{"id":3128,"date":"2008-11-07T10:11:33","date_gmt":"2008-11-07T10:11:33","guid":{"rendered":"http:\/\/scienceblogs.com\/principles\/2008\/11\/07\/whats-the-matter-with-making-u\/"},"modified":"2008-11-07T10:11:33","modified_gmt":"2008-11-07T10:11:33","slug":"whats-the-matter-with-making-u","status":"publish","type":"post","link":"http:\/\/chadorzel.com\/principles\/2008\/11\/07\/whats-the-matter-with-making-u\/","title":{"rendered":"What’s the Matter with Making Universes?"},"content":{"rendered":"

In a comment on a post from last week<\/a>, Neil B. Asks a good question about my snarky response to the “make-your-own-universe” kit<\/a>:<\/p>\n

\n

[Y]ou never explained why this “universe creator” could be considered based on a misapprehension. Considering the way multi-worlds QM theory is usually presented, IIUC; why would you (anyone?) say it doesn’t work as advertised?<\/p>\n<\/blockquote>\n

The short and unhelpful answer to this is “See Chapter 4 of my book when it comes out.” I spent a lot of time wrestling with the best way to understand this stuff, and I think it came out all right.<\/p>\n

The longer answer is, well, complicated.<\/p>\n

<\/p>\n

Let’s look at the description of the device again:<\/p>\n

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If two events are possible, quantum theory assumes that both occur simultaneously – until an observer determines the outcome. For example, in Schr\u00c3\u00b6dinger’s famous thought experiment, in which his cat may have been killed with a 50 per cent probability, the cat is both alive and dead until someone checks. When the observation is made, the universe splits into two, one for each possible outcome. For example, Schr\u00c3\u00b6dinger’s cat would be alive in one universe and dead in the other universe. <\/p>\n

According to the theory, any kind of measurement causes the universe to split and this is the basis of Keats’ new device. His universe creator uses a piece of uranium-doped glass to create a steam of alpha particles, which are then detected using a thin sliver of scintillating crystal. Each detection causes the creation of a new universe.<\/p>\n<\/blockquote>\n

For one thing, I think it’s a bit of a stretch to call this “quantum theory” as if it were the only thing out there. Many-Worlds is not “quantum theory”– it’s still just one interpretation of the theory among many interpretations (approximately as many as there are people who have thought deeply about this stuff).<\/p>\n

The bigger issue, though, is the claim that this device creates some discrete and countable set of universes– twenty trillion, or some such. This is presumably a count based on the number of uranium nuclei in the glass, assigning two universes for each possible decay. That’s not the right count, though, even in the sort of Copenhagen\/Many-Worlds mash-up interpretation they’re describing. The actual number of “universes” created here is infinite.<\/p>\n

Let’s think about a much simpler example– a single radioactive nucleus, inside a detector that will detect a decay with 100% probability. Let’s say you put the nucleus inside the detector, and then sit and watch it. One second later, you get a “click” from your detector (the canonical term, even though I don’t think I’ve ever made an actual measurement with a clicking detector). The way it’s described in the explanatory text, that counts as the creation of two universes– one in which you detected a decay, and one in which you didn’t.<\/p>\n

But if you think about it, that doesn’t really make sense. After all, the decay is probabilistic, so there was a chance that the decay would happen after only half a second. So there was a new universe created there. You just happened to be in the bit that didn’t notice the new universe.<\/p>\n

But there was also a probability of decay at a quarter of a second, giving rise to a new universe then. And there was a probability of decay at an eighth of a second, a sixteenth of a second, and all the way down the Zeno’s paradox regression to infinity. In the one second that you watched the detector waiting for the “click,” you actually “created” an infinite number of “universes,” one for each infinitesimal instant of time after the start of the experiment.<\/p>\n

You don’t need to hear a click, or see a scintillation to create a new universe– the act of not<\/strong> detecting a decay does the job as well. You’re creating new universes all the time.<\/p>\n

(You could make something that would fit the given description better, by using something with only two discrete measurement outcomes. A pair of polarized sunglasses<\/a> do the trick nicely– each photon hitting the glasses is either transmitted or not, leading to two universes per photon. If you’d like an active detection component, a polarized beamsplitter and two detectors would do.)<\/p>\n

Of course, an infinity of universes, each differing only in the decay time of a particular nucleus, is even harder to get your head around. Which is why the “creating universes” language is a little unfortunate– what we really have is a single universe, whose wavefunction is growing exponentially over time, spawning new branches every time a measurement occurs. Which is still a lot to swallow, but is at least self-consistent.<\/p>\n

The other problem with the explanation as written is that it seems a little Copenhagen-ish in the emphasis on measurement. It implies that new universes are only created when you watch the detector, which heads down the “Is the Moon there when I’m not looking at it?” path to madness very quickly.<\/p>\n

Anyway, that’s what I can offer by way of a relatively concise explanation of why I look askance at the “make-your-own-universe kit.” Others may be able to elaborate further, or just tell me that I’m an idiot in the comments.<\/p>\n","protected":false},"excerpt":{"rendered":"

In a comment on a post from last week, Neil B. Asks a good question about my snarky response to the “make-your-own-universe” kit: [Y]ou never explained why this “universe creator” could be considered based on a misapprehension. Considering the way multi-worlds QM theory is usually presented, IIUC; why would you (anyone?) say it doesn’t work… Continue reading What’s the Matter with Making Universes?<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"1","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-3128","post","type-post","status-publish","format-standard","hentry","category-physics","entry"],"_links":{"self":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/3128","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/comments?post=3128"}],"version-history":[{"count":0,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/3128\/revisions"}],"wp:attachment":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/media?parent=3128"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/categories?post=3128"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/tags?post=3128"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}