Russell Standish wrote:

On Thu, Aug 12, 2004 at 01:08:36AM -0400, Jesse Mazer wrote:

Also notice that in the analysis of Afshar's experiment by W. Unruh at
http://axion.physics.ubc.ca/rebel.html which scerir linked to, Unruh does
not dispute Afshar's claim that all the photons from the each pinhole end
up in a single detector. In fact, he offers a "simpler version of the
experiment" involving a multiple pass interferometer, depicted in figure 2,
and says that in this experiment you do know which path a photon took by
looking at which detector it hits: "By measuring which detector they
triggered, 5 or 6, one measures which of the beams, 1 or 2, the photon
traveled along". Since the experiment in figure 2 is just supposed to be a
"simpler version" of Afshar's experiment, it's pretty clear that Unruh
would not disagree that the lens insures that knowing which detector
absorbed a photon is enough to tell you which path the photon must have
taken through the pinholes. Unruh is a fairly big-name physicist and his
explanation of what's wrong with Afshar's conclusions about complementarity
are pretty detailed, while I don't know anything about Basil Hiley and his
criticisms are more vague.

After looking at Unruh's explanation, I can only say that he puts in a very persuasive manner, the substance of my post to this list: namely that the insertion of a diffraction grating (equiv. Unruh's path 4 absorber) destroys the "which-way" information.

Hmm, on rereading the last section of Unruh's article I see you're correct--in the second-to-last paragraph he says "However, while in the interference experiment, the presence or absence of the absorber in path 4 would seem to make no difference-- no photons travel along path 4 to be absorbed-- it makes a crucial difference in the interpretation. A critical part of the the interpretation is drawing the inference that IF the particle was detected in detector 5, THEN it must have come from path 1. Similarly, IF it was detected in detector 6, then it came from path 2. The presence of the absorber in path 4 does not change the experiment, but it destroys these inferential chains."


I'm a little confused by his answer though, because he says the presence or absence of the absorber along path 4 in figure 2 makes absolutely no difference to the outcome of the experiment, since interference insures no photons would travel along path 4 even without the absorber there. So if you placed a continuous screen in the position of the two detectors, you'd still get two distinct spots with no interference fringes between them...and doesn't complementarity say that the absence of interference between two spots on a screen in the double-slit experiment can only occur when you *do* know which hole the photons landing on each spot went through?

Along the same lines, I'm confused about exactly how your answer fits with Unruh's answer. You said:

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.

Is Unruh saying that in figure 2 without the absorber, the amplitude of a photon travelling along path 4 is zero, but with the absorber in place, there is some nonzero amplitude for a photon to travel along path 4 but then be scattered into the "wrong" detector? If he's not suggesting the possibility the absorber will scatter photons without absorbing them is relevant here, then I wouldn't think he'd say the possibility the wires will scatter photons without absorbing them is relevant to Afshar's experiment. When you say "the mathematics of quantum mechanics assures this", did you actually do a calculation of the effects of scattering in Afshar's experiment?


Jesse Mazer




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