Re: Quantum Rebel

From: Russell Standish <>
Date: Sat, 14 Aug 2004 10:45:45 +1000

On Thu, Aug 12, 2004 at 11:28:33AM -0400, Jesse Mazer wrote:
> Russell Standish wrote:
> 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?

"no difference to the outcome of the experiment", but a big difference
to the information obtained from the experiment. In the first case
(without the path 4 absorber), which detector a photon arrives at is
correlated with which path it took between path 1&2. With the path 4
absorber, one can no longer obtain that information. Weird perhaps,
but entirely in line with what QM predicts.

> 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?

The presence or absence of the absorber on path 4 changes the
wavefunctions involved, even though the amplitude of the wavefunction
along path 4 is zero. The same is true with Afshar's experiment. This
may seem nonsensical, but its exactly what QM says happens, and this
sort of thing has been seen often in quantum experiments. The
Aharanov-Bohm effect comes to mind.

More likely the source of the paradox is the incorrect belief that
placing an absorber/detector at a zero amplitude site will not affect
the experiment.

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

*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 (")
Room 2075, Red Centre          
            International prefix  +612, Interstate prefix 02

Received on Fri Aug 13 2004 - 20:52:44 PDT

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