I read about a double slit experiment where the detector clicks when a photon goes through the slit. When the detector doesn't click, it means the photon must have gone through the other slit, and a particle pattern still forms. I can't find the experiment that's being referred too though. Does anyone know how I can find it?

  • $\begingroup$ Do you mean "Double slit experiment for one photon at a time"? $\endgroup$
    – sslucifer
    Commented Oct 25, 2021 at 22:37
  • $\begingroup$ yes, and a detector on one slit. A detector that clicks if a photon passes. $\endgroup$ Commented Oct 26, 2021 at 19:02
  • $\begingroup$ @JenSanchez If a detector detects a photon than the photon must have been absorbed by the detective. A photon cannot pass the slit and be detected at the slit. As for the experiment, just go to YouTube and type double slit experiment one photon at a time. $\endgroup$ Commented Oct 27, 2021 at 1:02
  • $\begingroup$ I mean and experiment where there's a detector on one slit (say slit B). Photons are coming one at a time. If the detector fails to click, we know it must have gone through the other slit (slit A), and it acts like a particle. I read about it recently, but there wasn't a reference. $\endgroup$ Commented Oct 27, 2021 at 3:36
  • $\begingroup$ Don't go out of your' way. If It was done as described, i'll find it. I've found a few refrences to it. The idea that the knowledge of which slit, even without interaction, could change the results blows my mind. $\endgroup$ Commented Oct 27, 2021 at 3:40

1 Answer 1


This was a thought experiment .... it is impossible for a photon to produce a click and then carry on to the screen. If a photon makes a click it is detected and that's the end of the photon. For electrons an actual experiment was done using light to detect which slit ... and this caused the electrons to stop making the pattern.

Also you can read this answer for more insight: How does the double slit experiment ensure phase coherence?

  • $\begingroup$ I found an experiment that's pretty close to what I was looking for. I'm imagining photon going to two slits, with a detector at one slit. When the photon goes to the slit without a detector, it still produces a particle pattern. The wave would have to know the detector is there, and without setting it off, go through the other slit. Physics is trippy. Thank you! I'm learning. $\endgroup$ Commented Nov 7, 2021 at 6:00
  • $\begingroup$ Here is a paper that sort of measured which slit for photons, it uses "weak" measurement. physics.utoronto.ca/~aephraim/PWMar13steinberg-final.pdf. $\endgroup$ Commented Nov 8, 2021 at 17:29
  • $\begingroup$ If we are using photons you can not use a detector for which path. If you put a detector on one slit than that slit is blocked! if you put it over half the slit then a portion of the photons will get blocked and some will get by to make the interference pattern. $\endgroup$ Commented Nov 8, 2021 at 17:33
  • $\begingroup$ Every photon figures out a path .... the path is constantly changing as objects move about .... it is all due to the EM field, virtual photons and real ones. Example .. a photon leaves a star and travels for a million years, gets to earth and then a scientist places a mirror in front and sends it all the way back to the star! The photon reacts at the speed of light, figuring out its new path ..... $\endgroup$ Commented Nov 8, 2021 at 17:38
  • $\begingroup$ the DSE has been mysterious for over a hundred years. A lot of the bad science and confusion comes from being forced to think in classical terms .... like photons interfered and cancelled (violation of energy). Feynman and Dirac figured out that each photon must figure out its own path which is true ... but they were unable to say how. They could have spent more time on it if it was truly important but the nuclear age was dawning and the photons behaviour was well enough understood for their purposes. $\endgroup$ Commented Nov 8, 2021 at 17:49

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