn example of a well could be the trapping Coulomb potential of an electrically charged nucleus applied to the quantum description of an electron in an atom. Depending on the charge of the nucleus, the electron may or may not be trapped by the potential.\r\nSurpassing Obstacles and its Consequences\r\n\r\nA (simplified) diagram of Quantum Tunneling, a phenomenon by which a particle may move through a barrier which would be impossible under classical mechanics.\r\n\r\nLet us imagine that we have a particle in a finite box. In classical theory, if the particle does not have enough energy to climb the walls of the box, then it will simply stay inside forever. Only if its energy is greater than that of the trapping potential can it ever escape. In quantum theory, this is no longer true!\r\nTo understand why, we recall that the wavefunction is supported in any position in space. This means there is a non-zero probability of finding the particle outside of the box independently of its energy. The probability, of course, does depend on the energy, but the fact that it can be outside does not. This is exactly what quantum tunneling means. The surprising thing about this is that it defies our classical expectation. Even if classically, the particle is forbidden from escaping, quantum effects can save the particle from its fate.\r\nWhy is this important? Tunneling is essential for many fundamental processes like:\r\n\r\nNuclei in stars can overcome their electric repulsion via quantum tunneling, which enables fusion reactions to occur. These fusion reactions are important to sustain the star. Without tunneling, our Sun would not exist.\r\nTunneling also governs some radioactive decay channels. For example, alpha decay, which is the ejection of a Helium nucleus from a decaying parent nucleus, would not occur without tunneling. Without this process, some cancers would not have been able to be treated or pacemakers made.\r\nBodily processes rely on enzymes, which catalyze reactions that lead to digestion and respiration. If the protons and electrons found in enzymes could not tunnel, the reactions would occur too slowly to sustain life itself.\r\n\r\nWhile tunneling is extremely important for many aspects of life, it is important to remember that quantum effects become heavily suppressed when we consider bigger systems. This is why, for the electron, tunneling is commonplace, while for humans, trying to get past a locked door, not so much.\r\n"},"isAccessibleForFree":"http://schema.org/True"}</script> <script type="text/javascript" async="async" data-noptimize="1" data-cfasync="false" src="//scripts.pubnation.com/tags/af7c1dab-ebf1-4768-8b49-e4fe264a4ad8.js"></script> <style> /* MV CSS to be added to site */ @media only screen and (min-width: 1024px) { .wa-stickyAsideParent #adStickyAside { position: static !important; } } @media only screen and (max-width: 359px) { .pagination_list { flex-wrap: wrap !important; } #content_body h2 { word-break: break-word !important; } } /* End MV CSS */ </style>  <script> var _paq = window._paq = window._paq || []; /* tracker methods like "setCustomDimension" should be called before "trackPageView" */ _paq.push(['trackPageView']); _paq.push(['enableLinkTracking']); (function() { var u="https://matomo.yourwriterportal.com/"; _paq.push(['setTrackerUrl', u+'matomo.php']); _paq.push(['setSiteId', '1']); var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0]; g.async=true; g.src=u+'matomo.js'; s.parentNode.insertBefore(g,s); })(); </script>  </head> <body> <div id="wa_siteWrapper"> <div id="pageBody"> <article id="article_content" class="mod_content"> <figure class="article_featured_media content_media_fixed blog_featured_media article_flex_featured_media "> <picture style="padding-top: 52.03%;" > <source media="(min-width: 1024px)" sizes="1300px" srcset="https://www.worldatlas.com/r/w1300/upload/9f/0c/84/quantum-tunneling-01-01.png 1300w"> <source media="(min-width: 768px) and (max-width: 1023px)" sizes="1024px" srcset="https://www.worldatlas.com/r/w1024/upload/9f/0c/84/quantum-tunneling-01-01.png 1024w"> <source media="(min-width: 0px) and (max-width: 767px)" sizes="768px" srcset="https://www.worldatlas.com/r/w768/upload/9f/0c/84/quantum-tunneling-01-01.png 768w"> <img id="featured_media" src="/upload/9f/0c/84/quantum-tunneling-01-01.png" alt="quantum tunneling through a wall" fetchpriority="high" > </picture> </figure> <section id="content_body" class="content_body "> <header> <h1>Can We Quantum Tunnel Through A Wall?</h1> </header> <section> <p><a href="https://www.worldatlas.com/what-is-schrodinger-s-cat.html">Quantum mechanics</a> defies all intuition about our perceptible classical universe. The simple act of measuring a particle's position and then its momentum doesn't yield the same result as measuring the momentum first and then the position. This is a reflection of <a href="https://www.worldatlas.com/articles/what-is-heisenberg-s-uncertainty-principle.html">Heisenberg's uncertainty principle</a>. The more we try to sharpen our knowledge about the particle's position, the muddier our understanding of its momentum gets. </p> <p>What鈥檚 even stranger is that in the quantum world, particles are not confined to a single spot; their position is described by a wavefunction encoding the probability of finding it at some position. This wavefunction does not end abruptly anywhere in space; it decays exponentially, never reaching zero. If the wavefunction "leaks" through a barrier (or a wall), there's a non-zero chance of it appearing on the other side. This gives rise to the phenomenon of quantum tunneling, which is when a quantum system can cross barriers that are forbidden classically. In this article, we demystify quantum tunneling in simple terms and give reassurance to those wondering if they might instantaneously tunnel through a wall. </p> <h2>The Quantum Wavefunction </h2> <figure class=" content_media_fixed"><picture style="padding-top: 107.47%;"><source media="(min-width: 768px)" sizes="960w" srcset="https://www.worldatlas.com/r/w960-q80/upload/e8/21/82/wavefunction-01.png 960w"></source><source media="(min-width: 0px) and (max-width: 767px)" sizes="768w" srcset="https://www.worldatlas.com/r/w768-q80/upload/e8/21/82/wavefunction-01.png 768w"></source><img loading="lazy" alt="The wavefunction of a particle with spin-1/2" src="/r/w768/upload/e8/21/82/wavefunction-01.png"></picture><figcaption>The wavefunction of a particle with spin-1/2</figcaption></figure> <p>Quantum theory differs fundamentally from classical theory in many ways. The most basic way is that quantum mechanics is a theory of probability amplitudes, while classical mechanics is one of probabilities. Amplitudes are essentially square roots of probabilities, but weirdly, they can take on imaginary values. In classical theory, we can ask what the probability of observing the particle in one position or another is. The total probability of finding the particle in either position is simply given by the sum of the individual probabilities. Life is good. </p> <p>In quantum theory, the probability of finding the particle in either position is the square of the sum of the amplitudes. When we square two terms, we get mixed terms that signal <em>quantum interference.</em> This is radically different from classical theory, in which those mixed terms do not exist. </p> <p>To describe a system in quantum mechanics, we simply put all the probability amplitudes for finding it somewhere in what we call the <em>wavefunction</em>. This is in opposition to classical theory where the system can be described by a probability distribution. One can think of the wavefunction as a probability amplitude distribution. </p> <h2>Barriers, Potentials, and Walls </h2> <figure class=" content_media_fixed"><picture style="padding-top: 17.61%;"><source media="(min-width: 768px)" sizes="960w" srcset="https://www.worldatlas.com/r/w960-q80/upload/98/ab/25/particle-in-a-box.jpg 960w"></source><source media="(min-width: 0px) and (max-width: 767px)" sizes="768w" srcset="https://www.worldatlas.com/r/w768-q80/upload/98/ab/25/particle-in-a-box.jpg 768w"></source><img loading="lazy" alt="A particle in 1-D trapped in an infinite box" src="/r/w768/upload/98/ab/25/particle-in-a-box.jpg"></picture><figcaption>A particle in 1-D trapped in an infinite box</figcaption></figure> <p>The quantum description of a free particle is simple. Depending on its momentum, there will exist some probability of finding the particle in some position dictated by how heavy it is and how fast it is moving. The wavefunction which is sharply peaked at some position (momentum) is not valid which is a reflection of uncertainty. </p> <p>To make life more interesting, we can imagine placing the particle in a well. Now, there will exist different regimes for the description. The particle can have: </p> <ol><li>Energy less than that of the well (or barrier), in which case it is seemingly trapped and cannot escape.</li> <li>Energy greater than that of the well, in which case it does not even register the barrier and behaves as if it were freely propagating.</li> <li>Energy equal to that of the well, in which case it can go either way.</li> </ol> <p>An example of a well could be the <a href="https://www.worldatlas.com/space/the-electromagnetic-spectrum.html">trapping Coulomb potential</a> of an electrically charged nucleus applied to the quantum description of an <a href="https://www.worldatlas.com/space/what-is-spectroscopy.html">electron</a> in an <a href="https://www.worldatlas.com/articles/how-many-elements-are-there.html">atom</a>. Depending on the charge of the nucleus, the electron may or may not be trapped by the potential. </p> <h2>Surpassing Obstacles and its Consequences </h2> <figure class=" content_media_fixed"><picture style="padding-top: 62.5%;"><source media="(min-width: 768px)" sizes="960w" srcset="https://www.worldatlas.com/r/w960-q80/upload/f3/38/3d/quantumtunnel.jpg 960w"></source><source media="(min-width: 0px) and (max-width: 767px)" sizes="768w" srcset="https://www.worldatlas.com/r/w768-q80/upload/f3/38/3d/quantumtunnel.jpg 768w"></source><img loading="lazy" alt="A (simplified) diagram of Quantum Tunneling, a phenomenon by which a particle may move through a barrier which would be impossible under classical mechanics." src="/r/w768/upload/f3/38/3d/quantumtunnel.jpg"></picture><figcaption>A (simplified) diagram of Quantum Tunneling, a phenomenon by which a particle may move through a barrier which would be impossible under classical mechanics.</figcaption></figure> <p>Let us imagine that we have a particle in a finite box. In classical theory, if the particle does not have enough energy to climb the walls of the box, then it will simply stay inside forever. Only if its energy is greater than that of the trapping potential can it ever escape. In quantum theory, this is no longer true! </p> <p>To understand why, we recall that the wavefunction is supported in any position in space. This means there is a non-zero probability of finding the particle outside of the box <em>independently of its energy.</em> The probability, of course, does depend on the energy, but the fact that it can be outside does not. This is exactly what quantum tunneling means. The surprising thing about this is that it defies our classical expectation. Even if classically, the particle is forbidden from escaping, quantum effects can save the particle from its fate. </p> <p>Why is this important? Tunneling is <em>essential</em> for many fundamental processes like: </p> <ol><li>Nuclei in stars can overcome their electric repulsion via quantum tunneling, which enables <a href="https://www.worldatlas.com/space/what-is-nuclear-fusion.html">fusion</a> reactions to occur. These fusion reactions are important to sustain the <a href="https://www.worldatlas.com/space/stars.html">star</a>. Without tunneling, our <a href="https://www.worldatlas.com/space/the-sun.html">Sun</a> would not exist.</li> <li>Tunneling also governs some <a href="https://www.worldatlas.com/articles/what-is-radioactivity.html">radioactive decay</a> channels. For example, <a href="https://www.worldatlas.com/what-is-a-daughter-in-physics.html">alpha decay,</a> which is the ejection of a Helium nucleus from a decaying parent nucleus, would not occur without tunneling. Without this process, some cancers would not have been able to be treated or pacemakers made.</li> <li>Bodily processes rely on enzymes, which catalyze reactions that lead to digestion and respiration. If the protons and electrons found in enzymes could not tunnel, the reactions would occur too slowly to sustain life itself.</li> </ol> <p>While tunneling is extremely important for many aspects of life, it is important to remember that quantum effects become heavily suppressed when we consider bigger systems. This is why, for the electron, tunneling is commonplace, while for humans, trying to get past a locked door, not so much. </p> </section> <div class="article_credits_social"> <strong>Share</strong> <ul class="navSocial"> <li><button class="socialAction wa_badge_black icon-flipboard-icon" data-url="https://share.flipboard.com/bookmarklet/popout?v=2&title=Can%20We%20Quantum%20Tunnel%20Through%20A%20Wall%3F&url=https://www.worldatlas.com/science/can-we-quantum-tunnel-through-a-wall.html" aria-label="Share on Flipboard"></a></li> <li><button class="socialAction wa_badge_black icon-facebook" data-url="https://www.facebook.com/sharer/sharer.php?u=https://www.worldatlas.com/science/can-we-quantum-tunnel-through-a-wall.html" aria-label="Share on Facebook"></button></li> <li><button class="socialAction wa_badge_black icon-twitter" 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class="content_contributor">Shadi Ali Ahmad</span></a> November 24 2024 in <a href="https://www.worldatlas.com/science/">Science</a></span> </div> </section> <section class="article_after_content no-left-right-margin"> <h2>More in <a href="https://www.worldatlas.com/science/">Science</a></h2> <div class="mod_grid_list grid_read_stack"> <article class="grid_item mod_bclip bclip_full_16x9" itemscope itemtype="https://schema.org/WebPage"> <div class="bclip_container"> <a href="/space/uranus-mysteries-unraveled-by-analyzing-1986-space-mission.html" itemprop="relatedLink"> <figure class="bclip_media_box" itemprop="image" itemscope itemtype="https://schema.org/ImageObject"> <picture> <source media="(min-width: 768px)" sizes="400px" srcset="https://www.worldatlas.com/r/w400-h225-q40/upload/e6/7c/72/shutterstock-2310798441.jpg 400w"> <source media="(min-width: 575px) and (max-width: 767px)" sizes="575px" 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