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Schrodinger's cat questions

  1. Feb 22, 2009 #1
    Hi guys.
    Am a complete newbie to quantum mechanics but would love to find out a bit more about it!
    I have a few questions about the Schrodinger's cat experiment, which are probably very basic but I would greatly appreciate it if someone could just explain them to me!
    Basically, my understanding of the experiment is this:
    While the box is closed, nobody know whether it is alive or dead. Therefore, it is existing in a state of both. This chnges when it is observed by an observer, when it is forced to 'choose' a state.
    Firstly, is my interpretation of this experiment correct?
    Secondly, what/ who is this observer? does it have to be a human who can actually consciously decide whether the cat is alive or dead?
    Or could it be an animal, eg a monkey has been trained to open the box and observe the cat. It would be able to tell whether it was alive or dead, therefore does this force the cat to become one state or the other?
    so basically is it anything with eyes?
    further than that, what if there was a machine which could open the box and tell whether the cat was still alive or not. does this force the cat to be either alive or dead, even though no consciousness is present?

    This is probably a basic problem but I havent been able to find anything about it so thought I'd ask here! thanks in advance
  2. jcsd
  3. Feb 22, 2009 #2


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    Welcome to PhysicsForums, Nick-C!

    It might be easier to think of the example in a little less literal way. Generally, anything that has the effect of getting some information out of the box as to the state of the cat - even if that information is not itself observed - has the effect of collapsing the state of the cat to alive or dead. So a photograph would be enough.

    Now, there are some who do believe that the reality IS dependent on an actual observer. But I think that is more of a philosophical issue than a physical one.
  4. Feb 22, 2009 #3


    The correct answer is that it doesn't make sense to ask about the state of the cat in a closed box. The beauty of the Schroedinger cat paradox is that it goes to the heart of our understanding of what science is. Science is supposed to give us answers about results of observations. If science (physics, in particular) makes a statement, then there should be an experimental way to verify that statement. If the statement cannot be verified, then it doesn't belong to science (it belongs to philosophy, religion, or whatever). Any statement about the state of the cat in a closed box cannot be verified in principle (in order to verify it you need to open the box, then you deal with a different statement altogether). So, such statements are not scientific.

    Quantum mechanics does a good job at predicting what we'll see after opening the box. That's all we can ask from a sound scientific theory.
  5. Feb 22, 2009 #4
    Nick-C, you are asking right questions - how reality can depend of the type of the observer? Should it be conscious, intelligent being? This is why many people, like me, hate the 'standard', Copenhagen Interpretation. There are about 10 different interpretations: http://en.wikipedia.org/wiki/Interpretation_of_quantum_mechanics

    But before you go there, I recommend you an advanced version of the Cats paradox, designed specifically to learn the role of an observer:
  6. Feb 22, 2009 #5
    I agree that it is a mistake to make conclusion "because nobody knows the state of the cat, therefore the cat is in superposition", but I'm afraid that quantum mechanics indeed does predict that the cat will be in superposition of being both alive and dead, anyway.

    I don't agree. It makes lot of sense to try to make sense out of the laws of nature.
  7. Feb 22, 2009 #6
    The answer is simpler than most of physicists think. Schrodinger was wrong assigning to the cat only two and only pure states: |alive> and |dead>. Any cat is evolving rather than staying in a "pure" state |alive>. A cat at the time t is different from the cat at the time t+delta_t. And dieing is similar continuous evolving (although tragical). In other words, a macroscopic system with huge number of degrees of freedom and a lot of energy levels cannot normally be in a pure state but in continuous interaction with the environment (observed). Only at very low temperatures some macroscopic systems manifest quantum (pure) states (superconductivity, superfluidity, etc.). Everything macroscopic is evolving = everything macroscopic is being observed. See, for example, my publication on how classical phenomena are obtained from quantum mechanical ones in "Atom as a dressed nucleus", arxiv:0806.2635, or Central European Journal of Physics, Volume 7, N1, pp. 1-11 by Vladimir Kalitvianski.

    Vladimir Kalitvianski.
  8. Feb 22, 2009 #7
    Quantum mechanics does predict that if we open the box (i.e., perform a measurement) then sometimes we'll find a dead cat and sometimes we'll find an alive cat. QM even can calculate these probabilities accurately. However, I don't think that this is equivalent to saying that the cat in the box IS half-dead and half-alive. (Here we should possibly engage in an "enlightening" discussion of what is the definition of "is".)

    Quantum-mechanical formalism with its Hilbert spaces, state vectors, operators, superpositions, etc. is just a mathematical formalism. All these calculations make a contact with the observable world only at their final point - when probabilities of different events are obtained. These probabilities can be compared with what we actually measure. The mathematical ingredients of QM (Hilbert spaces, state vectors, operators, superpositions,...) are just formal abstract symbols. If you attempt to find their counterparts in the real world, you'll get into a logical contradiction sooner or later. The Schroedinger cat paradox is the best example of such a contradiction.
  9. Feb 22, 2009 #8
    How about a sick cat? Is it alive or dead? The paradox comes from the extreme simplification (abstraction from reality). The cat permanently evolvs (aging for example, eating, pissing, loosing hear). (Kot, da ne tot).

    Again, the paradox comes from the extreme simplification. There are systems that are "permanently observed" but "classically unchanged". Take, for example, the Einstein's Moon. It undergoes permanent changes, its observing is the inclusive quantum mechanical picture, but some think (Einstein is not exclusion) that it is a point-like body and three coordinates suffice to describe it.

  10. Feb 22, 2009 #9
    Sure, we need to learn the laws of nature. However, we shouldn't take them literally, when they concern something we do not actually observe.

    In the case of the Schroedinger cat our observations are limited to two events. First event: we place an alive cat in the box. Second event: after some time we open the box and find the cat either dead or alive. There are no observations at time points between these two events. So, in order to fill the gap we invent a "law of nature". This law tells us that the initial state of the cat is represented by a certain vector in the Hilbert space; that this vector evolves with time through action of an unitary time evolution operator; that at a later time this vector becomes a suporposition of the "dead cat" and "alive cat" vectors; that the probabilities of outcomes are given by squares of the superposition coefficients.

    You would probably agree that the "law of nature" I just described is simply an abstract and formal mathematical procedure. I don't think it is a good idea to say that this is what actually happens with the cat while it is in the box. I think that we will never know what *actually* happens with the cat while it is in the box, and we are not seeing it. I am not worrying about this lack of knowledge, because this "knowledge" is not verifiable by experiments, and, therefore, non-scientific.

    I think that the greatest lesson of quantum mechanics is that we should not ask questions about things that we do not observe. Otherwise we are going to be trapped in endless paradoxes and contradictions.
  11. Feb 22, 2009 #10
    Thanks dmitry I will check them out. Because, regarding Dr chinese's reply earlier (thanks btw!) about how a photograph would collapse the cats state, wouldn't, say, particles inside the box have the same effect? as they are interacting with the cat.
    or are we supposed to ignore that for the purpose of the paradox?
  12. Feb 22, 2009 #11
    These are exactly the paradoxes I was referring to.

    If you want sensible answers you need to ask sensible questions. You need to specify how the system is prepared and what and how is measured. Then quantum mechanics will tell you exactly what to expect (probabilities of measurements).

    If your question is "will the cat's wave function collapse if it is hit by a particle inside a tightly closed box?" then nobody can answer that, just as nobody can answer "how many angels can dance on the head of a pin?"
  13. Feb 22, 2009 #12
    Well I didn't really mean that as a question, more asking would that have the same effect as this hypothetical photograph being taken. If so, then the wave function would collapse almost instantly after the box as closed?
    As you say, perhaps this is not worth worrying about as we havent observed it by that point. It is interesting to think about though.

    As I said earlier, I'm a complete newbie to this subject so let me know if I'm talking rubbish!
  14. Feb 22, 2009 #13
    Don't worry about your (lack of) experience. The questions you are asking are very important. Many people spent their entire scientific lives trying to answer these questions.

    Quantum mechanics is very robust when it comes to predictions of observations. However, it is vague in everything that concerns "the mechanism of measurement". There are more than 10 different "interpretations" of quantum mechanics, which try to answer these questions. You can choose whatever interpretation suits better your philosophy. Similarly, there are several major religions (Christianity, Buddhism, etc.). And you are free to choose the one that you are most comfortable with.

    I have just preached my own "quantum-mechanical religion" (which is somewhat like Atheism). Feel free to disagree.
  15. Feb 22, 2009 #14
    I am sorry, this sounds rather rude. Like I am complaining that your questions don't make sense. This is actually not what I meant. My idea was that nature tells us her secrets only if we ask her proper questions. Nothing personal.
    Last edited: Feb 23, 2009
  16. Feb 23, 2009 #15
    To 'collapse' wavefunction you need to exchange few photons with it.

    Forget about the old 'wavefunction collapse',magicaland instantaneous. It is completely replaced with so called Quantum decoherence: http://en.wikipedia.org/wiki/Quantum_decoherence

    So it appears, after all these years, QM is not only able to predict the probability of finding dead or alie cat, but also explain, why we see only one of them, not a mixture of both. And to explain it we dont need something new, except old good QM.

    Strangely enough, some people are still repeating weird things about the so called 'collapse' as if quantum decoherence had not been discovered. Well, it is relatiely new thing of course, but anyway.
  17. Feb 23, 2009 #16
    "The correct answer is that it doesn't make sense to ask about the state of the cat in a closed box."

    Well I have to agree with the above statement. The problem is it is meaningless to talk about the fate of the cat without doing measurements. Remember that Niels Bohr said that a photon is a wave for some experiments and a particle for others; they both go together. In other words, it depends on the experiment.

    A word of advice; never try to approach a problem in quantum mechanics with a classical physics mindframe.
  18. Feb 23, 2009 #17
    What if I replace a cat with YOU?
  19. Feb 23, 2009 #18
    Ha no worries.
    I didn't realise that there are so many different interpretations of QM that people believe.
    Having a quick look through some of them, the problem that springed to mind is that many of them are impossible to prove, which in my mind makes them slightly pointless? Like the many worlds interpretation, I like the concept and how simple it is, but (as far as I can see) it cannot be proven. So does it actually help?
    So perhaps you are correct in saying that the only thing that really matters is what we can measure, what we can observe. What happens while, in this case, the cat is in the box is secondary.
  20. Feb 23, 2009 #19


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    The various interpretations, so far, have not led to any breakthroughs. If ever any make predictions that can be tested, that will be helpful. In the meantime, most physicists follow the orthodox QM in practice regardless of their personal beliefs.
  21. Feb 23, 2009 #20
    But then you've changed completely the experimental setup: the observer is inside the box, not outside of it. Remember that in order to apply QM you need to specify (i) what is your physical system (ii) how it is prepared (iii) what is observed. QM is not able to provide a full description of the entire universe. It is applicable only to a well-defined experimental setup.
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