Slashdot Mirror
A Mathematical Answer To the Parallel Universe Question
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."
It's just the Many Worlds interpretation of quantum mechanics, and I don't see anything in the article that's a shocking new revelation about it. The article's just a rehash of an idea that's been around since the 50s.
If it's for-profit but free, you're not the customer -- you're the product (e.g., the Slashdot Beta's "audience").
But can this explain why all the men have goatees?
I've fallen off your lawn, and I can't get up.
This is a fairly subtle point, so I'm not sure that I'm going to explain myself properly... but here's my best shot:
The Many-Worlds 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 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 particular classical 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 that the 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.
(1) Shit happens.
(2) Shit happens. Parallel universes are created. That isn't the choice. It is more like:
(1) (Copenhagen) The act of "observing" a particle at some point between the particle, the measuring apparatus, and your mind, somehow magically causes the particle to collapse from a wave state to a fixed one, without any other action on your part. Nobody has ever explained exactly what an observation is (we are, after all, made of particles too) nor when this happens.
(2) (Multiple worlds) Reality consists of particles in quantum waves of superimposed states. Period. When we observe a particles state, it's state becomes entangled with the state of the particles in our mind, and hence we observe the particle as collapsing to a single state "in each world".
I don't know about everybody else, but the fact that all states can exist, yet I can only perceive them separately, is no stranger to me than that all moments of time exist, yet I can only perceive each one separately.
Here is the New Scientist article being cited:
http://space.newscientist.com/article/mg19526223.700-parallel-universes-make-quantum-sense.html
However it is behind a paywall. See Google Groups 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. It's pretty technical but still a lot more readable than most physics papers.
I'm not sure if you were already aware, but there is indeed a concept called "Quantum Darwinism" which helps explain (using the results from quantum decoherence) why we observe things "classically" (single outcomes of experiments, non-entangled macroscopic states, etc.) despite the universe being fundamentally quantum.
Briefly, the theory shows (rigorously) how pseudo-classical states are the only ones that are robust against decoherence. Hence, those are the states that tend to persists for measurable periods of time. And those pseudo-classical states are the ones that give rise to other pseudo-classical states.
Moreover the main developer of these ideas (Wojciech Zurek) describes in his papers how what we typically term "memories" are inherently classical states (it's either "a" or "b"--not a superposition of both). He explains how macroscopic states will tend to be pseudo-classical, so of course any biological (macroscopic) creature will evolve to assume that reality is classical (it's an adaptive advantage and a good approximation of reality).
The point is that these larger-scale superpositions do indeed exist, but that local observers (e.g. instruments, or ants, or humans) can inherently only record/remember classical states, not quantum ones. So, our perception of reality (and memory of reality) is inherently a classical one.
Or maybe this. (Assuming the deep link works.)