{"id":12424,"date":"2015-03-11T08:56:06","date_gmt":"2015-03-11T15:56:06","guid":{"rendered":"http:\/\/www.preposterousuniverse.com\/blog\/?p=12424"},"modified":"2015-03-11T18:06:42","modified_gmt":"2015-03-12T01:06:42","slug":"what-happens-inside-the-quantum-wave-function","status":"publish","type":"post","link":"https:\/\/preposterousuniverse.com\/blog\/2015\/03\/11\/what-happens-inside-the-quantum-wave-function\/","title":{"rendered":"What Happens Inside the Quantum Wave Function?"},"content":{"rendered":"<p>Many things can &#8220;happen&#8221; inside a quantum wave function, of course, including everything that actually does happen &#8212; formation of galaxies, origin of life, Lady Gaga concerts, you name it. But <em>given<\/em> a certain quantum wave function, what actual <em>is<\/em> happening inside it?  <\/p>\n<div class=\"lyte-wrapper\" title=\"Philosophy of Cosmology\" style=\"width:640px;max-width:100%;margin:5px;\"><div class=\"lyMe\" id=\"WYL_5TFy6Ben0Ho\" itemprop=\"video\" itemscope itemtype=\"https:\/\/schema.org\/VideoObject\"><div><meta itemprop=\"thumbnailUrl\" content=\"https:\/\/i.ytimg.com\/vi\/5TFy6Ben0Ho\/hqdefault.jpg\" \/><meta itemprop=\"embedURL\" content=\"https:\/\/www.youtube.com\/embed\/5TFy6Ben0Ho\" \/><meta itemprop=\"duration\" content=\"PT48M47S\" \/><meta itemprop=\"uploadDate\" content=\"2015-03-06T15:00:44Z\" \/><\/div><div id=\"lyte_5TFy6Ben0Ho\" data-src=\"https:\/\/i.ytimg.com\/vi\/5TFy6Ben0Ho\/hqdefault.jpg\" class=\"pL\"><div class=\"tC\"><div class=\"tT\" itemprop=\"name\">Philosophy of Cosmology<\/div><\/div><div class=\"play\"><\/div><div class=\"ctrl\"><div class=\"Lctrl\"><\/div><div class=\"Rctrl\"><\/div><\/div><\/div><noscript><a href=\"https:\/\/youtu.be\/5TFy6Ben0Ho\" rel=\"nofollow\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/i.ytimg.com\/vi\/5TFy6Ben0Ho\/0.jpg\" alt=\"Philosophy of Cosmology\" width=\"640\" height=\"340\" \/><br \/>Watch this video on YouTube<\/a><\/noscript><meta itemprop=\"description\" content=\"Philosophy of Cosmology\"><\/div><\/div><div class=\"lL\" style=\"max-width:100%;width:640px;margin:5px;\"><\/div><\/p>\n<p>A surprisingly hard problem! Basically because, unlike in classical mechanics, in quantum mechanics the wave function describes superpositions of different possible measurement outcomes. And you can easily cook up situations where a single wave function can be written in many different ways as superpositions of different things. Indeed, it&#8217;s inevitable; a humble quantum spin can be written as a superposition of &#8220;spinning clockwise&#8221; or &#8220;spinning counterclockwise&#8221; with respect to the <strong>z<\/strong>-axis, but it can equally well be written as a superposition of similar behavior with respect to the <strong>z<\/strong>-axis, or indeed any axis at all. Which one is &#8220;really happening&#8221;?<\/p>\n<p>Answer: none of them is &#8220;really happening&#8221; as opposed to any of the others. The possible measurement outcomes (in this case, spinning clockwise or counterclockwise with respect to some chosen axis) only become &#8220;real&#8221; when you actually measure the thing. Put more objectively: when the quantum system interacts with a large number of degrees of freedom, becomes entangled with them, and decoherence occurs. But the perfectly general and rigorous picture of all that process is still not completely developed.<\/p>\n<p>So to get some intuition, let&#8217;s start with the simplest possible version of the problem: what happens inside a wave function (describing &#8220;system&#8221; but also &#8220;measurement device&#8221; and really, the whole universe) that is <em>completely stationary<\/em>? I.e., what dynamically processes are occurring while the wave function isn&#8217;t changing at all?<\/p>\n<p>You&#8217;re first guess here &#8212; nothing at all &#8220;happens&#8221; inside a wave function that doesn&#8217;t evolve with time &#8212; is completely correct. That&#8217;s what I explain in the video above, of a talk I gave at the <a href=\"https:\/\/www.preposterousuniverse.com\/blog\/2014\/10\/03\/ten-questions-for-the-philosophy-of-cosmology\/\">Philosophy of Cosmology workshop in Tenerife<\/a>. The talk is based on my recent <a href=\"https:\/\/www.preposterousuniverse.com\/blog\/2014\/05\/05\/squelching-boltzmann-brains-and-maybe-eternal-inflation\/\">paper with Kim Boddy and Jason Pollack<\/a>.<\/p>\n<p>Surprisingly, this claim &#8212; &#8220;nothing is happening if the quantum state isn&#8217;t changing with time&#8221; &#8212; manages to be controversial! People have this idea that a time-independent quantum state has a rich inner life, with civilizations rising and falling within even though the state is literally exactly the same at every moment in time. I&#8217;m not precisely sure why. It would be more understandable if that belief got you something good, like an answer to some pressing cosmological problem. But it&#8217;s the opposite &#8212; believing that all sorts of things are happening inside a time-independent state <em>creates<\/em> cosmological problems, in particular the <a href=\"https:\/\/www.preposterousuniverse.com\/blog\/2006\/08\/01\/boltzmanns-anthropic-brain\/\">Boltzmann Brain problem<\/a>, where conscious observers keep popping into existence in empty space. So we&#8217;re in the funny situation where believing the correct thing &#8212; that nothing is happening when the quantum state isn&#8217;t changing &#8212; solves a problem, and yet some people prefer to believe the incorrect thing, even though that creates problems for them.<\/p>\n<p>Quantum mechanics is a funny thing.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Many things can &#8220;happen&#8221; inside a quantum wave function, of course, including everything that actually does happen &#8212; formation of galaxies, origin of life, Lady Gaga concerts, you name it. But given a certain quantum wave function, what actual is happening inside it? A surprisingly hard problem! Basically because, unlike in classical mechanics, in quantum [&hellip;]<\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[28],"tags":[],"class_list":["post-12424","post","type-post","status-publish","format-standard","hentry","category-science"],"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts\/12424","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/comments?post=12424"}],"version-history":[{"count":4,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts\/12424\/revisions"}],"predecessor-version":[{"id":12428,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts\/12424\/revisions\/12428"}],"wp:attachment":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/media?parent=12424"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/categories?post=12424"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/tags?post=12424"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}