Alain Aspect settled the issue of hidden variables with the aid of Bell's
inequality in the crucial "Bell test experiments <http://en.wikipedia.org/wiki/Bell_test_experiments>
." I do not know how one can do similar types of experiments without access to
other universes. I agree that a multiverse is an interpretation of the usual
quantum mechanics however, it adds nothing new to our knowledge of Nature and so
the Copenhagen interpretation suffices.
?
Well, Aspect's experiment didn't deal with the entire issue of hidden variables.? It dealt with "local" hidden variables assuming all information travels in forward-time, but not with _non-local_ hidden variables (as in the Bohmian interpretation) nor with local hidden variables traveling _backwards_ in time (which would seem to be non-local since they would seemingly appear out of nowhere).? So there are many loopholes to hidden variables and it is still a very popular interpretation to QM among the Bohm camp.? As I will mention below, Aharanov introduced a version that uses the time-reversed signal.
The problem with CI is that it assumes the observer is classical and that the classical interaction causes the wavefunction collapse.? The problem is how do we treat the observer as a part of the QM system?? If we make the observer a wavefunction, then the wavefunctions of the observer and the?experiment?simply entangle and no wavefunction collapses, and we thus end up with MWI as the automatic outcome.? That is, unless we still put the "collapse" back into the system by hand, somehow.? This leads to the ad hoc theories about "wavefunction reduction" when the entanglement becomes greater than 10^10 particles, or similar ideas, in order to make the collapse still happen.
The problem with MWI is that it fails to explain probabilities.? If you have a quantum measurement where the particle will be "spin?up" 2/3 of the time and "spin down" 1/3 of the time, and if MWI predicts that both branches actually occur in nature, then why don't we observe each branch with 50% probability instead of the 2/3 versus 1/3 split correctly described by CI?? So the challenge in MWI is to make the number of branches in each outcome proportional to the observed measurement results as described by CI, and there is nothing inherent in the theory to do that.? So the debate is raging over this point, with critics of MWI saying that the versions of MWI that have been propped up to produce the correct percentages in the outcomes are ad hoc and therefore not real science.? Deutsch is claiming that he has solved this problem, and I'd like to read his paper if I can find time.
I think there is a bigger issue regarding non-locality in MWI.? Most versions of QM accept non-locality as an unfortunate reality, but now Deutsch is claiming that MWI can get around non-locality.? But I am skeptical of this claim and suspect that non-locality is simply being defined out of existence by undermining Einsteinian "reality" as a philosophical assumption.? His abstracts have led me to think that is where he is going, but it will take some hard reading to really find out.? In my opinion, MWI makes non-locality far worse than in any other interpretation of QM because when the branches of the wavefunction split, that split involves the entire cosmos.? That means that distant quasars are splitting into multiple copies of themselves every time you flip a coin in your hand.? How about that for non-locality???
There is another view of QM that I'm not really ready to discuss yet, which is more elegant than MWI (IMO) and disposes of all the basis scientifically or philosophically for multiple worlds in the wavefunction.? It is a view proposed by Aharanov back in the 1960's or 70's (his "Time Symmetric Counterfactuals").
I wish I had time to read Deutsch's papers, but there are too many other things I have to do, right now, and I am very rusty in QM as I have been working in classical stat mech for many years.? I'd like to hear from anybody who has tackled them.
Phil
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Received on Sun Sep 30 16:21:14 2007
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