Re: Quantum Rebel

From: Saibal Mitra <>
Date: Thu, 29 Jul 2004 02:58:38 +0200

Not me but Russell wrote that. I should have made that clear better when I
posted Russell's attachment (Sorry Russell!).

----- Oorspronkelijk bericht -----
Van: "Jesse Mazer" <>
Aan: <>
Verzonden: Wednesday, July 28, 2004 08:59 PM
Onderwerp: Re: Quantum Rebel

> 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
> 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 - 21:01:45 PDT

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