## A Mathematical Answer To the Parallel Universe Question 566

diewlasing writes to mention that Oxford scientists have proffered a mathematical answer to the parallel universe question that is gaining some support in the scientific community.

*"According to quantum mechanics, unobserved particles are described by 'wave functions' representing a set of multiple 'probable' states. When an observer makes a measurement, the particle then settles down into one of these multiple options. The Oxford team, led by Dr. David Deutsch, showed mathematically that the bush-like branching structure created by the universe splitting into parallel versions of itself can explain the probabilistic nature of quantum outcomes."*
## Why is this news? (Score:5, Informative)

## Well if you can't believe in God.... (Score:2, Informative)

## Re:Raises the question (Score:2, Informative)

Because you are in this one. If you were in a different one you would wonder the same thing.

That's the anthropic principle.

## Re:Ummm . . . (Score:5, Informative)

The Many-Worlds [wikipedia.org] concept of quantum mechanics was originally presented as an interpretation of the theory. It was viewed by many as being ridiculous, or "non-economical with universes" as the joke goes. Work in fields like quantum decoherence [wikipedia.org] has, over the last few decades, helped to explain how "normal" (classical) states emerge from quantum superpositions. Decoherence, briefly, explains how a superposition of quantum states evolves deterministically (no randomness!) into a discrete set of pseudo-classical states (due to entanglement with the many degrees of freedom available in the "environment"--i.e. the universe at large). This extension to quantum mechanics has been tested experimentally and verified.

The remaining issue in a theory of quantum + decoherence is that the classical states have the right probabilities, but there is still nothing to explain why we observe a

particularclassical state (photon measured spin-up instead of spin-down). However the (ad-hoc) postulate of wavefunction collapse, no longer being necessary to explain how the probabilities arise, can in fact be entirely removed if we allow thatthe global superposition never collapses.Thus, a local observer (e.g. an instrument or a human) perceives a single outcome only because they are a participant in this "global superposition" (the superposition of the entire universe). The wavefunction of the universe as a whole evolves deterministically.

Okay, that was a long-winded preamble, and I still have not answered your question. The answer is that the existence of multiple universes cannot be falsified per se. But, then again, in this formalism Many-Worlds is not an axiom: it is

a prediction. Given that it is a prediction of a thoroughly successful theory, we should be compelled to accept the prediction as correct even if we cannot directly test it. We can, at least, test other predictions of the theory. In principle, we can test for superpositions as big as we like (superpositions of entire galaxies, etc.), but we cannot ever test that final prediction: that the universe as a whole is also in a superposition. But, if we've tested the theory in every other way, can we really "throw away" the final prediction about the global superposition?Now, I know many of you will counter-argue that non-falsifiable predictions are not science, and should be ignored as metaphysics, or even "meaningless." Perhaps. But allow me to draw an analogy: One of the fundamental assumptions of science is that there is such a thing as "physical law." That is, we can extrapolate from one measurement to others. Put otherwise, we accept that the laws of physics are the same here as they are in a distant galaxy. Note that, because of the expansion of the universe and the speed-of-light-limit, there are some regions of the universe that we cannot ever explore (even in principle, assuming our current physics is correct). Thus, the prediction that "the laws of physics are invariant across the universe" is itself unfalsifiable, yet we generally accept it to be true.

Similarly, we need but extend this logic into quantum mechanics, where if assume that the laws of physics are the same everywhere in the universe (and everywhere within the wavefunction of the universe), then we should accept that the global superposition is probably correct: i.e.: Many Worlds "exist" (but are inaccessible to us). I agree that this conclusion is uncomfortable, but it appears inescapable given our current understanding of physics. (Note: As a scientist I'm of course allowing for the possibility of future measurements disproving some part of this logic--this is entirely based on our current understanding.)

As I said, the point I'm trying to make is not obvious. Hopefully I've not muddled it beyond understanding.

## More links (Score:5, Informative)

http://space.newscientist.com/article/mg19526223.700-parallel-universes-make-quantum-sense.html [newscientist.com]

However it is behind a paywall. See Google Groups [google.com] for the whole thing.

There is a great quote by physicist Max Tegmark: "The critique of many worlds is shifting from 'it makes no sense and I hate it' to simply 'I hate it'."

As far as the meat of it, traditionally the Many-Worlds Interpretation has had two technical objections raised. The first is called the basis problem, and the second is deriving correct probabilities. The basis problem is that when the universe "splits" it's not clear how it should split. The math allows for infinite different ways to split, but we only see one way. This has been solved in recent years by the study of decoherence, which in MWI terms is like looking at the splitting process up close. Turns out it can only happen one way in practice. So that one's done.

The article is more about the other one, deriving probabilities. Actually it's easy to derive probabilities in the MWI, but they're wrong. The right probabilities are what is called the Born rule, and it's been hard to get those. David Deutsch came up with a new idea in 1999 where he proposed tying it in to decision theory. He said that we really care about probabilities because they influence how we make decisions about what to do. If we can derive a reasonable decision theory within the MWI, then we've essentially explained probabilities. His work had some shortcomings but subsequent efforts have largely resolved those.

So now for the first time, the two traditional technical problems with the MWI have reasonably good solutions. Hence we are back to, as Tegmark says, "I hate it" as the main objection to the theory. Since that's not really a good argument, it can be said that the MWI should be considered the most compelling candidate for an interpretation of QM.

One final link, here is one of the papers that extends Deutsch's idea about decision theory and pretty much closes the holes: http://arxiv.org/abs/quant-ph/0312157 [arxiv.org]. It's pretty technical but still a lot more readable than most physics papers.

## read one of Deutch's papers (Score:2, Informative)

paper the article talks about.

http://xxx.lanl.gov/abs/quant-ph/0104033 [lanl.gov]

It's more of an information theory paper, it seems to me, and

not so easy to relate to any verifable theory of the

universe/multiverse.

## Better article on this subject (Score:2, Informative)

## Re:Why is this news? (Score:4, Informative)

I *am* a physicist, although I don't have Dr. Albrecht's credentials in this area, so he certainly has a more informed opinion than my own. However, based on my knowledge of the subject, the importance of this finding is in fact fairly over-rated. I don't think that it confirms anything unexpected *and* the theory is, as far as I know, not falsifiable. (I've never heard of a test which would differentiate between the Many Worlds view and the competing interpretations.) So you see, showing that you can't rule Many Worlds out is important, but it does strike me as really revolutionary.

## Oh really? (Score:3, Informative)

Math can prove that a mathematical system is consistent, and within that system can prove properties that result in that system.Oh [wikipedia.org] really [wikipedia.org] ? [wikipedia.org]

## Re:Well if you can't believe in God.... (Score:2, Informative)

The worst part is no one wants to correct this foundational work. There is not much glory in changing coefficients and contradicting published work only invites enemies.

My main point is that a certain amount of non-luminous matter is to be expected. I would not be supprised though, when that unglamorous job of correcting all these fits is done, that the quantity and location of 'dark matter' is revised downwards. That, or we find something more fundamentally wrong (like with the precession of Mercury's orbit).

Remember people used to believe in caloric.

## Re:Well if you can't believe in God.... (Score:3, Informative)

## Re:Why is this news? (Score:2, Informative)

In conventional interpretations, the wave function "collapses" upon observation, to a single eigenfunction.

In the Many Worlds Theory, it NEVER collapses. The magnifying effect of an observation of microscopic fluctuations into macroscopic changes in the world (i.e., a geiger counter clicking or a track in a cloud-chamber) causes the universe to split into distinct branches corresponding to eigenfunctions. These distinct branches continue to interact with each other (although extremely weakly, because they are "far apart" in Hilbert space).

This should be falsifiable, although the technology to do so might not exist now. One must repeatedly perform an extremely accurate measurement whose macroscopic effects are minimal (so the different branches do not drift too far apart). "Extremely accurate"=accurate enough to observe the interaction of the other branches of the universe. I believe Dr. Deutsch has proposed such an experiment that may be conducted within the next 50 years.

## Re:Universe means "one everything". (Score:2, Informative)

## Re:Yes... (Score:3, Informative)

There's always a different, random Futurama quote in an X-Fry: or X-Bender: header.

Example:

$ curl -I slashdot.org

HTTP/1.1 200 OK

Date: Tue, 25 Sep 2007 05:41:42 GMT

Server: Apache/1.3.37 (Unix) mod_perl/1.29

SLASH_LOG_DATA: shtml

X-Powered-By: Slash 2.005000175

X-Bender: They're tormenting me with uptempo singing and dancing!

Cache-Control: private

Pragma: private

Vary: User-Agent,Accept-Encoding

Connection: close

Content-Type: text/html; charset=iso-8859-1

## Re:Yes... (Score:3, Informative)

There's always a different, random Futurama quote in an X-Fry: or X-Bender: header.