From:
Caroline H Thompson [ch.thompson1@virgin.net]
Sent: 30
October 2004 20:04
To:
letters@newscientist.com
Subject: Entangled or not?
The Editor
New
Scientist
Dear Sir
Bob Millar
is quite right: supposedly "entangled" particles do not in fact have
instantaneous effects on each other after they have separated (Letters, p31, 30
October), it is just that the members of the pair started off with some shared
state (polarisation direction, for example) and once you know its value for one
you know it for the other. So why do
the quantum theorists claim that this is anything unusual? The problem arises because they think their
results infringe Bell's inequality, and if this is the case our simple
explanation *cannot* be correct. Our
"shared state" would be the "hidden variable" that Bell
showed to be incompatible with the quantum theory prediction. If the particles really did possess definite
states in this manner then his inequality would have to be obeyed and (they
claim) it is not.
All is far
from being lost, however! For if you
look at the fine print you will find that, though they infringe Bell's
inequality on a regular basis these days, it is by no means the strict version
that Bell himself derived. Various
different inequalities are used, all assumed to be equivalent but in fact all
demanding slightly different auxiliary assumptions. A common one is that the sample of observed coincidences is
"fair", and it is not hard to show that in practice this will rarely
if ever be the case. The selection of
the sample in the "Bell test experiments" is not at all like the
selection of people to take part in a survey, for instance. There is no way the experimenter can control
it. He cannot even reliably test for
it. All he can do is try and check that
there is no positive evidence for unfairness, and the only available test
(looking for constancy of the total coincidence counts) is neither sensitive
nor conclusive and involves considerable extra work.
To cut a
long story short, there are various possible "loopholes" in the Bell
tests, and no experiment yet has been free of them. This is not for want of trying!
New proposals for "loophole-free" tests are regularly posted
on the internet or in the journals.
Meantime, though, Bob Millar and I are perfectly justified in believing
that the two particles do *not* influence each other at a distance. Nobody has ever proved they do.
For more on
the Bell test loopholes see http://en.wikipedia.org/wiki/BellTestLoopholes
and related pages or my web site, http://freespace.virgin.net/ch.thompson1/
.
Yours
sincerely
Caroline
Caroline H
Thompson
ch.thompson1@virgin.net
http://freespace.virgin.net/ch.thompson1/