Re: Quantum Rebel

From: scerir <>
Date: Wed, 11 Aug 2004 23:11:36 +0200

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 nature).

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 from?

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,

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 Wed Aug 11 2004 - 17:18:39 PDT

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