CINXE.COM

<!DOCTYPE html><html lang="en-US"><head><title>Just a moment...</title><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=Edge"><meta name="robots" content="noindex,nofollow"><meta name="viewport" content="width=device-width,initial-scale=1"><style>*{box-sizing:border-box;margin:0;padding:0}html{line-height:1.15;-webkit-text-size-adjust:100%;color:#313131;font-family:system-ui,-apple-system,BlinkMacSystemFont,Segoe UI,Roboto,Helvetica Neue,Arial,Noto Sans,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol,Noto Color Emoji}body{display:flex;flex-direction:column;height:100vh;min-height:100vh}.main-content{margin:8rem auto;max-width:60rem;padding-left:1.5rem}@media (width <= 720px){.main-content{margin-top:4rem}}.h2{font-size:1.5rem;font-weight:500;line-height:2.25rem}@media (width <= 720px){.h2{font-size:1.25rem;line-height:1.5rem}}#challenge-error-text{background-image:url(data:image/svg+xml;base64,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);background-repeat:no-repeat;background-size:contain;padding-left:34px}@media (prefers-color-scheme:dark){body{background-color:#222;color:#d9d9d9}}</style><meta http-equiv="refresh" content="390"></head><body class="no-js"><div class="main-wrapper" role="main"><div class="main-content"><noscript><div class="h2"><span id="challenge-error-text">Enable JavaScript and cookies to continue</span></div></noscript></div></div><script>(function(){window._cf_chl_opt={cvId: '3',cZone: "www.aps.org",cType: 'managed',cRay: '919c50607f6f9e32',cH: 'YfJX1pae6JfprcuycP2gxsbB7h4k1HSeS4ZzBQhYFoU-1740870481-1.2.1.1-D0cisza.fHorXOJlRvKSWH04kaF8VkFUfNfmRGdB7PMZF32HGPJsa.me8Tf_Owup',cUPMDTk: "\/publications\/apsnews\/202012\/kirby.cfm?__cf_chl_tk=iuvsW5NGAXgADFqRB6SGFRW7BHz.Me7GKs9cVbwj9OU-1740870481-1.0.1.1-ndkr7Uy847uCUCQNKDT167gX4FGNUxRUITtwwxbJyQQ",cFPWv: 'b',cITimeS: '1740870481',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/publications\/apsnews\/202012\/kirby.cfm?__cf_chl_f_tk=iuvsW5NGAXgADFqRB6SGFRW7BHz.Me7GKs9cVbwj9OU-1740870481-1.0.1.1-ndkr7Uy847uCUCQNKDT167gX4FGNUxRUITtwwxbJyQQ",md: "nZYLOhVFuXwAzqDoOqQ.b4KsnpRRz3MgQPQ4m3c.8hs-1740870481-1.2.1.1-C9rgu8JQtfajQuBs6447rlodFdbINxhIgijYLoAZvBo2Oau2D_cGoRPDj890jFZoVopocn1GmI0um0ARkdGtm.36MaWB7pkMB_0di5etkgetn5nVx983emMaRCaEsUB_cKxhXKBGz5qSRK1pB.8RbQbk2VnWFu6QFVoN7nDWbDpCS5VKG1VAO6TijNe6zJ742vIcMIddDs_ifGzvo3yQwvq69Vo6ZLZEgJ9XwSqKsLQaSgUCWdbLDxLTNLLpQAquBE.2DewqXg._l2AhI7O6vpg6bE_h.YNpMXLsYhpKel4mKLVuvUAC8ldaTCGTYecA.XE6vb8QP6jQKzJA8Uq7tAdOT4Zg1Oq.3ZYCj0G9tgLZpXMpO34mEcl.iqS0uK_pbd6DgvuRLB1UHStQ2sLPbyL5kfL8QNcHtiFd5X1jV8MQpDXvocQIc7FLFIfsKvdcA9_P42K46WcwRYZah377o2mkX3Etr.M0EdyeTsX5VLH5fSS8IHD3XRCFieagONWakSi7MF21GslyVq1qIaiKW_aaF7jP2P.sAl9mXdjPSb5.dArCAfvMoK6fAnm5eDX5osK.GgkcczKuu37EGvzeF4tapHknLGgumEnLrgb5bWS3HnsRNGfxJL6z0IZOufPF6k2HK5E47D86euAMlr0gqdYHbMXOpqm8.Q0EkFzGC9vBo0r.EvqLnUpQmnWAHzfrysxbymO6trfPIdWytMhRZBn51ASk7S1IVXY_vO0YIAtw4i9GPMEGrGn1WF5JUEl_ht5mO4abcFUxamZKfyPVPSpeCrCyvfrlaVkRzqKPD5fuVbh8fN1BACIq.l1rv6RvYV.eG6.4iMnpQakojNcYXLoiOW5gEf6p6EKCNlmk5bii4RQ1TIMRSGhMFbfq2u16BUPEIHsdX2gWAuwCd8m4kiykiHQa3q0VdplFKipEAmPxrLZNKwJ0jhWPJkluUXbDGFMKvxi1LeJ8fg0Z8nbqaTi5Rq71Id04iYjMlOONivCqgBoR.PbFN0QzBoKpTSz_u6dC_p7oCiKZ6hpF1Df_v6cH_GFD.4PWuwPzhQHeFbsSxtGSEXfqMPgJBgQ6jf1T3LInT6FDM.OPGSI.yzM1MFN9z.0CLMbIzc1.IJM6Ek8ZPhlTssXC7SpaiCOS2TGqMVPsqxdnmSjAxGXzhikpWOtG3i3uSVf89VjLkHpucF060P1l2KrX8bEdgFhywlsfVtPeO3pIB8nDmWAVvTdQ_pglvzbCo_R3yeA0o7iN8nt.1kk.Gcg1cuoGqKiVD8hC",mdrd: "8DfMavdRyBfxKF_.n9gZawaEJrpJvltskGBKeHdae8Q-1740870481-1.2.1.1-zlVzzsB.98TIsO4gWrnv5g0lQ.Izg9ezq.A4xciZknW0RdetdUFfPNsTlYvqREzX_v2dHbuwQzWiF1aCOIJnnk8cBCOe81AEy8bkw_EkbU6QPllyQm7RsAjDggvePIODEqUUi8dskXjWJKTKYKrm5u5qjRvw1j6YUnkVsnDjYwBWf2.xUOklLTKeBm9wouZlJjZDvRRPmBTPLbjJ30qRIkGEnEWvmfFYXOT9PzCEfWxd4ajwrFDwssckepK_ZJaXv1GGYGrteAc.ihtLOlKJabk8peNPAoHBwvLVLhjGCtu0oQ.X0.g8A_Z2ztmpMbJl_5Yb1J0y3e_smpvKxXeDJhJeDZ1QATTJwBDs.zlHoMtKNPRVx8fW.vckLd6Gm.UfiWvqPWKTWIeM_qHNaudeWFCp5jvl.VKpBZbn6mLyzE6vePOfM8LKfXigHP9hHEPlI.0Jwgyn4rjgtdL7dmnbIAvTrLK9zFiCuZVarrxJipom3K6GnlwLd04f27HfDV.vJEBAranzk138ZL5Xpme_9QfYFsv7xk4niG64qMLo6rlskWrf4ykr1Fj3dxMxP.J_ZtZk1n5Idffa6XrmECOnapGhra.Z3oi2CMBBwpz.e2c6zG5kFwP5ZSN33QAV106qo2598un06wWdhUh9MEQnpfqlN5g7Vr9ER3y_C_deXPepnh.Zkx.0oHNRLlbGP9NIwYguVij3d._dh5bdfPY7OIH0xXcO0R.RqOThau7WLRHH.phR3_BECpSYpCqXhb.UoO8SnIUyt5dEmt9lcbRK1PwWb5Jy14B4BJpM2ymf9tkH9.DX7eHQG0.jeNFkhXTerZEiq6FgDxPk7TGthazKrV5uo8mrn2_WV4uHll5PP.f2ZtPlwWB_CmT99JYpRGI8RYI9iTSgsixJpIfIwknSE2aPEJE_Vz_FW79cPJCc6FPDWqu9ax5SYciJdxU8U6MsennJHLV_AkOnHBPsPnBAoO71F14HyMO6Xf4NorkhS5nH0lMeACKNUF.GehPJ_PIGfKkSMofVas5KwzrU.h7mNfBvy0Q4FKgw_gPWFPs7QPl36_g1yWMTFmJQYp0RQqAz15Ac7Co4ugnSUhbvoShsI8KyGa6rk5qVoDbiD4ZYdBmtoIb5_YV5f0fs3Pu7u1f5TejS_hQXp_6o2MpzGDOzVD6rQ7IdXIGlGizDSA._lgLNQLRobIJ5Rl9O0.E3CuIvdRveU5gZ73z.nHO1XyI6VExsn7VsaILDeQQjfH9vJ0Z1o0wl7xq2ez0WCL4lrdy.jVzMoOhAXpt8BfUgi2.KoN229cnLY8txRVXEmSKOe4.VrA3lNVB8iKOjVoXvbiqk2jU1AsrhKTf.JBdI42TS9DDIyPesJIKVwEhajSD6fNjpNJamaeb.NSAzopLEJ5dwkLNIyUSgdHbRLAJtrmTUyD4rLP7Tn6rSOP9eeXefKZu3miBtZoy7bRNxWQbk5cdSKa4uK4IP0hA7IhALw1wW46Ka.7Zpr.L8RzUJ_ZGUlO.CaNerwHLVFZgBkAcB49xHPdmVbAsb6Tz80i_NlBkrnwrkGvpL4IgOQ2C7BYjtIYZk1VIUjr4aJz919LhCRSSbPpSJDwfqaIiMc2Za6PvJ.1pIW8CPIWky8KyYRGWROPhdOI0UcCBD3giuf6rDy1XBJI3h75C0oDq.yewH1r7cO_IU.OP1su2pdrIgKdPuasM4zYgTjDb7rbRVvY74XWloZq9kn.1Vq_SrKSv1UrTHDoztRgY8Ie8C6o10fiiFWA5AWvpGk_3Ufv4LkBwfEoujvwldUJKn4hpoFTAPFhWyK4WadcclNB0VEzARN03Uuy6rGoiL1Ue2cHGvqCEhpMytstPUNEzJTgkSj2Sl23jua9NesLthYTFGDuS88azLa_.vzmikD0eGMWm.KfRdgPvQrN2gGtyBANxBLCZ3Eo.B6MFKFgKT.zEL31W0xLioCGM"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/b/orchestrate/chl_page/v1?ray=919c50607f6f9e32';window._cf_chl_opt.cOgUHash = location.hash === '' && location.href.indexOf('#') !== -1 ? '#' : location.hash;window._cf_chl_opt.cOgUQuery = location.search === '' && location.href.slice(0, location.href.length - window._cf_chl_opt.cOgUHash.length).indexOf('?') !== -1 ? '?' : location.search;if (window.history && window.history.replaceState) {var ogU = location.pathname + window._cf_chl_opt.cOgUQuery + window._cf_chl_opt.cOgUHash;history.replaceState(null, null, "\/publications\/apsnews\/202012\/kirby.cfm?__cf_chl_rt_tk=iuvsW5NGAXgADFqRB6SGFRW7BHz.Me7GKs9cVbwj9OU-1740870481-1.0.1.1-ndkr7Uy847uCUCQNKDT167gX4FGNUxRUITtwwxbJyQQ" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>