From: "John Collins"
 
> Essentially Ashfar's experiment involves fooling himself 
> (and perhaps a few others) with a new single-path photon 
> thoery, then undermning the new theory, whcih was not quantum
> mechanics..

The orthodox QM says that if we have the usual two-slit,
a "which way" detector, and the screen, the interference
pattern is destroyed as soon as the detector finds (with
probability 1) the "which way". In principle this is true 
also in the case of single particles. In practice you need 
many particles. (If the detector finds the "which way" with
probability < 1, the interference pattern is destroyed
partially, because there is a Greenberger & Yasin equation,
and so on ...).

The orthodox QM also says that is we put a "which way eraser"
between the (working) "which way" detector and the screen, 
the interference pattern is restored. But where it is restored?
At the screen of course.

Now in the Afshar experiment those lenses are (supposed to be)
"which way detectors". Now if we insert, after each of those lenses,
the "which way eraser", an interference pattern - according to
orthodox QM - will be restored. But where it will be restored?
At the screen (here - in Afshar exp. - I do not know if there is 
a screen, but in principle it could be so).

It seems to me that the "which way eraser" affects what happens
"after", but does not affect what already happened "before". But
if that is true for the "which way eraser", it must be true
also for the "which way detector". That is to say that any "which
way detector" does affect what happens "after" but does not
affect what already happened "before".

If the above is true - but I doubt it, since here is late, hot, and
dark ! - assuming (as Afshar/Cramer) that those lenses ("which way 
detectors") should affect what already happened "before", at
those little wires level, is not solid at all. 

s. 

Btw, it is not so simple to define "complementarity".
Between "waves" (are there "waves" in Matrix Mechanics?) and 
"particles"? Between localization and superposition of amplitudes 
(von Weizsaecker)? Between interference pattern and "which way" 
knowledge? Between continuous and discontinuous? Between 
separability and unitarity? Between reversibility and 
irreversibility? This one seems to be close to what Lawrence Bragg 
said: "Everything in the future is a wave, everything in the past 
is a particle"!


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