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.


*PS: A number of people ask me about the attachment to my email, which
is of type "application/pgp-signature". Don't worry, it is not a
virus. It is an electronic signature, that may be used to verify this
email came from me if you have PGP or GPG installed. Otherwise, you
may safely ignore this attachment.

A/Prof Russell Standish              Director
High Performance Computing Support Unit, Phone 9385 6967, 8308 3119 (mobile)
UNSW SYDNEY 2052                       Fax   9385 6965, 0425 253119 (")
Australia                [EMAIL PROTECTED]
Room 2075, Red Centre                    http://parallel.hpc.unsw.edu.au/rks
            International prefix  +612, Interstate prefix 02

----- Oorspronkelijk bericht -----
Verzonden: Wednesday, July 28, 2004 03:40 PM
Onderwerp: Re: Quantum Rebel

> Oops, I too was a victim of viral paranoia this AM and committed wholesale
> deletion of all attachment laden emails in my box including, apparently,
> Russel's. letter. Can someone send or forward me a copy? (of the letter
> a virus) ;)
> Thanks!
> > Please, Russell,
> > for the peace of our minds who believe in 'smart' viruses and have none
> > the software you indicated:
> > Could you at least put a word in the e-mail that thei comes from you?
> > "Safe from RS" or "from Russ" etc.)
> > I wanted to open this attachment in blind face and paranoid shiver,
> however
> > went first to FILE - Properties - 2nd page and checked the "from" line
> > the bottom.  Boring. Even there it may be a virus usiing your mailbox -
> > before you detect it.
> > I am not the only one suffering from virus-paranioa.
> > Cheerz
> > John Mikes

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