RE: Quantum theory of measurement

From: Jesse Mazer <lasermazer.domain.name.hidden>
Date: Wed, 12 Oct 2005 19:28:13 -0400

Ben Goertzel wrote:

>
>
>Thanks very much Jesse!
>
>You answered the question I *would have* asked had I rememberd my quantum
>physics better ;-)
>
>I think your answer is related to a paradox a friend mentioned to me.
>
>The paradox is as follows:
>
>"One does the EPR thing of creating two particles with opposite spin. Send
>one far away to Alpha Centauri and send the other through a
>Stern-Gerlach magnet and let the spin up and spin down outputs
>interfere to form a double slit. If the far away particle is measured
>up vs. down, our local particle must definitely go through the up hole
>or the down hole and we get no interference pattern. If he measures
>the far away particle sideways we get a superposition of states and we
>get interference. Thus by rotating his measurement he should be able
>to communicate to us faster than the speed of light. We should see our
>pattern blinking between interference and not. What's wrong with that
>argument?"
>
>Along the lines of your solution to my other, related puzzle, I'll try to
>analogize a solution to this puzzle.
>
>I guess the idea must be: there is no "change" to what a particular
>particle
>does
>when you observe its faraway coupled pair in a certain way. Its individual
>results do not visibly
>change from non-interference to interference. (If that did happen,
>you'd have the basis for a faster than light communicator, as you say.)
>
>Instead, when you observe some of the particles sideways and some
>vertically,
>you must be creating "correlational information" that exists only
>statistically
>as a correlation between that's happening in Alpha Centauri and what's
>happening locally.
>
>So, maybe there is some weird cancellation here, like in the case you
>described in your email.
>Perhaps, if one restricts attention to the cases where
>the faraway particle is measured "right" then interference is seen, and if
>one restricts
>attention to the cases where the faraway particle is measured "left" then
>interference
>is seen; but if one looks across all cases where the faraway particle is
>measured
>sideways, then the peaks and troughs of the different cases might cancel
>out
>and
>you'd get no interference?
>
>-- Ben

Yeah, I'd agree with your guess about what would happen here. Certainly you
won't see interference vs. non-interference on the screen depending on what
measurement was made on the other particle, since it's been proven that
quantum effects cannot be used to transmit information faster than light;
so, I think you would see no interference in the total pattern on the screen
no matter what, but if you looked at specific subsets of cases once you
found out the result of the measurement on the second particle, you'd
probably be able to see interference patterns in those subsets where the
measurement of the second particle did not allow you to determine which slit
the first one went through.

Jesse
Received on Wed Oct 12 2005 - 19:29:41 PDT