This video shows the change of a photon's interference pattern in real time of the Young's single and double slit experiment.

In this video it is claimed that by adding a detector to view how the photon moves through the slits the wave collapses and the screen will not show the interference pattern. The photon now behaves as a point particle.

Where can I see video evidence of an experiment showing this happening "real time"? That is, run the experiment like in the first video, showing the interference pattern, and then turning on the slit detectors, showing the interference pattern change instantly.

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    $\begingroup$ "The photon" never behaves as a point particle. It always behaves like a quantum. By adding a detector in one of the slits we are simply performing a different quantum experiment (a single-slit experiment), so the outcome is different. You can perform single slit and double slit experiments at home. I like to make the slits by scratching the backs of a cheap mirror with a scalpel knife, but you can also cut them out of aluminum foil. With a bit of experience it's a sixty second experiment. It also doesn't teach you anything about wave function collapse, which does not exist. $\endgroup$
    – CuriousOne
    Commented Jun 19, 2016 at 2:17

2 Answers 2


The answer by Craig Gidney is quite adequate for the question, but I want to address the word "collapse" in the title, since search engines will be homing in on it. From webster.com

1: to fall or shrink together abruptly and completely : fall into a jumbled or flattened mass through the force of external pressure <a blood vessel that collapsed>

2: to break down completely : disintegrate <his case had collapsed in a mass of legal wreckage — Erle Stanley Gardner>

3: to cave or fall in or give way <the bridge collapsed>

4: to suddenly lose force, significance, effectiveness, or worth <fears that the currency may collapse>

5: to break down in vital energy, stamina, or self-control through exhaustion or disease; especially : to fall helpless or unconscious

6: to fold down into a more compact shape <a chair that collapses> Definition of the word "collapse", Webster's dictionary

Note how the word describes a physics situation.

A wave function is a mathematical formula with complex numbers, posited for all particles in the quantum mechanical framework, from which the classical dimensions we live in emerge. It is a mathematical expression of a very successful model which, when squared with its complex conjugate gives a probability density distribution for observing with real numbers the problem at hand. In the case of the double slit experiment, the probability of finding the photon at the specific (x,y) of the screen.

The wavefunction exists in our copy books and our computers as a mathematical formula valid continuously. It is an unfortunate label that the word "collapse" has been attached to any property of the wavefunction. The wave function does not break down in any of the senses of the definition of the world collapse. It is always there, in our copy books and computers. A single measurement picks up an instance, and accumulation of measurements gives the probability distribution that the wavefunction so successfully models.


Take this paper which gives Probability of delivery within x days of a given date, the date given by the doctor from the data the mother gave.


The birth of a baby will be an instance of this plot, which accumulated with more instances should verify the distribution shown. Is anything in any logical way collapsing, according to the definitions of Webster?

I hope this makes clear that collapse is a wrong word to use for a mathematical distribution, attributing reality values. It is at worst an anthropomorphic word, giving human attributes to a mathematical formula, at best a misguided identification of the complex mathematical formula to the real formula of a collapsing balloon. The wave function is not a balloon either.

  • $\begingroup$ +1 Excellent answer! To support your point that the double slit experiment does not have much to do with the collapse of the wavefunction may I point to this answer of mine: physics.stackexchange.com/questions/259075/… $\endgroup$ Commented Jun 19, 2016 at 11:20

Note that your first video is showing a simulation of the experiment. From the transcript:

We don't have the equipment to do it for you, but we can show you a simulation of what you would see. Right now, you are watching an animation showing what we would expect to see if we could do the double slit experiment with only a small number of photons in a system at a time.

Also note that "detectors prevent interference" is a prediction of quantum mechanics. It's not exclusive to collapse interpretations.

With those noted...

A paper that does an actual experiment and records the results is 'Controlled double-slit electron diffraction' by Roger Bach et al from 2013. They use a blocker instead of a detector, though.

The final measured pattern for various positions of the blocker (false color):

Various distributions as slit is blocker varied

The electron-by-electron build-up of one of the patterns:

electron-by-electron build-up of one of the patterns

I realize that the first gif is probably what you had in mind when you talked about the interference pattern disappearing "in real time", but the actual real-time experience is more like the second gif.

I realize that you wanted a recording of an experiment with a detector, instead of a blocker. The problem with showing a recording of those experiments is that they tend to not be quite so visual. I doubt there's anything nearly as good as the two gifs above.

For example, Kim et al.'s delayed choice quantum eraser experiment used electronic coincidence counters instead of screens and required separating the "with-erase" and "with-reveal" runs after the fact. There wasn't any screen to record, just some photon counters being put into various positions and clicking together at various rates.

There are lots of youtube videos showing the detector-removes-interference idea in principle, such as this one using polarizers, but because they don't limit things to one photon at a time they don't require quantum mechanics to explain.

  • $\begingroup$ After I wrote my comment to the OP, I took a knife and a piece of aluminum foil and I cut two slits using a business card as a straight edge. That took me all but two minutes, one minute of which was spent on finding the business card. Then I held a cheap red laser level behind the newly created double slit and what I observed looked eerily like the first video. Now, what did I learn about the "collapse of the wave function" from that, though? $\endgroup$
    – CuriousOne
    Commented Jun 19, 2016 at 3:39
  • $\begingroup$ @CuriousOne I didn't call out the asker's misconceptions about "collapse" (the comments and the other answer are already doing that), but I did try to point them at useful resources and focus on what actually happens. $\endgroup$ Commented Jun 19, 2016 at 6:50
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    $\begingroup$ I apologize if I misconstrued your answer. I did, indeed, try the experiment to prove to myself that I didn't misrepresent how long it takes to perform, but after decades of trying I still can's see the slightest hint of quantum fields in the double slit experiment... or anything that would, in the faintest, allow us to derive quantum mechanics from it. I think that's worth pointing out. $\endgroup$
    – CuriousOne
    Commented Jun 19, 2016 at 8:29
  • $\begingroup$ The polarizer experiment doesn't demonstrate that it is the knowledge of the photon's path (i.e. the detector) that results in removing the interference. Rather it shows that horizontally and vertically polarized photons do not cause an interference pattern. $\endgroup$
    – Cloudyman
    Commented Dec 4, 2021 at 13:03
  • $\begingroup$ And the 'eraser' of a 45-degree polarized filter, doesn't demonstrate that erasing the knowledge of which slit the photon passed through re-creates the interference pattern, but rather simply that photons that have the same polarization do cause an interference pattern. i.e. the photons, after being horizontally and vertically polarized, then recombine to have the same polarization, which then results in an interference pattern again. $\endgroup$
    – Cloudyman
    Commented Dec 4, 2021 at 13:03

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