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

Schrödinger's cat

  1. Dec 2, 2009 #1
    This may have been posted or questioned before but I have yet to see it anywhere. I must also warn you that I am not a physicist... I just find the subject interesting.

    The Schrödinger's cat experiment has never felt right to me not because I had trouble grasping the concept of something existing in two states until observed it was something else that I couldn't quite put my finger on until recently.

    It hit me a couple nights ago that the cat is its own observer therefore preventing itself from ever existing in both the dead and alive state at once. This sounds to simple to be true but I cannot see another way around the fact that the cat knows its alive based on its senses and if it doesn't then its dead.

    It might be argued that the cat is incapable of knowing whether its dead or alive but it doesn't need to comprehend its state. The cat may not see or hear anything but its aware of its own weight on its paws its aware of the temperature in the box etc etc

    I would appreciate any input on this and I will be posting this on other forums as well to see what others think.

    - Jeffrey Rodriguez
  2. jcsd
  3. Dec 2, 2009 #2
  4. Dec 2, 2009 #3
    From the Wikipedia Schrödinger's cat article:
  5. Dec 2, 2009 #4
    Last edited by a moderator: Dec 2, 2009
  6. Dec 2, 2009 #5


    User Avatar
    Science Advisor
    Gold Member

    I don't know if this makes it more clear or not:

    When a particle is in a superposition, that is fundamentally different from saying it is one state or another. If you *DIRECTLY* observe the property of "is cat alive" you will always get the answer: it is alive OR it is dead. But if you *INDIRECTLY* observe that same property (using inference about its state without actually observing it), you will always get the answer that it is HALF ALIVE. (You could also say that it is both DEAD and ALIVE.) Obviously, that is a completely different answer. That is because they are completely different states. The first is an eigenstate and the second is not.

    If you have a hard time seeing this difference, consider the two slit experiement. Does the particle go through one slit or the other? If you DON'T see interference, then you know the particle was NOT in a superposition when it went through the slits (we can know which slit). If you DO see interference, then you know the particle WAS in a superposition when it went through the slits (and we don't know which slit). Same with the cat.
  7. Dec 2, 2009 #6
    This is what I have a problem with... you state that if you directly observe the state you get the answer but indirectly observing causing it to have the answer of "HALF ALIVE". The problem I have is that it is already being "*DIRECTLY*" observed by the cat itself... kind of like the two slit experiment being measured preventing the particle from being in the superposition forcing it to go through one slit or the other not both.

    Once again I am not a physicist :)
  8. Dec 2, 2009 #7


    User Avatar
    Science Advisor
    Gold Member

    It doesn't act the same. That is the issue. If it did, we wouldn't need to express the state differently.

    Now, you must recall that the cat analogy is just an analogy. No one is actually trying to say that an entire cat is half alive. But if we talk about a quantum particle, then it really is "half alive" (i.e. in a quantum superposition) and it cannot observe itself.
  9. Dec 2, 2009 #8
    For those who believe in Copenhagen Interpretation, "wavefunction is not an objective reality, but just observer's knowledge of a system", so different observers (experimenter and a cat) should not always agree on if wavefunction collapsed or not.

    But you should not focus on the collapse theories, I recommend checking non-collapse interpretations, where this problem is solved in much more elegant way.
  10. Dec 2, 2009 #9


    User Avatar
    Gold Member

    The problem with any interpretation that requires an 'observer' is that you have to define what an observer is. Why would some physical entities be considered observers and not others? Humans, cats, worms, bacterias, computers, measuring equipments? How do you tell the difference? You'd have to introduce some notion of 'consciousness' which, in my opinion, has no place in physics.

    There has to be an objective and rational explanation to explain the decoherence in a way that doesn't require 'conscious observers' and prevents the cat from being in a superposition of states.
  11. Dec 2, 2009 #10
    Cat can be quite big, like billions of electrons in a superconductive ring - in superposition.
  12. Dec 2, 2009 #11
    Yes, and there is such explanation, called 'Quantum Decoherence' :)
  13. Dec 2, 2009 #12
    If you take the cat out of the equation then it makes perfect sense.

    As far as I am concerned the cat is an observer because the thing to be observed is it being alive or dead and what better to observe that then the cat itself....

    Edit: So maybe what qualifies something as an observer is what is to be observed...
  14. Dec 2, 2009 #13


    User Avatar
    Science Advisor
    Gold Member

    That is not accurate, in my opinion. It is in a superposition, and that is different that an "unknown but definite" state (which is what you get when you perform an observation but don't look at the results). If you have the possibility of knowing the results, there is collapse.

    Note: I agree there are some interpretational issues with Copenhagen and collapse (it is sometimes called the measurement problem). However, it does not make any incorrect predictions. MWI was created in part to resolve the issue that different observers might see collapse differently, and many consider this a good solution to the measurement problem. Of course, you end up trading one thing for another in each interpretation. The key is that a superposition acts differently when you put all the facts together. That won't be the case if you take a classical perspective (in which there are no superpositions).
  15. Dec 2, 2009 #14
    Could see this if the cat was in a coma lol
  16. Dec 2, 2009 #15


    User Avatar
    Gold Member

    Yes, the cat is an observer. The cat observes that the poison did not release and thus it lives.
    The cat also observes that the poison did release and thus it will die.

    Both those states exist within that box until the box is opened by you.

    But it doesn't have to end there. You too are in a larger box, and you too are in superposition until the box you are in is opened and observed. In one superposed state, you open your Schrodinger's box and the cat is alive. In the other superposed state, you open the box and the cat is dead. (You only experience one of these.)

    Like a russian doll, this can continue. The observer who opened the box that you are in is likewise in his own box and he too is superposed.
  17. Dec 2, 2009 #16
    I think part of the confusion may stem from how you understand the thought experiment. A cat is a macroscopic entity and does not exist in a superposition of states like, say, electrons can. You can think of the cat as being in one state at all times. What actually exists in a superposition of states is the radioactive substance that decays and (possibly) sets off the Geiger counter, which eventually kills the cat. The "observation" is not watching the cat, but watching the radioactive substance that holds the cat's ultimate fate. It's a very common misconception in quantum theory that the human mind is what determines measurement. You get lots of crackpot ideas from that premise, but always be careful to watch out for it.
  18. Dec 2, 2009 #17
    I thought that the act of observing knocked it into existing in only one state like the double slit experiment.
  19. Dec 2, 2009 #18
    Wouldn't the cat existing in one state or the other force the decaying radioactive substance out of superposition and into one state?
  20. Dec 2, 2009 #19
    The cat does not exist in linear superposition of states. It's the superposition of the radioactive substance that eventually forces the cat to be dead or alive. When the substance decays and triggers the Geiger counter, then it transitions into a new state.
  21. Dec 2, 2009 #20
    So the experiment is to show that the radioactive substance is in superposition before its detected by the counter and that's all?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook