non-separability of 2 particle systems in quantum mechanics

richardconers_at_yahoo.com
Date: 10/12/04 

Date: 12 Oct 2004 16:59:19 -0700

I'm reading "Quantum mechanics and experience". It's starting to sink
in, and I hope I can use this formum to clarify some points as I go
along (I finished reading it, but I'm reading it a second time because
there's no way I get it entirely in the first round <g>.)

My big hang-up so far is understanding non-locality/non-separability
of 2 particle states. Albert doesn't explain how such a state comes
about in the first place. Can I assume that all combinations of 2
particles at a time are NOT in a non-separable state? Then what puts
them into that state? And what causes the end of non-separability
(i.e., separate reactions of the particles independent of the changes
going on in the other particle)?

I raise the questions because I don't understand why Albert makes such
a big deal out of non-locality. If two particles are in a
"non-separable state" where the color of A is opposite the color of B
(A+B = 0) is the quantum state of the two particle system, then
non-locality seems to follow logically.

In this state neither A's color nor B's color is determined, just the
state of the two particle system. So if A is then measured, A-B is
still undetermined because B has not been measured yet. When B is
finally measured, it will turn out to be "minus A or the opposite of
A", unless something is done to change the two particle system, which
meausring alone won't do, since A-B is already in a determined state.
So B, at the time of measuring of A, is still undetermined, but there
is no way (I assume) to confirm that B is undetermined without
measuring it. As soon as you measure B, it must be "minus A" per the
2 particle system state.

If the above is true, nothing in that description calls for any
"direct, physical" contact between A and B, so non-locality is
inherent in having something called a "non-separable" state. You
don't even need Bell's theorem to conclude this, do you?

I raise this question because this non-separability seems to me to be
very, very much different from incompatible properties of a one
particle system. It seems like quantum mechanics is asserting right
from the start that there are pairs of properties of a single particle
which are 100% incompatible. This seems to be inherent in the
physical makeup of the universe.

In contrast, I don't think Albert makes explicit under what
circumstances non-separability of two particle systems obtains. Is he
saying that non-separability is inherent in all two particle pairs,
just like incompatibility is inherent in pairs of properties of one
particle systems? Even so, that wouldn't mean that all 2 particle
pairs are in a non-separable state, since only specific pairs of
properties of a one particle state are incompatible, not all pairs of
properties.

So that leads me back to the question of what generates
non-separability of two particle systems--and which properties become
non-separable under those circumstances (hopefully it doesn't mean
that all properties are non-separable in that state)?

Thank you for your explanation(s)!!!

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