# Re: Quantum Mechanics

From: John W Burgeson (burgytwo@juno.com)
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)

www.burgy.50megs.com

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