Quantum Rebel

From: Russell Standish <R.Standish.domain.name.hidden>
Date: Wed, 28 Jul 2004 12:16:23 +1000

I just read the New Scientist article "Quantum Rebel" last night about
Shariar Afshar's work on the double slit experiment. Ingenious as the
experiment is, I really don't think it says anything about different
interpretations of QM. Indeed, the outcome of the experiment is just
what I'd expect from quantum theory, regardless of which
interpretation is used.

OK - so the claim is that Bohr's complementarity principle (CP) is tested
by this experiment and found wanting. I decided to go back to the two
text books I learnt quantum mechanics from - Leonard Schiff's book
which is the older and more traditional of the two, and Rammamurti
Shankar's book which has the more modern approach, but which I found
explained things better. Shankar doesn't mention the CP at all, and
for Schiff, the CP is basically a restatement of the Heisenberg
uncertainty principle, a principle not tested by Afshar's experiment.

In the double slit experiment, how I understand the CP to work is that
one cannot measure which slit a photon passes through, and retain an
interference pattern. Assuming it is possible to do this, one could
divide the measures data into those photons that passed through slit A,
and those that passed through slit B. The resulting distribution of
photons arriving at the screen of
the two slit experiment is then the sum of the distributions of the
two subsets of data. However, the two sub distributions do not have
inteference patterns so how can the sum have an interference
pattern. Hence any such measurement of which slit the photon passes
through must affect the photons so as to destroy the intereference

Now in the article, Afshar claims to have measured which slit the
photon passed through and verified the existence of an interference
pattern. However, this is not the case - without the wires in
place to detect the presence of the interference pattern, photons
arriving at detector A have passed through slit A, and vice-versa with
detector B and slit B. However, with the wires in place, some photons
are scattered, indeed some photons which passed through slit A will
arrive at detector B. With both slits open, and the wire placed
exactly at a null point of the interference pattern, the photons
passing through slit A and arriving at detector B exactly counteracts
the photons passing thoguh slit B that have been lost through
scattering. The mathematics of quantum mechanics assures this,
coincidental this may seem.

It may be a question of "interpretations of interpretations of QM",
however on the basis of the New Scientist article, I don't believe
Afshar have shown a problem with the complementarity principle.


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A/Prof Russell Standish            	 Director
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Received on Tue Jul 27 2004 - 22:20:45 PDT

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