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Quantum Cat question

  1. Feb 15, 2015 #1
    OK here's a hypothetical situation in order to tease out a query that is bugging me...

    We have a cat, this cat has a bomb in it that is wired to it's brain and when brain death is detected, the cat explodes (you probably see already where I'm going with this). Unknown to the experimentalists, this cat has been substituted for the cat they were going to use in the thought experiment they were setting up with the box, the cyanide and the radio active atom plus detector and switch.

    In the room are only the scientists who do not know the cat has been switched, outside the room in a sound proof cubicle are some other scientists who do know the cat has been switched.

    Obviously if the cat is dead there's going to be exploded cat and no one is going to have to actually peer into the box to collapse the wave.

    What is the situation here, with regards to the superposition of the dead and alive cat, where the box has not exploded and those in the room don't know about the bomb, and how does it change if someone who does know walks into the room.
     
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  3. Feb 15, 2015 #2

    Nugatory

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    You have been victimized by one of the more common misstatements in popularizations of quantum mechanics.

    The cat is never in a superposition of dead and alive. It is always either dead or alive, and which it is has nothing to do with whether or not a conscious observer has made an observation. When Schrodinger first proposed his cat experiment involving the cat and the cyanide and the radioactive decay, he was not suggesting that the cat might be in a superposition. Instead, he was pointing out a problem: Although there was never any doubt that the cat would always be either dead or alive (just as a tossed coin is either heads or tails even if we don't look at it), in the then-current (75-odd years ago) understanding of QM there was no reason why it had to turn out that way.

    The explanation only became clear some decades later with the discovery of quantum decoherence (google for that term, but be warned that the math is a bit daunting - you may also want to try a layman-friendly book entitled "Where does the weirdness go?"). Unfortunately, by then the notion of the cat with an uncollapsed wave function, both dead and alive at the same time, had taken root in the broader culture.
     
  4. Feb 15, 2015 #3

    bhobba

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    Just to expand on Nugatory's excellent answer the reason conciousness came into this thing is an early analysis of the measurement process done by Von-Neumann in his classic Mathematical Foundations of Quantum Mechanics. QM is basically a theory about observations that appear in a common-sense classical world. What Von-Neumann showed is the quantum classical cut can be placed pretty much anywhere. With no place better than any other he decided to place it at the only place that was different - human conciousness.

    While this an issue in principle, in practice it never is eg in the Schroedinger's cat experiment the obvious place the observation occurs is at the particle detector - everything is classical from that point on and the cat is dead or alive.

    Still we have the in principle issue and further research was required to resolve it. The resolution is in decoherence:
    http://www.ipod.org.uk/reality/reality_decoherence.asp [Broken]

    These days the cut is usually assumed to be just after decoherence. This also allows a fully quantum definition of what a observation is - it's when decoherence happens. The reason for conciousness introduction was no longer valid and it's now a very backwater idea.

    Thanks
    Bill
     
    Last edited by a moderator: May 7, 2017
  5. Feb 16, 2015 #4
    Ok thanks for those excellent replies.

    These days the cut is usually assumed to be just after decoherence.

    I presume this is referring to Heisenberg's 'schnitt'

    I have been listening to Amir Aczel's 'Entanglement', Manjit Kumar's 'Quantum' and Louisa Gilder's 'Age of Entanglement' and I see that I may be stuck in that era. I will most certainly now focus on this 'decoherence' business. Thanks for the heads up.

    PS, that ipod.org site looks like I've struck gold.
     
  6. Feb 16, 2015 #5

    bhobba

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    Its also known as the Von-Neumann cut. I suppose its who you were most influenced by determines what you call it. My background is math so its Von-Neumann in my case.

    It doesn't resolve the measurement problem which is surprisingly subtle and has a number of parts. Some parts, such as what is called the preferred basis problem it resolves. Others, such as the problem of outcomes (it the main one - colloquially called why do we get any outcomes at all - technically how does an improper mixed state become a proper one) it leaves untouched.

    If you want to pursue it further THE textbook is the following:
    https://www.amazon.com/Decoherence-Classical-Transition-Frontiers-Collection/dp/3540357734

    In particular see section 2.5.

    A freely available cut down version is the following:
    http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

    Thanks
    Bill
     
  7. Feb 16, 2015 #6

    RUTA

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    You have to be careful when talking about the cat in a superposition state. If it's truly in a superposition of dead and alive, then it can't be interacting with the environment, i.e., the floor, the box, the air in the box, etc. That's where decoherence comes in, as mentioned above. However, decoherence doesn't solve the measurement problem, which is the non-unitary evolution from the superposition state to a definite outcome. You have to add something to decoherence to make that happen. http://arxiv.org/pdf/quant-ph/0112095v3.pdf
     
  8. Feb 21, 2015 #7
    RUTA, you can't kill a cat while avoiding decoherence. The air etc. is not relevant.
     
  9. Feb 21, 2015 #8

    RUTA

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    If you assume the cat is in a superposition state of live and dead, then you've "killed it" and "not killed it" before decoherence. I think it was in this experiment that air-caused decoherence was an issue for example, Gerlich, S., et al.: Quantum interference of large organic molecules. Nature Communications 2, 263 doi: 10.1038/ncomms1263 (2011).
     
  10. Feb 21, 2015 #9
    The "live" and "dead" states are by definition not in coherence.
     
  11. Feb 21, 2015 #10

    RUTA

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    The assumption is an entangled state of live and dead, as I understood the OP. If it's merely a matter of not having checked for evidence, then it's an epistemic problem, not a quantum problem.
     
  12. Feb 21, 2015 #11
    i always say this when i come across this topic....the cat is dead when the experiment takes longer than say 8 minutes.
     
  13. Feb 21, 2015 #12
    RUTA, my point is that such a state is a contradiction in terms, except in Collapse models, where superposition can be meaningful post-decoherence.
     
  14. Feb 21, 2015 #13

    RUTA

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    But, if there is truly no way to know whether the cat is alive or dead, e.g., no evidence exists because the cat is not interacting with anything -- air, floor, box, etc, then quantum mechanics says it's neither alive nor dead. Keep in mind this is not a claim about the knowledge of observers, this is a claim about interactions of the cat with its environment that could be used to ascertain its true state, regardless of whether or not anyone chooses to do so.
     
  15. Feb 21, 2015 #14

    RUTA

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    So, for example, if you set up twin-slit interference with electrons then scatter photons off the electrons and collect the scattered photons to discern the electrons' paths, the electron interference pattern is destroyed. But, if you hide the results of that photon scattering, say with a lens in front of the photon detector that ruins the which way information, then the electron interference pattern is restored. Having the information available at the photon detector (before you destroy it with a lens) destroys the electron interference pattern whether or not any conscious entity chooses to look at the photon detector.
     
  16. Feb 21, 2015 #15
    That's not correct. Quantum mechanics does not say anything at all about states that cannot be observed. There are only two ways to correctly talk about superposition in physics:
    1. Before decoherence, where the elements of the state can potentially interact and affect future observations.
    2. If you specify that you are working in a psi-ontic model, and that you are interested in the philosophical rather than the physical implications of superposition. This only really works in collapse models, because in hidden variable models there are no superpositions, whereas in MWI the two states are in different "worlds" following decoherence.

    Scattering photons off electrons will almost always cause decoherence and ruin interference patterns, even without a photodetector. "Information available" is not important.
     
  17. Feb 21, 2015 #16
    Actually, there may be an exception: I believe that Roger Penrose, in his Objective Reduction scheme, intended superpositions to have physical effects even post- decoherence through some sort of gravitational influence. I can't provide a reference for this and I also don't think a model for it has ever been developed.
     
  18. Feb 21, 2015 #17

    RUTA

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    See page 95 of this Sci Am article on quantum eraser for the electron-photon reference http://www.arturekert.org/quantum-eraser.pdf [Broken]
     
    Last edited by a moderator: May 7, 2017
  19. Feb 21, 2015 #18

    RUTA

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    We're not talking about a state that cannot be observed. We're talking about a state prior to "observation" (no consciousness needed -- see quantum eraser article in previous post, all that matters is that the experimental apparatus allows for the information).
     
  20. Feb 21, 2015 #19
    Sorry, badly written article. That's yellow science for you. As for the specific case of scattering off electrons, I don't know, there may be a clever way to do it with one electron and one photon; that's why I wrote "almost always". But if that article is your only source I'd assume it's their mistake.
    The important thing to realize is that decoherence has nothing to do with knowledge, potential or actual. It basically means that the state is too complicated to create interference.
     
  21. Feb 21, 2015 #20
    Just for fun...

    "Seems Schrodinger demonstrated a keen sense of humor picking the cat, anticipating difficulty of experimental preparation using the only creature with nine lives (a hidden variable), the uncertainty principle enjoining certain knowledge which of its lives the cat would be living during the experiment, thus confounding the "catlapse of the wave function", and therefore with great foresight anticipating the "Many Cats" interpretation of QM, long before Everett, Dewitt, et al... "

    manycats-jpg.57519.jpg
     
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