Re: Why the fuss about Afshar's experiment?

On Feb 18, 1:57 pm, "Dave" <n...@xxxxxxxxxxx> wrote:
"Gerry Quinn" <ger...@xxxxxxxxxxxxxxxxxxx> wrote in message

news:MPG.20426a8f60772d3b98b4a7@xxxxxxxxxxxxxxxxxxx



I read in the New Scientist of an experiment by Shahriar Afshar in
which he modified the standard two-slit experiment (using photons). He
installed a lens, mirrors and detectors behind the slits to tell which
pinhole each photon went through (by testing what direction it came
from). So far, so normal. We expect that the photons will now behave
as particles, and will not show an interference pattern.

He then inserted wires between the slits and the lens to block off the
regions where, if the photons were displaying an interference pattern,
the pattern would show dark bands. He found that there was no
diminution of the photon count, suggesting that in some sense the
interference pattern must still be there.

On the face of it, this obviously disagrees with Bohr's complementarity
principle. Apparently it caused quite a fuss and some hostility.

But the complementarity principle, surely, is not a law of physics,
just a description of a consequence of the laws of physics, which in
certain subtle cases may be incomplete.

As far as I can see, Afshar didn't actually make any *measurement* that
standard quantum theory doesn't allow him to.

I'm prepared to bet that if he sets up photographic plates at the two
detector positions, images of the wires will always be just out of
reach due to scattering of the photons. If he makes the wires wider,
the photon count at the detectors will drop and he won't be measuring
all of them as particles, meaning that the point of the experiment will
be vitiated. Certainly if he can 'photograph' the very narrow wires in
this fashion, quantum theory will have a real problem, but he hasn't
done that yet.

Am I missing something?

- Gerry Quinn

i am not familiar with that experiment, but did he find something he wasn't
expecting? or that he thought disproved some other theory? it sounds
perfectly expected to me, i would expect photons to display both wave and
particle properties at the same time, so even if they were detected going
through one slit or the other they still make an interference pattern.

Well yeah. The interference pattern was apparently detected even
though it was known which slit the photons passed through.

.

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