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


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