Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does Schrodinger's cat know whether it's dead?

  1. Jul 6, 2010 #1
    I've read several descriptions of the Schrodinger's cat thought experiment, but I've never heard anyone discuss whether the cat qualifies as an observer that can collapse the superposition of states. This seems like too simple a question not to have been raised before, but I've never head anyone raise it.

    If it is, how is the thought experiment valid? If the cat being aware of its own death or continued life can collapse the superposition, then the cat is either alive or dead and not in a superposition of states.

    If it is not, that raises the question of what qualifies as an observer that can collapse the superposition. Do you have to be human to be able to observe a superposition of states such that the superposition will collapse? That seems like an absurdity.
     
  2. jcsd
  3. Jul 6, 2010 #2
    Ill take a crack at this question...

    Schrodinger's cat box......is......a horrible horrible analogy.

    I think that half the reason people cant grasp high level physics is that physicists are terrible at creating analogies. I remember back in circa 1988 I read an old book on "relativity"....I was cross-eyed for a week. Anyways...

    You cant ascribe the "cat" to having "consciousness" or have "observer" properties. In other words.....there is no such thing as the damn cat! What this analogy is attempting to describe is a "unit of space" within the probability wave of an (electron/proton)...for a given "unit of space" here....the (electron/proton) "does" and "does not" exist....it is a "probability"...this (existence/non existence) per "unit space" for the electron dose not have whiskers...does not have a shiny coat..does not have sharp claws...and certainly does not have consciousness...incidentally it cannot be an observer either. Does that help?
     
  4. Jul 6, 2010 #3
    The cat, as I see it, is a quantum object and not an observer in the experiment.
    One doesn't have to be a 'living' being to be an observer either. ;)
    The cat could always know whether it's dead by appealing to complimentarity!
     
  5. Jul 7, 2010 #4
    The answer for this question depends strongly on interpretation of QM. In fact, this is the very essence of "interpretation" - how microscopic QM laws interact with macroscopic classical world.
     
  6. Jul 7, 2010 #5
    One major issue here is that for any object, macroscopic or otherwise, to be entangled, it has to be cooled close to it's ground state, such that the motion that comes from heat does not decohere the quantum state. In the case of the cat, the very brain functions that we would use to define the consciousness of the cat, is enough to cause decoherence, and one thus needs to cool below the limit where these occur.

    Therefore I think one could never talk about the cat being conscious of it's own superposition, as a lack of consciousness seems to be a prerequisite for any object that is to be a part of a quantum state.
     
  7. Jul 7, 2010 #6
    The first point of reference with Schrödinger’s Cat is to understand that Schrödinger himself did not intend to create a philosophical monster that would leave Stephen Hawking reaching for his firearm. Schrödinger intended his thought experiment to highlight what he perceived as the absurdity of what is known as the Copenhagen Interpretation of quantum theory. But in point of fact, the thought experiment is useful in demonstrating the real point.

    The second point of reference then, is to grasp that a great proportion of what is spoken about Schrödinger’s cat is pretentious nonsense, hence Mr Hawking’s eroding patience. A significant further proportion of the philosophising is perfectly earnest, but just as misguided. And the key thing is to realise that all of this is just philosophising, it is not science.

    I am reading a book at the moment called ‘Quantum, A Guide for the Perplexed’, by Jim Al Khalili, a man who is beginning to emerge as one of the key conduits between cutting edge physical science and genuinely interested lay men and women such as myself. The book as a whole is a wonderful overview of exactly what is the source of some of the more challenging theories, such as the ’many worlds theory’. But he also includes an excellent demystification of Schrödinger’s cat. At risk of breaching copyright, I will mention that he suggests replacing the cat with a human being, and replacing the poisonous gas with some anaesthetic. The point not being sensibilities about killing a human being, the point being that the idea of a superposition of dead and alive is replaced by a superposition of being conscious and not conscious, and the key new element being, after the experiment is over, you can ask the experimental subject what happened. The point is, a bit like length contraction and time dilation being only relative to the stationary observer, the superposition only exists for the observer outside the box. As Al Khalili says, if we dig up some ancient rock and find a fault line caused by the release of an alpha particle when a radioactive nucleus decayed, the idea that that fault has existed in a superposition of having been there and not having been there until we dug it up and observed it is easy to dismiss. Clearly, at some point in geological history, the nucleus decayed, the alpha particle cut its fault trace, and it lay there unobserved until we dug it up.
     
  8. Jul 7, 2010 #7

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    "The cat is alive" is a random true/false variable with some probability of being true and some probability of being false. (we don't have enough control of the environment to look at the experiment in another fashion)

    We know the following true/false variables are true with near certainty:
    • If the cat is alive, it knows its alive.
    • If the cat is dead, it doesn't know it's dead.
    (given, of course, the presumption that cats are self-aware, etc)

    If the cat is in a superposition, that doesn't change anything.

    • If we opened the box to see a live cat, then all future experiments we do will be consistent with that observation.
    • If we opened the box to see a live cat, then all future experiments we do will be consistent with that observation.

    These are also random variables that are true with near certainty. If the cat is in a superposition, that doesn't change anything.


    It is an impossibility to observe multiple "branches" of a superposition. The experiments we do to measure superposition is not to directly confirm it, but prepare lots of identical experiments, and show the results could only be explained if there was a superposition involved.

    Superpositions only seem weird when you don't realize there's a difference between what we, as hypothetical "godlike" observers with access to all of the mathematical details of a wave-function can see, versus what an observer stuck within the universe described by a wave-function is capable of seeing.

    I.E. the cat is both dead and alive. No observer will see both. There is the hypothetical possibility of an incredibly complex (or unlikely) experiment whose results depend on the superposition -- but such an experiment will never see the cat as being both dead and alive: it will just be studying a different aspect of the problem that cannot be analyzed in terms of a dead branch and a living branch.

    (Of course, in reality, collapse interpretations might turn out to be right and superpositions really don't propagate)
     
  9. Jul 7, 2010 #8
    The double slit experiment done with fullerenes and the quantum eraser give support to the idea that there is no cat before you inquire about it, i.e. open the lid and look.
     
    Last edited: Jul 7, 2010
  10. Jul 7, 2010 #9
    I concur with this answer.
    I was totally baffled by the cat problem for many years until I read about decoherence theory, then quantum theory all made sense to me.

    The point is that if an entangled state of live-cat/dead-cat were ever to form, then on microscopic timescales it would immediately decohere into either a definite macroscopic dead cat or a definite macroscopic live cat. As such the language of macroscopic physics applies to the cat, and we can make an objective statement about the state of the cat.
    It does not depend on subjective observation.

    As such the question as posed does not apply.
     
  11. Jul 7, 2010 #10

    If quantum theory makes sense to you, that's a very sure sign you still have a lot to learn. Or probably think and contemplate. Or a combination of both.




    I'll cut right to the chase on this. There is no coming back to the old concepts of objects with properties in time and space, this is certain. We are in the mids of a paradigm shift and although it's not exactly clear how everything should fit in a coherent picture, it's clear how everything will NOT.

    This common-sense view of reality that made sense to you is a total misconception.

    Any time you use a concept that involves or implies the term "superposition", you aren't talking of objects with properties in time and space(which happen to be the picture of reality that you appear to have made sense of).

    A system that goes into a superposition, or that can be put in superposition, or that might EVER potentially be in superpositional states, is NOT something to be made sense of. The human mind DOESN'T and CANNOT comprehend de-localized, physical objects with indefinite properties. Anything that can be in superposition is not an object, it's an event, and there isn't anything in this 'universe' that cannot potentially be in superpositional state.

    If you still think qt makes sense to you, you need to go back and re-think "the physical matter in superposition" issue. Today, tomorrow or in a month, when you have figured out what is really happeneing, you'll surely go down the drain into a blind alley from which nobody has escaped, as Feynman liked to say.

    Realism, in the physics sense, is dead and so is the Newtonian universe of our perception. It's not to be made sense of, unless you are ready to believe the unbelievable.
     
    Last edited: Jul 7, 2010
  12. Jul 7, 2010 #11
    The cat is either dead or asleep.
     
  13. Jul 8, 2010 #12


    At the microscopic level I agree with you - quantum physics has revealed a form of reality totally remote from our intuitions. OK - I dont really 'understand' this - but I get how the maths works.

    But we were talking about a macroscopic object (the cat). A necessary requirement of any interpretation of quantum mechanics is that it MUST recreate the "common-sense view of reality" at the macroscopic level with "objects with properties in time and space".

    Decoherence theory seems to provide this missing link making quauntum theory more understandable. Feynman, for all his genius, was as far as I know unaware of this theory.
     
  14. Jul 8, 2010 #13

    OK, but none of the interpretations do. We are used to thinking in terms of physical structues in space and time, whereas the whole notion of "physical structures" goes to trash assoon as you introduce superpositions. There are very obvious conceptual problems with the notions of space and time as well, and it will take nothing short of demolishing and re-bulding the entire 'universe' anew with wholly different basic constituents to restore the newtonian universe of perception.





    Only slightly more 'understandable', yes. Take a closer look and the common-sense reality is still not there, it's as far away as it can ever be. As i said earlier, "objects" that can be put in superpositions cannot be regarded as objects. They are registered events(detections).
     
    Last edited: Jul 8, 2010
  15. Jul 8, 2010 #14
    I've not read decoherence. In my short conversations with a physicist I asked what occurred during reduction to what seems to be discrete-state during measurement. He stated that what's occurring is the wavefunction is simply bunching up to a pointer like state during thermal interaction.

    Doesn't this mean that there still isn't this nice, understandable point particle universe even at the large scale? And doesn't this fact actually support Georg's view of the conceptual confusion that should be here? Aren't there still large scale superpositions with decoherence, just not in the same way (at all) as a molecule may be superposed in near absolute zero temperature?
     
    Last edited: Jul 8, 2010
  16. Jul 8, 2010 #15
    Wow, you're such a drama queen. It means rebuilding the theory to include quantum weirdness and our everyday experience. That doesn't mean our everyday experience isn't real, or the way we describe it is no longer valid. This is what you call 'throwing out the baby with the bathwater'. A paradigm shift doesn't mean starting from scratch. It means... shifting to a different framework... changing some assumptions.

    Gravity doesn't stop working just because we don't have a TOE.

    QM can give us insights, but the newtonian universe we experience isn't going anywhere, its just not the complete picture. In fact, what we experience is just as essential to understanding the complete picture.

    And, Schrodinger's cat is an analogy, which means it can be useful, but is not definitive.
     
  17. Jul 8, 2010 #16


    If that theory is going to include physical structures with definite properties in space and time, that theory is going to be WRONG, as it won't be confirmed in experiments. As far as i am aware, there is no such theory in development, and there is no hint of motivation to pursue something that appears so patently absurd, given the experimental verification of qm and gr.

    Your common-sense is generally the least reliable tool to understand reality, it's on par with the biblical account of creation, if not much worse(given that there are "common-sense" theories that the Earth is flat, or that the Sun orbits the Earth.)


    True, it isn't going anywhere, but the newtonain picture of the universe is actually SPECTACULARLY WRONG, as long as we are talking about the God's veiw of the universe. Actually, the single most expensive and elaborate piece of equipment on Earth - the LHC - works because relativity is incorporated into it(very high speed and very high energies is not where relativity is simply visible, it's where relativistic effects are PREDOMINENT).





    You have not understood the cat analogy, and if you think i am a drama queen, you've hardly understood anything. The upcoming paradigm shift is not going to be about minor adjustments to current theories. But if you wish to believe you live in a newtonian universe, so be it, there are people who believe in the flat Earth.

    BTW, everybody is confused. Think about it.




    When you discard the realism assumption, it means starting from scratch. One of the hardest problems before string theorists(according to Greene and others) is to make ST background-independent and to account for the emergence of spacetime from ???.




    Would you want to "rebuild the theory" to also include yet another idea from our everyday experience - that the Earth is flat?

    You can try here and browse the experimental evidence along with their Wiki and their newly opened flat-Earth shop:

    http://theflatearthsociety.org/cms/


    The 'drama part' or what might seem like strong wording is supposed to challenge those who believe in realism and objects with properties in space and time. I am genuinely curious what others have to say in defence of such a position.
     
    Last edited: Jul 8, 2010
  18. Jul 8, 2010 #17
    You seem to have grasped exacly the same issues out of decoherence as I have (despite claiming to not have read it!)

    So yes, I think you, I and Georg can agree that decoherence makes things slightly better, but is still baffling.

    This is as far as I got with it before I gave up:
    - Decoherence does still leave superpositions at the macro-scale.
    - The good news is though that you dont get the nasty coupling terms which makes quantum theory so wierd (I rely on you understanding what i'm talking about here)
    - What this does is make things look a bit more like standard statistical physics (there is a 50% chance the cat is alive, 50% chance the cat is dead -but then that is no different from any statistical physics when the inputs are unknown)
    BUT
    - we still need to try and make sense of where the macro-state which didn't happen went (ok so we later learn the cat is actually dead, so what of the live cat).
    -it is so tempting at this point (and this is actually what I do to stay sane) to invoke the multi-universe interpretation : so I really believe another universe exists somewhere with a live cat (and then another me observing that and so on).
    BUT
    -I know I shouldn't : talk of such multiverses is metaphysical nonsense: how could I ever verify this. Popper would turn in his grave.

    So that I where I leave it for other to enlighten someday.
     
  19. Jul 8, 2010 #18
    You're missing the point. Newtonian mechanics describes the world enough for most people. Its not a common sense argument, its a utility argument.
    Since when are we talking about god?? Or do you have a QM interpretation for gods?
    isn't useful if my car runs out of gas.
    Actually, it slopes down towards a lake where I live.
    Except you of course, you KNOW I'm wrong. Not that you have understood even a bit of what I said. You're too focused on being right.
    That's crap.

    http://en.wikipedia.org/wiki/Naive_realism

    You're just an obsessive reductionist.
    Space and time are descriptive of observation, which means they are real enough. The fact observations on the quantum level require different descriptions should neither be surprising or problematic.

    Even something as basic as atomic theory shows that everyday objects are not as they appear.... that is... they are not 'solid' in the way people used to think of them. That doesn't mean they aren't solid in some fashion.
     
  20. Jul 9, 2010 #19

    Yes, exactly. I don't deny that they seem solid in some fashion, it would have been ludicrious if i had. My point was about realism as defined in physics - that objects have definite properties at ALL times(the elements of reality as per the EPR argument). On the one hand we have much experimental evidence and a mathematical theorem that this is not the case(the cat is not always there), on the other hand we have common-sense and everyday experience that contradict this. It seems one has to leave one's sanity at the door, or make many new assumptions(though now is the least appropriate time for more controversial assumptions) - like the many world hypothesis, the ftl pilot-wave hypothesis(relativity theory wrong/incomplete), etc.

    BTW, the God's view of the universe is the same as saying 'reality', as the term 'universe' is becoming increasingly misleading. Gotta love how there is always talk of 'reality' in the quantum physics forum and talk of 'universe' in the cosmology forum.
     
    Last edited: Jul 9, 2010
  21. Jul 9, 2010 #20
    I think you are defining realism very narrowly.
    Its philosophical history is much more complex.
    Which means our understanding of QM and everyday experience, and how they relate is lacking. Not that one obliterates the other.

    When our understanding of matter shifted from particles to wave/particles, it didn't mean that describing matter as particles was no longer useful, it just means the some assumptions changed and our understanding became broader.
    Again, 'reality' is another word loaded with history. Idealism and phenomenology have been around for quite a while. QM is really just part of an ongoing discussion.
    Its a paradigm shift, it happens often when you move from one science.... one scientific framework... to another. Or really any field of study... to another.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Does Schrodinger's cat know whether it's dead?
  1. Schrodinger's cat (Replies: 5)

  2. Schrodinger's Sno-Cat (Replies: 0)

Loading...