Re: Quantum Rebel

From: Jesse Mazer <>
Date: Thu, 12 Aug 2004 11:28:33 -0400

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
>> 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&quot; 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
>>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

Jesse Mazer
Received on Thu Aug 12 2004 - 11:39:51 PDT

This archive was generated by hypermail 2.3.0 : Fri Feb 16 2018 - 13:20:09 PST