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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

pattern.

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.

Cheers

Received on Tue Jul 27 2004 - 22:20:45 PDT

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

pattern.

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.

Cheers

-- *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 R.Standish.domain.name.hidden Room 2075, Red Centre http://parallel.hpc.unsw.edu.au/rks International prefix +612, Interstate prefix 02 ----------------------------------------------------------------------------

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