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Stern Gerlach Experiment

  1. Dec 14, 2003 #1
    Last edited: Dec 14, 2003
  2. jcsd
  3. Dec 17, 2003 #2
    Question about SGE

    What if we do a S-G experiment with a uniform magnetic field?

    Each atom will still have to "decide" to point up or down, only we won't be able to tell them apart at the output of the apparatus because there will be no vertical deflection. Now, is this not a different situation from the input beam? At the input, (if I understand correctly) each atom is essentially "unpolarized" i.e. in a 50-50 superposition of both states. At the output, we have a 50-50 mixture where each individual atom is definitely "up" or "down".

    My questions:
    (a)is the above reasoning correct

    (b) if so, is there an experiment that we can do to distinguish a beam that has been "combed out" by a uniform magnetic field, from a beam that has not passed through any field.
  4. Dec 17, 2003 #3


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    Re: Question about SGE

    I'm not a QM guru, so this won't be very specific:

    Let's say we have a beam of electrons that's been polarized s.t. all spins are pointing up at orientation 0. Then if we slap a detector at an angle of θ to 0, then the probability of getting an electron with spin up orientation θ is cos(θ/2)2.
    The probability for the other orientation from the 'combed pair' is cos((π-θ)/2)2=sin(θ/2)2
    So, along any orientation, you would still get a 50-50 distribution of spins since you get .5*(sin2+cos2) probability of spin up.

    So the splitting and recombining does not unscramble the electrons. In fact it scrambles them. You can do the following:

    Let's say we've got a beam of electrons that is all spin-up in orientation 0. Now, if we split it into beams along orienation π/4, and then recombine it, we will get a beam with a 50-50 orientation split along spin orientation 0.
  5. Dec 17, 2003 #4


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    Oh, there's a nice easy experiment for you to check this.
    It takes 3 polarizing filters and a light source.

    Take the two polarizing filters, and line them up so their polarization is perpendicular. You should get almost no light coming through.

    Now, if you put the third filter between them, you'll get *more* light coming through from before.

    Analagous experiments are , I believe, possible with birefringent materials such as calcite, which can act as a beam splitters instead of eliminating a particular polarization.
  6. Dec 20, 2003 #5

    Doc Al

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    Re: Re: Question about SGE

    I'm not exactly sure what you are saying, but you can certainly recombine the exit beams of a SG magnet by putting them through a second (reversed) SG. If you can maintain the coherence of the beams, you will not be able to distinguish the final beam from the original: it will be in its original pure state. That's (part of) what's weird about this stuff.
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