An experiment that’s causing somewhat of a stir

Ah, it’s been a while since I heard ‘Copenhagen-interpretation’. You may say that said interpretation is the quantum mechanics sister to our well-known I only believe/It’s only real what I see. In terms of particle / wave duality, the interpretation states that I can only make an observation of the wave or particle aspect of a subatomic spacetime entity at any time.

Anyway, via Newscientist I heard of one Shariar S. Afshar performing a new take on the double-slit experiment with photons. He sets up an experiment by which he figures out through which slit a ‘photon’ passed and proves at the same time that an interference pattern is given.

Interesting stuff, and there is some commenting going on at Kathrin Cramer’s weblog. btw, here is a good picture of the double-slit experiment as seen in the current Newscientist issue.

I have actually two questions about this experiment:

  • While the Newscientist picture does suggest that the feat can be achieved to find out from which pinhole the photon came, isn’t the path that the photon took (between detector and pinhole) still unknown?
  • When Newscientist says that we are “…seeing evidence of light behaving as a wave and a particle at the same time”, is this statement based purely on our ‘classical’ understanding of the photoelectric effect in the detector? What I mean is; Although I know the two pinholes, and I may even be able to say where a photon was detected, what can I really say about the path that any ‘particle’ may have taken to get to the detector?
  • Is it correct that previous double-slit experiments measured the position of the photon directly at the pinhole (i.e. before the generation of the interference pattern) while Mr. Afshar determines the position after generation of the interference pattern? Could this matter in ruling out the buggy Copenhagen interpretation?

I posted these questions as comments at Kathrin’s weblog in the hope to get pointers.

Either way, I always thought that saying something like particle/wave duality tells more about our capabilities than it actually tells us about nature.


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Frank Quednau 2022