Re: Quantum Rebel

From: Jesse Mazer <>
Date: Wed, 28 Jul 2004 14:59:17 -0400

Saibal Mitra wrote:

>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.

A poster on sci.physics.research elaborates on this point a little with a
nice thought-experiment involving enlarging the wires until they are almost
touching, at which point you just have a new set of "slits":

>Now I haven't done any calculations or read the New Scientist article
>except looking at the lab setup graphics, but if I would hazard a quick
>guess, it would be that it will turn out that even if the wires are
>placed in the interference fields valleys, the finite width of the
>wires will diffract just enough photons to erase the which-way
>information that was gained by focusing the detectors at the holes in
>the wall through the lens.
>Consider the limiting case with wires placed with their centres in the
>interference fields valleys as before, but expand their width so much
>that they almost touch each other. What you have now is yet another
>wall with a bunch of slits in! Obviously, almost all which-way
>information is lost after the wavefronts pass these almost
>infinitesimal slits since they will diffract the photons equally no
>matter from which hole in the *first* wall they originated, so any
>detector placed after this obstacle will be like running a new
>multiple-slit interference setup (although with the lens now severely
>defocusing the too-closely placed new slits). And since the which-way
>information from the first wall is erased, interference is free to
>happen between the first and the second wall. After the secondary wall
>the detectors can pick up which-way information causing them to behave
>as if there was little subsequent interference.
>Conversely, the other limiting case is with no wires (or secondary
>wall) present. Then all which-way information is present and again the
>detectors behave as if there was no interference.
>The experiment shows a case in between these limits and the effect I
>guessed at above could (and should, according to traditional QM) turn
>out to always cancel any attempt to find both 100% interference and
>100% which-way information. This would be better showed with some
>calculations of course...
Received on Wed Jul 28 2004 - 15:05:54 PDT

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