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Copenhegen interpretation or Many World Interpretation? 
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#1
May2913, 06:26 AM

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Can there be an experiment (even a thought experiment), which could settle the debate CI vs MWI? Now a days many scientists seem to favour MWI. I find it interesting because it perhaps allows time travel (the possibility of an observer going back to his/her own past) while avoiding the grandfather paradox, since, having visited your past, you can come back to a "different present" in a parallel universe. This is what I read in one of the popular science books.



#2
May2913, 06:50 PM

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PF Gold
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No, there's no such experiment.
We would need a quantum theory of gravity before we can even try to make an argument for that time travel thing. So that popular science book is doing some wild speculation. 


#3
Jun213, 03:22 PM

P: 34

In principle... probably, yes. Every interpretation of a model has implications along with it, and it is just a matter of time before someone clever enough comes along and sees a way to measure the reality of one set of implications verse another.
I am not that someone, however. All I can add is we have no mathematical understanding of the collapse of a probability into a specific outcome, which means if our quantum model reflects "actual reality", then it follows that either our model is incomplete, or else the other information produced by our model (the other outcomes and their probabilities) should also be realized somehow. The amazing accuracy of quantum predictions is a fair vote in the "actual reality" column, however the failure to accomodate gravity is a contradictory vote in the "incomplete model" column. That is more or less the juncture on which this debate stands. 


#4
Jun213, 03:59 PM

P: 718

Copenhegen interpretation or Many World Interpretation?



#5
Jun213, 05:29 PM

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That's how I interpret Bohr anyway. I know that there are lots of people who interpret his statements as saying that the laws of nature are such that measuring devices are fundamentally classical. But I find it very unlikely that Bohr believed anything that silly. 


#6
Jun213, 05:53 PM

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#7
Jun213, 06:14 PM

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Now that we know about decoherence, there's no reason to think that there's such a thing as a nonunitary collapse. The decoherence approach is to apply QM to a larger system that includes an environment. The result is an apparent collapse. Interactions with the environment put the system in a state that's practically indistinguishable from a collapsed state. This doesn't contradict Copenhagen, because we're just using the same theory (and if we want to, the same interpretation) to a larger system. 


#8
Jun213, 06:45 PM

P: 718

Fine, we won't say the Cword since it bothers you. We'll say that the answer to, "Can Copenhagen and Many Worlds be empirically distinguished?" is "no" on the grounds that "Copenhagen" is not a welldefined model of QM, being subject to disagreement on its definition. Instead, we'll ask, "Can Many Worlds be empirically distinguished from other welldefined models of QM?" The answer to that is yes. MW is empirically distinguishable from (and has been experimentally preferred over some) variants of QM of involving wavefunction collapse. Whether it can be empirically distinguished from other noncollapse variants is not yet clear. If you think this view is compatible with Copenhagen, then I'll reiterate that I think your use of the word is very nonstandard. However, the point remains: if we put aside "Copenhagen interpretation" as a notveryhelpful term, then various purported models of quantum mechanics—models that are usually, though perhaps inaccurately, called "interpretations"—may still be experimentally distinguished from one another. Many Worlds—which you are apparently already on board with, even if you haven't realized it—can be distinguished from other mainstream models. 


#9
Jun213, 06:46 PM

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a) does explain why we observe a classical state of the cat, but b) it does not tell us which state (either "dead" or "alive") will be observed. So  even taking decoherence into account  in some sense there is still a collapse of the density matrix "dead or alive" to one pure state, e.g. "dead". 


#10
Jun213, 06:54 PM

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#11
Jun213, 07:06 PM

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#12
Jun213, 07:19 PM

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#13
Jun213, 08:24 PM

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PF Gold
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There seems to be at least as many different Copenhagen interpretations as people who use that term, probably there are more. For example, in two classic articles on the foundations of quantum mechanics, Ballentine (1970) and Stapp (1972) give diametrically opposite definitions of “Copenhagen.”Source: http://arxiv.org/abs/quantph/9910078 


#14
Jun213, 08:25 PM

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#15
Jun213, 08:30 PM

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#16
Jun213, 08:34 PM

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#17
Jun213, 08:38 PM

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P: 2,150

You can certainly make a theory of wave function collapse so that it's interpreted as a literal event, but I don't think Copenhagen does that. Actually, it seems to me that the intuitive idea of a wave function collapsing as a physical event has serious difficulties if you have more than one particle. With more than one particle, the wave function is a function on configuration space, rather than physical space, and so it's not possible in any straightforward way to interpret "collapse" as some rapid or instantaneous change of a field. 


#18
Jun213, 08:39 PM

P: 718

Perhaps it is possible to work out a less extreme experimental difference between MWI and your Copenhagenish interpretations, but the existence of one in principle is sufficient to answer the OP's question in the affirmative. In the meantime, I will amuse myself by imagining physicists being hurled at high velocity at a double slit in the hopes of observing an interference pattern 


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