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Schrodinger's Cat and looking in the box.

  1. Jul 9, 2013 #1
    My knowledge of Quantum Mechanics is not nearly as complete as may be needed to address this problem but I am still curious as to what would happen.

    As I understand it if we go along with the Schrodinger's Cat thought experiment we can say that the cat is in a state of superposition. And that the cat is both alive and dead. That is until someone looks in the box and observes inside. At that point the wave function breaks down and we observe the cat as either alive or dead. I have then heard if there is no one else in the room to observe other than the single initial observer. Then to someone on the outside of the room with the initial observer and the thought experiment then the entire room is considered to be in a state of superposition until someone looks in to observe.

    Anyway my question is if there is nobody but the initial observer and the unobserved thought experiment in a completely isolated and sealed room (No one can observe in upon the room) and the initial observer looks in the box and observes the cat as alive or dead then reseals the box and by some arbitrary method entirely forgets what they have observed, is the cat in the box put back into a state of superposition where it is both alive and dead? Or does it remain in the state that the initial observer observed it in before forgetting?

  2. jcsd
  3. Jul 10, 2013 #2


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    Staff: Mentor

    Schrodinger proposed this thought experiment to focus attention on a weakness in the way people were thinking about QM early last century. Thus, it's not that interesting to consider the various ridiculous extremes to which this thought experiment can be taken - of course they're ridiculous! The whole point of the exercise is to demonstrate that something wasn't right in the state of QM eighty years ago.

    Instead, we can consider some of the responses to Schrodinger.

    Bohr probably would have said that we can't perform the "perfect forgetting" experiment you've described, so it's meaningless to consider what might happen if we did perform it.

    Decoherence, not known at the time, suggests that the half-dead/half-alive state will very rapidy evolve into a state that's more analogous to a flipped coin before we've looked at - we don't know if it's heads or tails, but there's no doubt that it's one or the other even before we look, and likewise the cat is unambiguously dead or alive.

    And there are a number of other possible explanations for what's going on. They're called "interpretations", and they all have one thing in common: they all predict the same result for the probability that we will find a live cat when we open the box.
  4. Jul 10, 2013 #3
    The actual thought experiment involves a radioactive substance, a Geiger counter, and a poison vial. After a set length of time, the substance has a 50% change of decaying, which trips the Geiger counter, which breaks the poison vial glass, etc. If you open the box to observe whether the cat is alive or dead, you have, according to the model, collapsed the wave function which yields either a dead cat or a living cat. If you then put the cat back in the box, the cat is going to remain as you observed it whether you forget about it or not since you have already collapsed the superposed wave function for that scenario.

    In order to get the state of the cat back to a life/death superposition, you would need to restage the scenario with the apparatus (Geiger counter and poison vial, etc.) If you did that, then you could once again say the cat was in a superposition between life and death.

    Edit: Oh, and the above argument only holds if the cat is found alive after the first "go-around." If the cat is found dead and you put the dead cat back into the box and forget about it. Guess what, the cat's still going to be dead next time you look in there, whether you restage the scenario/apparatus or not. We're not talking pet sematary here :eek:
    Last edited: Jul 10, 2013
  5. Jul 10, 2013 #4
    Nugatory: Saying it's not that interesting seems a bit subjective. But interesting reply though.

    DiracPool: How is it you can know that what would occur in the situation I described would behave so deterministically? From the point of view of anyone in the universe it would be thought that the box had never been opened since nobody would know that it had been opened. Thus from the point of view from anybody who had not viewed it it would still be in a state of superposition. The main question is would it re-enter a state of superposition for the person who had viewed it and then completely forgot it.

    From what I've been told even if someone were to observe the cat and see it as either alive or dead, if they were say the only person in the building, then to an outside observer of the building the system would still be in a state of superposition. So if you could cause the initial and only observer to forget the observation then there is no possible way to discern whether or not an observation had ever occurred. Thus could the cat still be considered to be in a state of superposition?
  6. Jul 10, 2013 #5
    First of all, I didn't say that the whole Shrodinger's cat thing isn't absurd on its face, I was just telling you what the traditional model was.

    I already answered "the main question" in my last post, please re-read. The short answer is no, unless you restage the scenario.

    Don't get me wrong, though, I do understand where you are coming from, and it is a legitimate query. What may ease your dis-ease is the more modern "interpretation," as Nugatory puts it, of the Shrodinger cat experiment (is it officially called a paradox? I'll just use "experiment" here). The traditional notion behind the experiment is that it is the conscious act of a human looking at the cat which collapses the wave function. I think this is what your conception of it is, and that's why you are, understandably, confounded by what happens in all these special conditions.

    However, the modern interpretation of the experiment is that it the collapse of the wave function is accomplished by some kind of "measurement" upon the system. That measurement could be some human looking at the cat, or it could be something as faint as a molecule of air bumping into the cat. Once that occurs, the superposition has been "measured," the wave function is collapsed, and the cat becomes dead or alive whether or not the guy standing outside the box witnesses it or not. Does that make more sense? So, if this is the case, then it doesn't matter whether the guy outside the box forgets it or not, the cat has already been measured and is either alive or dead. In like fashion, all the observations of all the people outside the hermetically sealed building make no difference either for the same reason.
  7. Jul 10, 2013 #6
    To simplify, let's assume the cat is going to be brought from his lab cage and humanely sedated before the experiment, so the cat's awareness of his surroundings during his time in the box is out of the picture. He will still either live or die, just won't know about it...

    There are two things here that seem to be going on:

    1] Measurements don't exist independently apart from a context that attaches logical meaning to the operation of the measurement. This context is chosen and defined by the experimental design, and is why the death of the cat (the result of an active purposeful designed experimental situation and process with mechanical results that can kill the cat) is a measurement... and why the continuing life of the cat is also a measurement.

    But, say the lab tech was sent to the animal lab to retrieve the cat before the experiment and found the cat already dead in his cage... was something measured? If he found the cat to be alive, would that be a measurement?

    2] The other thing is at what level does the assumption of contrafactual definiteness become met? This is the "cat has to be either live or dead at all times including before the box is opened"...

    But which either / or measures apply to the lab tech on his way to retrieve the cat before the experiment?

    Maybe the cat died before the planned start of the experiment. (cat is alive or dead?)

    Maybe the animal lab manager's girlfriend took the cat home. (cat is present or gone?)

    Maybe the cage positions in the rack got mixed up during cleaning and the cat originally selected for the experiment is still in the animal lab and the wrong cat was retrieved to be used in the experiment. (original cat or different cat?)

    Each of those is a potential context within which to derive meaning for some kind of measurement... I know I'm stretching the "boundaries" of the closed system of the formal experiment, but that is sort of the point... the boundaries of the closed system define the meaning of the measures; which ones are considered, and which potential ones are excluded.
  8. Jul 10, 2013 #7
    Just to make it clear the cat never goes into a living/dead superposition state. The evidence to back this up is much broader than any evidence which comes from QM. This often neglected evidence can be classified under the titles general knowledge and common sense.
  9. Jul 10, 2013 #8


    Staff: Mentor

    Not this one again.

    The solution to Schrodinger's Cat is trivial, utterly trivial. The weirdness is at the particle detector - everything from that point on is classical - the cat is never in any kind of superposition - it is alive or dead - that's it.

    It was never a problem for Copenhagen because Copenhagen divided the world into quantum and classical. It assumed knowledge of the quantum world occurs when it makes its appearance in the classical - which it does at the particle detector.

    What it does however is focus on the division of the world into classical and quantum in Copenhagen. This is obviously a blemish - the world is entirely quantum. What it pointed to was the need for an entirely quantum theory of measurement without this division.

    Since then this has largely been accomplished with the fleshing out of the phenomena of Decoherence but a few issues do remain. It is generally thought (not by all, but generally) it is crossing the t's and dotting i's sort of stuff. Then again that was thought of classical physics at the end of the 19th century and we know how that turned out - so one never knows.

  10. Jul 10, 2013 #9
    We need this one, though, bhobba. It keeps the kids coming back and maintains there interest in science :smile:
  11. Jul 10, 2013 #10
    I always thought that Schrodinger's cat experiment is a way of saying that its pointless to talk about something that will never interact with us ?
  12. Jul 10, 2013 #11
    DiracPool: You are partly correct and partly incorrect I think. The collapse of the wave function (if that is what happens some different interpretations would disagree) is not caused by some random air molecule bumping into it. I know this was but a part of a statement regarding a range of things that could collapse the wave function but I thought I should point that out. It is considered to be a paradox I believe. In any case the collapse of the wave function is caused by the Geiger counter. The original experiment assumes that the Geiger counter is not an observer and thus would not collapse the wave function but it has been shown that just the measuring of the system causes the collapse of the wave function. Thus until the Geiger counter gets a measurement the system is in a state of superposition. Though I could see how it could be argued that because the Geiger counter is the mechanism by which the cat would be killed that if the Geiger counter had not observed the cat, the cat would only be capable of being alive. Unless it for some reason died of old age during the middle of the experiment. I don't mean this to point out anything relating to the argument just thought I'd point it out. In post number 9 I believe you mean "their" rather than "there" as you're not referring to any sort of place.

    Secondly, while subtle, I can't help but feel that by saying " It keeps the kids coming back and maintains there interest in science" you are making very subtly either an ad hominem fallacious statement or name-calling. Since you didn't directly call me a kid, rather you subtly implied it, it makes it slightly more ambiguous. Though I am inclined to say you committed an ad hominem. Due to the fact the term kid is generally used to refer to someone of a young age specifically usually a child. Likening my intelligence to that of a child. There is no need for it, especially on a board such as this. Granted it was extremely subtle and thus I can't outright say that's what you meant. So in the event of that's not what you meant, please disregard my prior statement.

    Dadface & bhobba: I believe you may be incorrect. According to the Copenhagen Interpretation of Quantum Mechanics the system is in a state of superposition until an observation takes place. In this case observation would mean the Geiger counter getting a measurement. Though I said something above that I think applies here so I'll quote it. "Though I could see how it could be argued that because the Geiger counter is the mechanism by which the cat would be killed that if the Geiger counter had not observed the cat, the cat would only be capable of being alive. Unless it for some reason died of old age during the middle of the experiment." It is interesting (to me at least), I'll look into it some more.

    Nick666: Kinda, Schrodinger meant the thought experiment to be a Reductio ad absurdum. He tried to point out some of the flaws he felt were in the Copenhagen interpretation of quantum mechanics by pointing out absurdities that he saw in the interpretation at the time.
  13. Jul 10, 2013 #12
    nobody knows.
    maybe rules MWI or maybe rules Collapse Models.
    we have to test Macrorealist Models
    (Quantum Vs Macrorealism)

    http://aspelmeyer.quantum.at/docs/82/downloads/exp.pdf [Broken]
    "This is at the heart of the so-called “quantum measurement problem”, also known as Schrödinger’s cat paradox. Another question is whether quantum superposition states of massive macroscopic objects are consistent with our notion of space-time or whether quantum theory will break down in such situations"

    Last edited by a moderator: May 6, 2017
  14. Jul 10, 2013 #13


    Staff: Mentor

    I can't quite follow your reasoning. Do you believe the particle detector is not an observation? Do you believe an observation requires an observer? If so be aware that was never the intention in Copenhagen where when quantum effects make their appearance here in the classical macro world then that is an observation and obviously that occurs at the particle detector. The reason this observer thing came into it was the early fully quantum analysis of the measurement by Von Neumann where he showed the cut (where an observation occurs) could be put anywhere including the consciousness of an observer, and since this was the only thing different this is what he and Wigner latched onto as the place to put it.

    Since then a lot of work has been done on Decoherence and it is now pretty obvious that is the place to put the cut. When Wigner heard of some early work by Zurek on Decoherence he recognized immediately consciousness was no longer required and abandoned it. Today this is one of the least favored interpretations with very few holding to it. Using that decoherence occurs between the particle detector, environment and particle source so that a particle is either there or not in a time interval prior to actual detection - it is not in a superposition - specifically it is in an improper mixed state. From that point on everything is entirely classical - the cat is not in a superposition - it is alive or dead. Technically the cat and the other paraphernalia is entangled with the source but because they are macro objects have no interference terms.

    BTW an issue does remain - it is decoherence transforms the superposition into an improper mixed state and on that key word here - improper - hinges if decoherence solves the measurement problem or not - but that is another discussion. There are others as well such as the so called factoring problem but the status of that issue remains open right now with more work required.

    Last edited: Jul 10, 2013
  15. Jul 10, 2013 #14
    bhobba: I apologize for being unclear. Though your question is a bit ambiguous regarding what one would consider to be an observer. From what I've been reading the particle detector would be an observer. I did answer whether or not I think the Geiger counter is an observer in my previous post but I didn't outright say it so I can understand the confusion. The Geiger counter is an observer. Or at least that's what part of a wikipedia article said. It cited this as it's source: http://web.archive.org/web/20061130173850/http://www.ensmp.fr/aflb/AFLB-311/aflb311m387.pdf

    As for your second question it depends on what you mean by observer. If by observer you mean anything that measures something in the system be it human, machine, or otherwise then yes. That's what I've been lead to believe. Observation requires an observer.
  16. Jul 10, 2013 #15


    Staff: Mentor

    From the link:
    'Our results are consistent with the idea that a measurement from the Geiger counter is sufficient to collapse the quantum state, most likely because the counter involves amplification processes that are irreversible. Conscious perception of the outcome of a quantum measurement is not a prerequisite for the collapse of a quantum wavefunction.'

    That pretty well sums it up - although I would evoke decoherence. At the particle detector collapse has occurred - everything classical from that point - cat is alive or dead - not in some weird superposition. As I said nearly all the mainstream interpretations Copenhagen, MWI, Consistent History's, Ensemble Interpretation and probably quite a few others as well its utterly trivial. About the only one for which its an issue is the consciousness causes collapse but right from the outset such a wired view of the world has problems with objective reality that this thought experiment readily demonstrates.

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