I have been watching a lot of videos of the double slit experiment and trying to wrap my head around it. I found examples of where the experiment has been done, however the videos cut out and only simulate the effect an observer has on it. I understand that the wave interference pattern collapses when an observer is put into play.

Is there a video that displays the observer effect in comparison to the regular double slit experiment? I would like to see how observing the particles before and after they go through the slit changes the resulting patterns.

Follow up question:

I watched this video https://www.youtube.com/watch?v=Iuv6hY6zsd0

This video demonstrates the interference pattern created by sunlight in a box. I understand that this is how light behaves (photons? Particles? I am not sure ...)

Question: would it be possible to do this experiment, and somehow "measure" the light before or after it goes through the slits, to create the pattern that is created when observing?

And if not, why is that?

  • 3
    $\begingroup$ Possible duplicate of The observer detail of double slit experiment $\endgroup$
    – stafusa
    Mar 12, 2018 at 16:22
  • $\begingroup$ The results of this experiment, in a strange sense, tell you what you already know. If you don't know which slit the photons are going through, you get a wave result in the interference pattern. If you put a detector on one or both slits, you know which slit the photons are going through, and you get a particle result in the interference pattern. In effect, the observer becomes part of the experiment whether he wants to or not! $\endgroup$ Sep 18, 2018 at 1:45
  • $\begingroup$ You are not allowed to watch the videos of the experiment without an observer. $\endgroup$
    – M. Enns
    Sep 29, 2019 at 2:03
  • $\begingroup$ In this research paper there are some videos that seemingly captured by a low-noise camera: Video recording true single-photon double-slit interference, American Journal of Physics 84, 671 (2016) by Reuben S. Aspden and Miles J. Padgett. $\endgroup$
    – MimSaad
    Nov 2, 2021 at 7:26

2 Answers 2


Great question! I suspect the reason you can't find videos (although I haven't looked for them myself) is because most of the videos of interference will be videos of photon interference, since that is the easiest kind of interference experiment to do. However, the only kind of measurements we can perform on photons in this experimental context are what we call "destructive" measurements: for example, you could just block off one of the two slits, and then for any of the photons that get through you will know which slit they went through... but you will be destroying the other half of the photons.

This is therefore not as compelling an illustration of the collapse of interference patterns as an experiment in which you can measure which slit the particles go through while still allowing them to pass through the slits. For this, you need to use some other kind of particles, such as electrons. This has been done, and the interference pattern indeed collapses when you measure which slit each electron passes through, but I still don't know if there is a video...

  • $\begingroup$ One option to do this with photons is to use a pair of polarization filters. Each incoming photon is measured either horizontally polarized or vertically polarized, then passes through the corresponding filter, producing a pair of overlapping single-slit patterns. Removing the filters then produces the double-slit pattern. $\endgroup$ Oct 31, 2021 at 15:23
  • $\begingroup$ @JohnDvorak Can you sketch or link a drawing of your setup $\endgroup$ Oct 31, 2021 at 16:25
  • $\begingroup$ @BillAlsept traditional double slit setup, but a polarizer can be placed before each slit. One slit gets a vertical polarizer, one gets a horizontal one. Clockwise and anticlockwise also work... although on second thought with linear polarizers you can turn off measurement just by turning the filter 90 degrees. $\endgroup$ Oct 31, 2021 at 16:37
  • $\begingroup$ @JohnDvorak Yes if you have polarizers before the slit that are perpendicular to the slit then nothing will get through that’s slit. $\endgroup$ Oct 31, 2021 at 16:48
  • $\begingroup$ ... huh. Would circular polarizers after the slit work? $\endgroup$ Oct 31, 2021 at 17:20

I know it's a while since you posted your question but I wonder if you ever found the type of video you were looking for. I ask because I have found the same thing - that it is hard to find an actual video of the interference pattern disappearing when a measuring device is introduced, surprisingly hard to find given the significance of the effect.

One reason may be the nature of the experimental results, e.g. link below to delayed-choice quantum erasure experiment shows the experimental results indicating interference and non-interference:


If you have found a suitable video in the meantime I would be very interested in hearing.

  • 1
    $\begingroup$ I think you will always at least get a single slit diffraction pattern. There is never a case where only two clean lines appear behind the slits. I had seen animations where two lines appear but never a real photo. As long as we’re talking about visible light this is the case $\endgroup$
    – Lambda
    Nov 24, 2018 at 16:28
  • $\begingroup$ @Lambda You’re absolutely right! You always have single slit interference which also has nothing to do with an observer or not. Observation is just a measurement and measurement is intercepting the photon before it gets to the detection screen. The more photons you take out of the experiment the more the interference pattern goes away. $\endgroup$ Oct 31, 2021 at 16:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.