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Schrödinger's cat and many world interpretations

  1. Jan 10, 2012 #1
    In the Schrödinger cat's experiment if the observer decide not to interact with cat directly by opening the box but rather by bringing the interaction of Geiger counter and radioactive substance to see if the cat's alive then why the de-coherence will occur?
     
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  3. Jan 10, 2012 #2

    Simon Bridge

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    Because you've made an observation ... the method of the observation does not matter.
    You can get a robot to do it - the sentience of the observer does not matter.

    Note - the Geiger counter won't detect the disintegration that kills the cat - the radiation from that event gets absorbed by the hammer that breaks the vial. But even if you could put the cat on a heart monitor, probe the box with x-rays, anything - it's all observing the cat.
     
  4. Jan 11, 2012 #3
    but in the other case i haven't had a direct interaction with cat's wavefunction
     
  5. Jan 11, 2012 #4

    Simon Bridge

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    Doesn't matter.
    You don't have to have a direct interaction with the cats wave function.

    I think you need to explicitly state what you think decoherence means in this context.
     
  6. Jan 11, 2012 #5

    Ken G

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    To add to that, the reason you don't need to interact with the cat's wavefunction is that the cat doesn't have a wavefunction. Wavefunctions are qualities of entire systems (and that only holds if you break from the Copenhagen interpretation and hold that cats can participate in wavefunctions in the first place, because it is not so easy to demonstrate this)-- so includes anything you could measure about that system. That's the wavefunction you are interacting with. If you project that wavefunction onto the "cat" substate, you don't have a wavefunction any more, you have what is known as a "mixed state", described by a "density matrix." This matrix will reflect various entanglements between the cat and the other things you can measure, and it is through those coherences that your understanding of the cat is affected by your understanding of its environment. Put simply, if you choose to treat the cat as a quantum system, then it is entangled with the things you are measuring, and if you choose to treat it as a classical system, then the issue doesn't even come up (which is the purpose of the Copenhagen interpretation).
     
  7. Jan 11, 2012 #6
    Fundamentally the whole system will be described by Schrodinger equation, which doesn't entail any form of collapse.
     
  8. Jan 11, 2012 #7

    Ken G

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    If that is "fundamentally" true actually depends quite sensitively on your preferred interpretation! For example, Bohr would most likely say that the Schroedinger equation was never meant to apply to the whole system, and there is no experimental evidence that it does. The problem with it is that if you never get collapse, you have a real problem explaining the perceptions of the observer.
     
  9. Jan 11, 2012 #8
    Yes, unless the wave function, as given by the Schrodinger equation, is not everything, as suggested by de Broglie/Bohm.
     
  10. Jan 11, 2012 #9
    Yes indeed, in regards to Bohr's interpretation. GianCarlo Ghirardi does a nice job of offering a thought experiment to differentiate whether collapse occurs at the macroscopic apparatus or not.

    Perhaps when we perceive a definite state, the system is still in superposition? After all, to calculate the correct probability of a future state - if it can come from either state1 or state2 - then you need to calculate it as if it evolved from both state1 and state2 - at least that is Brian Cox's stance on the matter, in his quantum book.
     
  11. Jan 12, 2012 #10

    Ken G

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    Can you summarize or link to it? It sounds interesting.
    I'm not aware of a probability that requires we calculate it as if it evolved from both state1 and state2 in any situation where state1 has been observed. What you refer to sounds more like a two-slit kind of situation?
     
  12. Jan 12, 2012 #11

    Simon Bridge

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    iirc you can get it with particles that can decay into each other cyclically: the observed particle is understood as a composite state so that observation just puts the wavefunction in a particular superposition rather than collapsing to a unique substate.
     
  13. Jan 13, 2012 #12

    Ken G

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    It sounds like you are talking about the interesting case of a neutrino with a measured flavor (say, a muon neutrino or some such thing). As this is not a state of definite energy, it will not be stationary, and will oscillate among the various flavor eigenstates. But that doesn't mean that when we observe a definite flavor state the particle is "still in a superposition"-- at that moment it's in a definite flavor state, not a superposition of flavors. However, it will evolve into a flavor superposition, as time passes. To make future predictions, we only need that it is in state1 now, not a superposition of state1 and state2, even though it will later on be in a superposition of state1 and state2. So I don't think that says much about the MWI approach, where the initial state1 is not really all there is to it-- we don't need any of state2 in that initial state, we only expect the superposition to appear at a later time, and the Copenhagen interpretation would just say it evolves from a state of flavor definiteness to a state of flavor indefiniteness (and back again).
     
  14. Jan 13, 2012 #13

    Simon Bridge

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    I think the first mention I had of it was with K mesons.
    There is a standard treatment of neutral particle oscillations which represents the flavor-state as a composite of virtual states.

    How it evolves from a state of flavor definiteness to a state of flavor indefiniteness (and back again), appears well modelled by evolving the virtual particle states... which would provide an example to your query end of post #10 wouldn't it?

    This is a bit different from preparing a system in a superposition of eigenstates where a measurement collapses the system to one and all subsequent measurements yield the same result.

    I'd like to point out that this sort of argument just keep coming up with respect to Schrodinger's Cat ... it's kind-of the point. I don't think we can come up with the one right path i a forum discussion when so many reams have already been written on the subject. The best we can do is attempt to illustrate the different positions.
     
  15. Jan 13, 2012 #14

    Ken G

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    I don't think it does, but I understand what you are saying. To me, the issue is what has been established by experiment (the term "observed" was used in post #10), versus what remains fundamentally indeterminate. You are invoking a more sophisticated version of this principle, involving virtual states, but I don't necessarily see a fundamental difference there than with something like the simple example of a two-slit experiment that does not determine which slit the particle goes through. Then we can treat "going through slit A" as a type of virtual state as well, and we certainly agree that the result of the experiment will involve a superposition of slit A and slit B virtual states. But those slit states have not been observed, so that's why they can remain in superposition-- in post #10, I was asking about a reference to a situation where we do have an observation, yet we still need to include a superposition that includes elements that are contradictory to that observation (such as people talk about in MWI). Even with virtual states, I don't think that's ever necessary.
    I agree, and that's why I was trying to distinguish between essentially philosophical debates about the meanings of superpositions, versus an actual calculation of a probability of a future event that requires invoking a superposition. The latter is pretty concrete, so if there was actually a calculation that required we include a superposition of state1 and state2 (at some given time) even though we had actually observed state1 (at that time), then we would be forced to adopting an MWI-type view of the situation, it would no longer be an issue of philosophical priorities. But that's just what I don't think has ever been found to be the case.
     
  16. Jan 13, 2012 #15
    Who is watching us watch the cat, again?
     
  17. Jan 13, 2012 #16
    I still didn't see anyone really comment on the Many World Interpretation, so I'm going to give it the layman's try. In Many Worlds, there is no wave or decoherence at the particle level. Instead, all results actually happen. So in this case, there are Universes where the cat lives and Universes where it does not. 'You' have access to all these universes up until the moment that the information about which universe you are in reaches you. That information is bound by the laws of Physics and can not travel faster than the speed of light. David Deutsch describes it as a 'Decoherence Wave', if I remember correctly from his latest book 'The Beginning of Infinity'. Anyway once the information reaches you, there becomes more than one you, each belonging to a different universe. You'll have to read one of the experts to learn how this happens without violating conservation of mass, energy, etc. The way I understood it, all the universes split and merge all the time, but it is a bit confusing.

    Anyway, what gets Duestch all riled up all the time is that no one wants to buy into the Many Worlds Interpretation because (in his opinion anyway) it makes us feel less important if there are near-infinite versions of each of us experiencing all these different outcomes. We all want to feel 'unique' so we dismiss the evidence of MWI. His explanation of this evidence includes things like Quantum Computing calcuations that have already been performed; i.e. we know the calculation requires x number of bits, yet in our universe fewer than x bits exist, so where is the rest of the calculation being performed? His answer: another universe. Same thing with the double-slit experiment. We see evidence of two photons interfering, but since only one exists in our universe where is the other? He hates that we 'made up' a wave to explain the dilemma, when the simpler explanation (the extra photon is in another 'very close' universe) is abandoned because again we don't like the idea that multiple copies of us exist in these Universes. Reading Duestch is fascinating stuff, and the politics of the Physics/Science community is also fun to read about. I get the impression that most other popular scientific authors don't like Deustch because among other things he is constantly telling other authors that they are wrong (in The Beginning of Infinity he hammers some of the theories of Dawkins and Jared Diamond among others...)
     
  18. Jan 13, 2012 #17

    Ken G

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    I admit that there's a certain lure to the idea that photons in other universes are what is interfering there, but here are some of the problems with Deutsch's argument:

    1) He dislikes saying that a "wave" is doing the interfering, rather than copies of the same particle, but this suggests that he feels it is natural for particles to interfere with themselves-- where does he get that? Nothing about particles suggests that they "should" interfere, so it's not like saying "oh, as soon as you recognize there are copies of this particle, it is obvious why they should all interfere." The entire idea that a particle can undergo interference comes from its wave nature, as waves are the way physics treats interference, and classical waves exhibit interference in a perfectly natural and easily mechanistically understandable way (that's what motivates the wave language in the first place). So it seems misplaced to me to want to replace the wave concept with the particle-interfering-with-itself concept, there's just no improvement there and we certainly lose contact with the nice classical analogies.

    2) I don't think anyone actually dislikes MWI because it makes us seem less important in all those universes. Rather, it is the complete absence of empirical evidence of the existence of those other universes! I know a lot of astronomers, and we have all come perfectly well to terms with the idea that we are a very very tiny part of a very very large, possibly even infinite, universe, yet none of them are favorably inclined toward MWI. It has nothing at all to do with our place in the cosmos, it's all about the absence of evidence. Absence of evidence is not evidence of absence, but it is a good reason to reject an interpretation that seems unnecessarily bizarre.
     
    Last edited: Jan 13, 2012
  19. Jan 14, 2012 #18
    If the there is a photon heading towards the slits, but it exists in two universes, what would cause the copy in universe A to interfere with the copy in universe B? I thought these universes cannot communicate?
     
  20. Jan 14, 2012 #19
    What Duestch is saying is not that the particle is interfering with itself, he is saying that there are two particles interfering with each other. From my understanding, it is possible to recreate the results of the double-slit experiment simply by having multiple photon-sources. The photons from the different sources will interere with each other. (At least that is what I gathered from what Deustch said). So his explanation is that there are multiple sources, even when we think there is only 1. The universes CAN communicate as long as local coherence is in place. In his model decoherence is a local phenomenon that 'spreads' at the speed of light (or near) as information is spread. He also postulates that local decoherence can be 'undone' and the universes can be sort of 're-entangled' by loss of information, or additional information that 're-aligns' or 'merges' the universes. I'm sure if he read this he would say I'm not expressing it exactly right, again this is a lay interpretation of his theory.

    In the end, it comes down to the idea that 'knowledge' has physical properties and a physical impact on environment. Also you need to be willing to accept that our consciuosness is a result of a physical paradigm, and thus can be duplicated. Tough stuff to digest. If Deustch weren't such an angry bugger, he wouldn't get ignored by the rest of the popular scientific community. I'm in the middle of Lisa Randall's new book 'Knocking on Heaven's Door', and while many of the concepts she is pushing are very similar to what Deustch presented in 'Beginning of Infinity' there is literally no mention of him or his work, whereas she notes the work of Dawkins and others several times....
     
  21. Jan 14, 2012 #20
    Well, I have no doubt that consciousness is a physical manifestation or 'brain state.'

    It's a pretty simple thing to test: put a human into a medically induced coma, and they are no longer conscious. Return the brain function, and voila, consciousness returns.

    Anyway, clarify this: how are there 'multiple sources' to interfere with one another? If I fire one photon out of a gun at two slits, what mechanism would cause that photon to simultaneously exist in another parallel universe according to the MWI?

    Further, how many other worlds does the photon exist in, and what determines that number? Are there three other copies of that photon in 3 other worlds, 3 trillion, infinite?

    Finally, when we measure where the photon is in our world and collapse it's wave function, what happens to the copies in the other worlds? Our measured photon can no longer interfere with them, so we must be changing their reality as well.
     
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