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Perhaps any "the best" question should be a community wiki.. –  HDE Feb 28 '11 at 16:27
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MW is inherently untestable/unfalsifiable by its nature, and I would venture to say that is less Physics and more Philosophy. –  Justin L. Mar 1 '11 at 2:46
    
@JustinL. That is precisely what I don't like about it. –  Ali Oct 13 '13 at 2:19
    
@JustinL.: MWI is philosophy, but it might be better to say that all interpretations of quantum mechanics are philosophy. –  Ben Crowell Oct 13 '13 at 3:13
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@KaziarafatAhmed The popular article you linked and the research it describes has nothing to do with many-worlds whatsoever!! –  Mark Mitchison Oct 13 '13 at 9:00
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13 Answers

For:

  1. Collapse is an awkward non unitary concept in the C.I. and likely to be the source of most of the so called paradoxes (like Schrodinger's Cat etc.). Many worlds does not have collapse and thus avoids those paradoxes. It frees one from assuming the "add on" of collapse.

  2. Quantum entanglement finds a "local" meaning.

  3. Removes randomness from quantum theory (although for an observer it does not remove any randomness).

Against:

  1. Huge proliferation of universes for explaining the observations of an observer. In a way it does not respect the Occam's razor. (Many worlders claim MWI respects it since this interpretation has an economy of principles).

  2. The problem of preferred basis.

  3. Defining a suitable measure of probability to achieve Born rule.

  4. Other universes can not be observed. (A variation of saying it does not respect Occam's razor)

  5. It is rather a psychological way of thinking about Q.T. rather than a real ontology.

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real ontology, could you explain it further? –  HDE Feb 28 '11 at 16:26
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@HDE: Since it is not a philosophy community, I shall restrain myself from explaining it deeply. Just would like to mention that the word "ontology" means "the nature of being" that is the "thing in itself" rather than our knowledge and suppositions which is called "epistemology". –  user1355 Feb 28 '11 at 16:32
    
I see, then the word ontology is a synonym of realism, and as I understood ralism is questioned since Bell theorem.. –  HDE Feb 28 '11 at 16:34
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@HDE: As far as MWI is concerned it does not question "realism". Bell's theorem is about different predictions of local hidden variable theories and quantum theory. Experimental verdict favored quantum theory. It means local hidden variable theories are out. –  user1355 Feb 28 '11 at 16:39
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I've heard the preferred basis problem many times, but it seems people have different views on how it is a problem for MWI. Some claim it mean you can't have ontology in MWI and hence doesn't explain our observations. Other's say it put MWI in problems with relativity. –  Qurious Feb 28 '11 at 19:54
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in my opinion, the best argument is that removes the need to postulate a collapse of wavefunction, and explains the source of the randomness of quantum events as a single observer splitting in multiple version of the same observer entangled to each eigenstate of the observed system

the weak aspect of the many worlds interpretation is that it doesn't give a natural explanation of probabilities as the magnitudes-squared of amplitudes, this is still required to be assumed

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I think Decoherence remove collapse need too –  HDE Feb 28 '11 at 16:25
    
interesting. Is there a reference you want to share that explains this? –  lurscher Feb 28 '11 at 16:54
    
@lurscher: Yes, the "decoherent histories" or "consistent histories" approach enjoys the virtues of both worlds, C.I. and M.W.I. although there are problems of this approach too. You have to make some additional assumptions (like maximality etc.) to make it work. –  user1355 Feb 28 '11 at 17:05
    
@lurscher ipod.org.uk/reality/reality_decoherence.asp –  HDE Feb 28 '11 at 17:52
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@lurscher Actually, the Born's rule (i.e. $p(x)=|\psi(x)|^2$) can be derived in a very natural way, see Zurek's paper arXiv.org/abs/quant-ph/0405161. –  Piotr Migdal Mar 1 '11 at 14:23
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It is called "interpretation" for a reason: Ultimately, it all describes the same physics, i.e., so far there is no statement "X" that would be true in the MWI and false in Bohmian QM or the Classical Interpretation. Now, if two ways of explaining something lead to entirely the same results, then for all practical purposes these two ways are identical. In my opinion, it is therefore purely a matter of taste to which of the available interpretations one ascribes.

It is only when results between different interpretations become inconsistent that we can rule one of them out.

Which reminds me of a funny text I read on this topic: You can prove to yourself that the MWI is correct by committing "quantum suicide": Think of Schroedinger's cat, but you are the cat. If the MWI is correct, then there will always be a world in which you aren't dead yet. You should notice that you aren't dead, and after repeating the experiment for a sufficiently long time you can conclude that, with high probability, the MWI is correct. If, however, the classical interpretation is correct, you'll just die with high probability :-( Thus, I say "Don't try this at home!"

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Arguably in Copenhagen this "quantum suicide" will give the same result: after a suicide the collapse stops so there is always an amplitude for you to being alive, and for this amplitude collapse starts again. –  Anixx Feb 7 at 8:26
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Having studied some David Deutsch material recently here are some other points:

For

  1. A Multiverse framework for considering whether there is "something beyond" traditional quantum theory. EDIT Thus providing a framework for investigating quantum gravity. Deutsch sees the Multiverse as an as yet undiscovered theory, motivated by Everett, Quantum Computation and certain properties of General Relativity.

  2. A possible "explanation" for Quantum Computation speedups (EDIT ADDITION) That is for those quantum computations in the BQP Complexity class.

Against

  1. Those Probability and Preferred Frame problems

  2. The fact that the theory has split into many sub-theories (e.g. Decoherence, Many Minds, Multiverse, etc) that there is no single model to evaluate any more.

  3. (Related to 2 perhaps) Everett did not publish enough to clarify exactly what the theory actually said in several respects.

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@Roy: I don't understand your "something beyond" point. MWI is nothing beyond Q.T. –  user1355 Feb 28 '11 at 15:53
    
@sb1 : If you follow the Deutsch discussions you will see in more detail. His argument seems to be that "Multiple Universes" is a bit classical ie simplistic view of what is really happening. What is really needed is a "Quantum Multiple Universe" concept which he believes needs further equations to fully define. So the Multiverse becomes a research framework into QM, in my view (of his view). –  Roy Simpson Feb 28 '11 at 15:58
    
@Roy: It may be David Deutsch's view (I have huge admiration for him). But as far as MWI is concerned it does not go beyond quantum theory in my humble opinion. –  user1355 Feb 28 '11 at 16:02
    
@sb1 : It all depends on what "beyond" means of course. I have added some further EDIT points on that. –  Roy Simpson Feb 28 '11 at 16:20
    
Another against: misleading intuition about quantum computation speedups. –  Peter Shor Feb 28 '11 at 18:54
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Everettians are trying to pull wool over your eyes... If it only were that simple that our world keeps branching with time with a preferred basis, and we subjectively find ourselves in one branch. There are two major problems with this interpretation.

  • Different branches can and will recombine in the future. This is seldom emphasized, but this throws a wrench in the interpretation. In fact, for a system in thermal equilibrium, branching and recombination happens at an equal rate. It's only thanks to the fact that locally, we are out of equilibrium that one-way branching makes any approximate sense at all.

  • There's no canonical preferred basis in general, not even macroscopically. Decoherence works most of the time at macroscopic scales, but with many important exceptions. If decoherence were universal at macroscopic scales, do you think we'd be able to observe double slit experiments or superconductivity or quantum optics? Even more troubling is the fact that the basis to be chosen can depend contextually upon future decisions, as in the delayed choice experiment.

  • In the many minds interpretation, subjectively fixing the conscious state of the observer still leaves most of the rest of the universe in an indeterminate superposition. Only those coarse-grained properties of the "world out there" corresponding to our internal conscious states will be determined by entanglement. Those Everettians who try to tell you subjective experiences are what causes the entire universe to split into many worlds are bluffing.

  • If the other worlds out there have some objective existence, how come we can't extract information from them, except in very special cases where we have a coherent variation in the phase and amplitudes between the many branches which then recombine? Not only that, after recombination, the separate worlds lose their separate identities. Besides, a coherent variation rules out the possibility of a complex intelligent observer, at least in the part of the wave function which varies coherently.

Do you really see?

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Dear Coward, this is actually relatively accurate as a criticism, except for the conscious state argument. The conscious state will fix the wavefunction all the way throughout the universe relatively sharply, and this is not a bluff. The reason your intuition is failing is that every particle is entangled with every other (for example, every atom here collapses the Andromeda galaxy, the wavefunction is global) –  Ron Maimon Nov 25 '11 at 0:52
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The largest problem with "Many Worlds" interpretations is they do not currently offer testable hypotheses.

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MWI is an example of misunderstanding of what the wave function is about and it introduces unnecessary "universes" that do not follow from experimental data. It does not solve any conceptual problem. On the contrary. Besides, it is not verifiable. Briefly, it is a funny example of nonsense in science.

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@Vladimir: I don't agree at all. I accept it is rather psychological. But it is one of the best ways to think about quantum theory. –  user1355 Feb 28 '11 at 15:50
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Quantum Wave is not observable neither and physics deal with it anyway because of its observable effects –  HDE Feb 28 '11 at 16:29
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Sure. The question is: Which does Occam's razor favor? Proponents of both the Statistical Interpretation and the Many World Interpretation could claim that their concept is the more economical one. The former doesn't need infinitely many universes, the latter doesn't need the collaps of the wavefunction. As far as I am concerned, I believe it's (currently, until more evidence comes up) a matter of taste. –  Lagerbaer Feb 28 '11 at 16:51
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;-) photograph of the probability? ;-) then be sure some day some experimental "physicists" will come with a photo graph of a multiverse ;-) Can those "physicists" take photograph of irritation? –  user1355 Feb 28 '11 at 17:26
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I've read about the Ensemble Interpretation, but I seem to remember that the main objection against this is that you can then never use it for single particles? –  Qurious Feb 28 '11 at 20:41
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A good argument against is Occham's razor. Another is the fact that it is not an experimental testable hypothesis. The best argument I think against it is the fact that the only reasons for Many-worlds are based in human language, whose intuition is only developed for a classical setting, as such any human meta-reasoning cannot be expected to apply to non-classical areas, there are no mathemtical or scientific reasoning involved.

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Sorry, by I see little sense it your post. Actually MWI eliminates measurement (which is very not natural in many aspects and non ), which Ockham's razor. It's better to calculate rules than naively calculate beings (whatever they mean). Well, as we are familiar with classical world, it's intuition puts as biased against MWI, not for (BTW: talk with non-physicits, and they will cal MWI quirky; talk with quantum physicists, and for them MWI is natural). –  Piotr Migdal Mar 1 '11 at 14:31
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There's no experimental proof one way or the other. The only way to tell is Occam's razor.

There's two interpretations of Occam's razor. One says that the number of laws should be small. The other says the number of entities should be. I can't prove one of them is correct philosophically, but given that the laws could fit readably on a single note card, and there are 10^80 particles (which are distinct entities), I'd go with the former.

The many-worlds interpretation has strictly fewer laws. That is to say, the laws that make it up exist entirely within the Copenhagen interpretation, where they govern entangled particles. As such, no matter what language you use, the many-worlds interpretation will come out smaller.

Edit: My mistake. I can prove one is correct philosophically. The proof of Occam's razor is basically that all the prior probabilities have to add to one, and if it didn't decrease as complexity increased, they'd add to infinity. This gives a lower limit to how fast probability must decrease.

It applies to number of laws and number of entities, but it just means that exactly 10^80 particles is 80 orders of magnitude less likely than 1. 10^80 to 10^81 is still about as likely as 1 to 10.

When you deal with laws, you're not talking about how many bits the theory is. You're talking about the theory itself. If it takes 267 bits to express, and it's an exact set of 267 bits, that's analogous to there being exactly 10^80 particles.

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Has the MWI produced any new physics? What I mean is not people who subscribe to it doing new physics, but people doing new physics because they were thinking within that interpretation. If the answer is nothing, then this, in my eyes, is a very strong argument against it. Of course this could apply to other interpretations.

I am not sure if I am clear. What I mean is for example, is there a calculation or a derivation or anything like that, which would have not been done (at least for some time) had there been no MWI.

Edit: I take the comment about David Deutsch as an example of what I was asking for, hence my answer should be viewed as a point in favour for the usefulness of MWI.

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this is not a place to do follow-up questions. However the answer is clearly no; MWI is just an alternative axiomatic framework, if you like, for copenhague quantum mechanics, which is shown to be entirely equivalent in predictions –  lurscher Feb 28 '11 at 17:25
    
I didn't mean it as an actual question. I phrased it as a question because I cannot be 100% sure that there aren't any examples. So it is a statement with a reasonable amount of uncertainty. –  MBN Feb 28 '11 at 17:31
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David Deutsch invented (co-invented?) quantum computation by thinking about how you could experimentally test the many-worlds interpretation. –  Peter Shor Feb 28 '11 at 18:53
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Please use the Post answer button only for actual answers. You should modify your original question to add additional information. –  Sklivvz Apr 3 '11 at 17:28
    
@Skivvz: Was that comment for me? This answer was written more than a month ago, why now? And why are you telling me that! –  MBN Apr 4 '11 at 2:58
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The conservation of energy: This is exactly what I have been thinking about. In MWI the total energy of universes is constantly increasing as we are multiplying ourselves along with universes. This clearly violates the law of conservation of mass and energy law. While in case of CI there is always one reality after collapse of all other possibilities thus conserving the mass. (AFTAB KHAN) _6÷

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http://lesswrong.com/lw/r5/the_quantum_physics_sequence/

One of the best introductions for MWI out there. Answers queries relating to Occam's razor.. and why it's more than our fantasy..

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Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. –  Sklivvz Apr 3 '11 at 17:29
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I don't understand why nobody has mentioned the apparent violation of conservation of energy. Am I naive here for thinking that a quantum mechanical trigger that allows the bifurcation of world lines, violates the conservation of energy?

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Yes, you are. The overall energy over all "worlds" is still conserved, because it is weighted by measure -- just like standard superpositions, because that's all MWI is. Within each "world", there is a local "relative energy" that also appears to be conserved. –  wnoise Apr 4 '11 at 15:37
    
But then an observer following one world line would see a shift in energy from 100% to 50% after the bifurcation. In the end after billions of bifurcations the total energy available to any one observer is going to approach zero. MWI still makes no sense to me. –  ja72 Apr 4 '11 at 16:46
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Not at all. An observer in each world is also cut down in measure, so the effect of a given energy is the same, so is measured the same. –  wnoise Apr 4 '11 at 17:29
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