On Fri, Aug 13, 2004 at 11:43:10PM -0700, Fred Chen wrote:
...

> 
> A better (and far simpler) way to challenge complementarity would be to
> use a low-intensity interferogram in a photographic film or CCD. At
> first the photons being detected are few so the shot (particle-like)
> aspect is more obvious. As more photons are integrated, the classical
> interference pattern is observed. Can there be a transition region where
> both aspects are observable?
> 

This does not challenge complementarity. Consider a double slit
apparatus with the photon source's intensity down so low that each
individual photon can be observed hitting the screen one at a
time. But when one plots the distribution of positions where the
photons strike the screen after observing many of them, the
interference pattern results. This is simple and uncomplicated, but is
not what the complementarity principle is about.

Now consider that you have information about which slit the photon
passed through before hitting the screen - ie each photon is labelled
1, 2, 1, 1, etc, according to whuch slit it passed through. Therefore,
you can separate the observed photons into two sets, according to
which slit the phtons passed through. The distribution of each subset
corresponds to a single slit experiment, and the final distribution
must be the sum of the two single slit experiements. But single slit
experiments do not have interference patterns - hence the sum cannot
have an interference pattern either.

Consequently, if you have any way of knowing which slit the photon
went through (the "which way" information), then you cannot have an
interference pattern. This is what the complementarity principle means.

                                        Cheers
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