RE: Quantum Rebel

From: Fred Chen <>
Date: Fri, 13 Aug 2004 23:24:06 -0700

Yes I think this is correct.

The theoretical zero amplitude region in the interference plane of the
wires is also of zero width, while on the other hand the wires are
obviously finite width. The wires do interact with the photons in
reality, or equivalently, diffract the waves coming from the slits. So
in the end each detector will detect photons coming from both slits. If
you covered up one of the slits, but left the wires in place, both
detectors will detect photons originating from the slit.

So complementarity (if defined as exclusive particle/wave observation)
has not been disproven. The particle-tracking information from each slit
is destroyed by interaction with the wires.


-----Original Message-----
From: scerir []
Sent: Wednesday, August 11, 2004 2:12 PM
To: Everything List
Subject: Re: Quantum Rebel

It seems (to me, et al.) that the (supposed) information
Afshar gets about the "which way" at the screen, is
different from the usual information about the "which way"
one gets at the slits, or from the (probabilistic) information about the
"which way" one can have even before the slits (in case of asymmetry).
This seems to be the point.

So there is a problem here. Because we know that information about the
"which way" at (or before) the slits does destroy the interference
pattern completely (partially if the information has a probabilitic

On the contrary the "which way" information Afshar thinks he gets (via
those lenses) at the screen, does not destroy the interference pattern
(at the wires, which is a different place and time from the screen). So
how Afshar gets this information? Via optics, geometrical optics,
straight lines.

That is to say that a photon goes following straight lines between the
slit and the detector, but (apparently) goes "interferential" at the
wires. (I wrote apparently because at the wires, imo, there is no
measurement, but a "negative" or "non-demolitive" or "weak"
measurement). All that seems not consistent. So the information Afshar
gets at the screen, via those lenses, is not real, or proper. Because
the conservation of momentum (the conservation of distribution of
momentum) forbids all that, imo.


[fwding what Basil Hiley thinks ...]

Thanks for the copy of the New Scientist's article about Afshar's
experiment. Unfortunately it is no challenge to Bohr's position. How on
earth does he know the photons arriving at detector 1 come from pinhole
1 when both pinholes are open? You cannot use ray optics in the region
where the light from the two pinholes overlap so you cannot draw any
such conclusion.

Lets look at the claim a little more closely. Afshar is quoted as saying
"According to my experiment one of the key assumptions about quantum
theory is wrong." Ok what assumption? The article doesn't say. It
mutters something about Bohr and goes on to say "When faced with a
classical apparatus these mysterious quantum entities will either show a
particle-like or a wave-like face." Where has this sloppy thinking come

Bohr never said any such thing. What Bohr actually said was 'However,
since the discovery of the quantum of action, we know that the classical
ideal cannot be attained in the description of atomic phenomena. In
particular, any attempt at an ordering in space-time leads to a break in
the causal chain, since such an attempt is bound up with an essential
exchange of momentum and energy between the individuals and the
measuring rods and clocks used for observation; and just this exchange
cannot be taken into account if the measuring instruments are to fulfil
their purpose. Conversely, any conclusion, based in an unambiguous
manner upon the strict conservation of energy and momentum, with regard
to the dynamical behaviour of the individual units obviously
necessitates a complete renunciation of following their course in space
and time'. [Bohr, Atomic Theory and the Description of Nature, pp. 97-8,
Cambridge University Press, 1934]

Notice the phrase 'following their course in space and time'. This is
not a discussion about particles 'behaving only like waves or only like
particles'. It is about 'following' or 'looking at' the process using
some suitable instrument. So if you 'look at' each photon as it passes
through one pinhole you won't get an interference pattern. Even the Bohm
approach agrees with that.

OK after that little rebuff we finally come to Afshar's real claim. It
is that his experiment is "recording the rate at which photons are
coming through each pinhole". If this statement was correct then it
would imply that "there should be no interference pattern" where the
beams overlap. "But there is, Afshar says".

Sure there is an interference effect simply because Afshar's experiments
do not 'follow' anything and they do not 'look at' each photon as it
passes through a pinhole. He is simply collecting and counting the
distribution of photon arrivals at his two detectors. Then he makes
inferences about what could possibly be going on and concludes,
incorrectly that a photon detected in the 'photon detector for pinhole
1' came from pinhole 1. However that conclusion is based on the
assumption that the rays emanating from pinhole 1 arrive at the 'photon
detector for pinhole 1'. But the ray picture breaks down as soon as you
enter the region of overlap of the two beams and you cannot conclude
that the photon entering pinhole 1 arrives at the 'photon detector for
pinhole 1'. You haven't measured which pinhole each photon passed
through so you have not contradicted Bohr.

Unfortunately Afshar's conclusion, "According to my experiment one of
the key assumptions about quantum theory is wrong" is incorrect. His
conclusion is wrong simply because he doesn't understand the physical
optics that lies behind the experiment he is doing.


Just 2 more little papers about it.

W. Unruh

M. Pitkaenen
Received on Sat Aug 14 2004 - 02:26:17 PDT

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