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Schrodinger's Cat

  1. Apr 9, 2004 #1
    I'm sure you've all heard of Schrodinger's thought experiment regarding Quantum Mechanics.

    The question is: does the cat constitute an observer?
     
  2. jcsd
  3. Apr 9, 2004 #2
    What if the cat is dead? Well, the cat won't die and come back to life if you open the box, so something else has to be going on here.
     
  4. Apr 10, 2004 #3

    chroot

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    According to decoherence theory, even a stray photon "counts" as an observer.

    - Warren
     
  5. Apr 10, 2004 #4
    Yeah I know, I am observant. Wait, you weren't talking about me. Sorry. :biggrin:
    So dead cats can observe, and the box can observe? Or is it something special about photons?
     
  6. Apr 10, 2004 #5

    chroot

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    Any particle interacting with an object in a mixed state will collapse the superposition and puit the object into a pure state.

    - Warren
     
  7. Apr 10, 2004 #6
    Ah. Now i know exactly what you mean.
     
  8. Apr 10, 2004 #7

    chroot

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    The idea is called "decoherence," and it's the reason we don't see quantum-mechanical effects [often] on the macroscopic level. All the billions of particles interacting with the system -- atoms of air, infrared photons, etc. -- all serve to keep macroscopic objects like cats out of quantum superpositions.

    Plenty of good sites abound:

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

    - Warren
     
  9. Apr 10, 2004 #8
    It's rather odd though. That a single particle would determine the cats fate. That theory implies that if the cat breathed, it's fate would be determined. The cat would have to be in a complete vacuum to stay alive/dead. :biggrin: (alive/dead) sounds weird...)
     
  10. Apr 10, 2004 #9

    chroot

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    He'd also have to be at absolute zero, which isn't physically possible. Really, a cat is a pretty poor laboratory for quantum-mechanical effects, no matter how you kill it.

    - Warren
     
  11. Apr 10, 2004 #10
    I went with curiosity and never looked back. :biggrin:
     
  12. Apr 10, 2004 #11

    chroot

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    BTW, there are some ways to observe macroscopic quantum mechanical effects, but they don't involve house pets -- are you familair with superfluids?

    - Warren
     
  13. Apr 10, 2004 #12
    Sure, I love orange juice!
    But seriously, what are they?
     
  14. Apr 10, 2004 #13

    chroot

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    Well, I'll give you a bit to start with. If you take a bucket of liquid helium and cool it down to within a few degrees of absolute zero, it undergoes a phase transition called Bose-Einstein condensation. All the atoms hurry to enter the same quantum state, because that minimizes their total energy. So you wind up with a bucket full of atoms that have all agreed to be in the same state. What happens when you try to use it like a normal fluid?

    Well, you'll notice it flows without viscosity. That's right, it flows without resistance through even the very smallest pores in your container, and through even the smallest pipettes. Why? Because all the atoms are already in their lowest energy state. Since they're all doing the same thing, though, the walls of a pipette can't smack them around too much -- you can't smack around one, you have to smack around every single last trillion of them.

    Superfluids also will only permit certain values of angular momentum, e.g. 3 or 5 or 7 rotations per second. Even if you spin the bucket at 4 rotations per second from now to eternity, the helium atoms won't care. Angular momentum is quantized for their collective quantum state, and they'll only rotate at 3, 5, or 7, and never, ever at 4.

    And the list goes on. You can basically consider a bucketful of liquid helium to be like one giant macroscopic quantum object.

    - Warren
     
  15. Apr 10, 2004 #14
    That is very odd sounding. I never really took the time to think of a way for liquid to flow without viscosity. Makes you wonder what Schrodinger was thinking about the cat.
     
  16. Apr 10, 2004 #15

    chroot

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    Quantum mechanics is pretty "weird," for sure, if by "weird" I mean "contrary to our everyday experience." It just takes more specialized apparatus than a cat to let you see it with your own eyes. :smile:

    - Warren
     
  17. Apr 10, 2004 #16
    I don't think that that counts as a BEC. This announcement would seem to imply otherwise. I'm pretty sure BEC is a transition in a gaseous state, without becoming a liquid or a solid.
     
  18. Apr 10, 2004 #17

    chroot

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    Sorry, you're wrong. Superfluid liquid helium is a BEC. Please do some more reading.

    - Warren
     
  19. Jul 2, 2008 #18
    That also fascinates me.

    But you say 3, 5 or 7. Does it extend to 1 and -1, or is it an approximation? Is zero a valid angular speed?

    And is there a phenomenon of superposition of different speeds?
     
  20. Jul 2, 2008 #19
    Instead of liquid, gas, solid, you should be thinking that helium-4 is indeed a boson (why is that? because it has an overall integer-valued spin!) which means that macroscopic quantities of the stuff follow Bose-Einstein statistics, which means that an indefinite number of them can collapse into the same state (unlike fermions which obey the Pauli exclusion principle) and so we can create macroscopic chunks of matter for which all the constituent particles are in the same state, which gives the Bose-Einstein condensate its unusual properties.

    Yes, 1 and 0 are valid values for angular momentum. Usually we talk about the absolute value, but whenever direction matters we see negative spins like -1 along with positive ones.

    Yes! It might surprise you to know that any quantum state can be written as a superposition of different speeds. But the same is true if the word speed is replaced by "Energy", "Position", or any other physical variable you can think of!
     
    Last edited: Jul 2, 2008
  21. Jul 2, 2008 #20
    Yes. Any macroscopic system is an "observer".
     
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