[extropy-chat] more on Afshar Experiment, as of 7 weeks ago

scerir scerir at libero.it
Sun Sep 19 21:51:32 UTC 2004


> anyone have anything even more up to date on this 
> important challenge?

As told by S.Afshar his paper will appear soon on 
PRL. Afshar wrote something, here and there, see
below. There are several problems of interpretation,
of his experiment.

-------------------

The (supposed) violation of "complementarity" is this.

S.Afshar assumes that a photon - whatever it is - goes, 
following straight lines (geometric optics), between 
the slits and the detectors. So he uses mirrors & lenses
to reveal which slit the photon passed through. (Notice
that also A.Zeilinger used, in 1999, similar assumptions
and optics, in a different experiment). But this assumption, 
motivated by the principle of conservation of momentum, 
was put in question, since many think that the photon is 
not a ball, and between the slits and the detectors 
- the no "collapse" zone - the photon behaves like a wave, 
or the like. So, in that zone, photons don't go straight. 
But, in that zone, between the slits and the detectors, 
Afhar also located many little wires, located exactly 
at the (calculated) minima of interference (supposing
this time that, in that zone, the photon behaves *also*
like a wave, or the like).

The results of the experiments are that photons passing
through the slits (also one at a time) are recorded by
the detectors (they also record - in assumption - the 
specific slit the photons passed through) and they are not 
intercepted by those little slits (meaning that they are 
also "waves"). Wave-like and particle-like properties of
photons in the same experiment. Complementarity violated (?).

-----------

[S.Afshar speaks here]

"It is true that before the measurement is made at the image 
plane, one can use the superposition, however, upon the measurement, 
the which-slit information is established and retrodictively, one 
must assign a particular slit to the photon. When you collapse 
the superposition state, only one of the slits will be found as 
the origin, (that is to say the Cat is found either to be dead or 
alive...), I will address this issue in a theoretical paper on the 
theory of measurement in a couple of months. At issue is the
law of conservation of linear momentum, [...]" 

"The issue of retrodiction and so forth has its origins in 
Heisenberg's own writings, later expanded by Einstein, von 
Neumann and many others, (lately Griffiths, Gel-Mann etc.). 
Again at stake are the conservation laws..."

"I (and every quantum theorist) agree that the quantum formalism 
in my experiment predicts an earlier interference and a later 
which-slit determination at the image plan. As much was agreed 
upon even between Einstein and Bohr (See Wheeler ref.s on his 
delayed-choice experiments). The issue is "measurement". According 
to CI, "measurement" of the which-slit information for a photon, 
makes it (non-locally) to have behaved like a particle, (i.e. a BB), 
thus incapable of having interfered."

"The source of the contention is not the quantum-mechanical 
formalism; it is the ad-hoc assumptions injected to the QM by 
the measurement theorists, the first of whom were Bohr, Heisenberg 
and Pauli."

"Here's a quote from Bohr himself discussing the welcher-weg 
experiments:
<...we are presented with a choice of either tracing the path of the
particle, or observing interference effects.we have to do with a 
typical example of how the complementary phenomena appear under 
mutually exclusive experimental arrangements>" 
N. Bohr, in: Albert Einstein: Philosopher-Scientist, P. A. 
Schlipp Ed., Library of Living Philosophers, Evanston, Illinois, 
1949"

---------------

Comments.
1) Those little wires, located at the (supposed) minima of
interference (assuming that photons behave like waves, at
least in that zone) do not perform a measurement (they
perform a negative measurement). Thus the existence of waves 
(or interference) is an inference, a deduction.
2) "At issue is the law of conservation of linear momentum"
says Afshar. This issue is not new at all. Many wrote papers
speaking of possible retro-actions (backwards in time, if
"time" has a meaning here) from the detectors to the momentum
distribution function at the slits. (Riedtijk, Zurek, Wootters;
also Klyshko, and Bennett).
3) Wave-like and particle-like behaviours are revealed in
distinct points and times (at the wires, at the detectors). 






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