Re: Quantum Mechanics

From: John W Burgeson (
Date: Tue Apr 17 2001 - 12:09:46 EDT

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    Who are you addressing? Ph.Ds in literature. Ph.Ds in
    physics? Ph.D. in biology? It makes a lot of difference. >>

    These are seminary students. Some are familiar with philosophy; others
    are still at the MDiv level.

    "As I recall, Alan Aspect's experiment was the first
    experimental test of the Bells' inequalities. These
    are related to the Einstein Podovsky and Rosen paradox.

    In essence, this states that the electron has two states:
    spin up and spin down. Now suppose that you can somehow
    isolate an electron pair (creating an entangled electron
    pair) and if you can somehow separate them in such a way
    as to put one electron in one box and the other electron
    in another box without otherwise "measuring" them (and
    therefore losing that entangle condition you isolated).
    At this point, you don't know which box has spin up and
    which box has spin down. (The box must be able to somehow
    isolate these spins from interactions with the world ---
    a rather idealized thought experiment!

    Anyway, now suppose there is some button on the box that
    allows you to chose the time that you want to measure the
    spin in either of the boxes. So you a given one box, and
    your friend is given another one. You friend travels to
    alpha Centari (sp?) with his box, and you stay at home
    (or vice versa). Four when your friend arrives at alpha
    Centari many years later, he measures the spin in the box.

    According to the quantum entanglement concept, then whatever
    the direction of the spin is measured in your friend's
    location, that direction immediately decides the spin on
    your location. What's more, if you became impatient waiting
    for your friend to arrive at alpha Centari and you decided
    to make the measurement, that would immediately determine the
    spin direction of your friend's box.

    Actually, the successful way to show this is by optical
    methods. It seems that optics is less interacting with
    the world, so it is possible to isolate individual photon
    pairs and show that measuring one photon spin immediately
    determines the direction of spin of the other photon. The
    measurements have been done with as much separation as 11
    kilometers, and it does seem that they really do act
    essentially instantaneously.

    The reason this does not defy the principle of relativity
    (allowing faster than light communication) is because you
    can't determine which spin direction the entangled photon
    will take along the two different channels. So the actual
    information (which direction was measured by your friend
    on alpha centari) must be propagated in the normal way at
    the speed of light, but when you compare your results with
    your friend's, they will corroborate with each other.

    What makes this spooky is that the "measurement" is vastly
    separated by a large distance. Hence, when the measurement
    is made, there appears to be no requirement that information
    about the direction of the spin direction must be propagated
    to the other entangled spin to make this property observable
    at the other location. It in essence suggest that the
    universe is "connected".

    There are a number of articles in Nature in recent years on
    that as well as Phys. Rev. I have not been following it closely, but
    some introductions to these ideas and maybe a clearer explanation
    can be found in the back issues of Physics Today as well. For a 15
    minute introduction, that is probably the best place to start I would
    think. >>

    That's a good description; I thank you. Gribbin's book has a longer essay
    on this -- he calls it the "two kitten" experiment. I used to read
    PHYSICS TODAY regularly; I probably should resume doing that.

    >>Just because someone has a PhD, it doesn't mean they know everything.

    Knowledge as mere knowledge makes one proud, but knowledge with
    a heart of wisdom one humble. >>

    I'll go along with that.

    Thanks again.

    Burgy (John Burgeson)

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