I just recently watched a video where they were explaining the double slit experiment.

I understand that the interference pattern would be observed if you send photons or electrons one after the other. The conclusion that the author seems to make is that it is as though the particle/packet of energy travels through both the slits after somehow splitting into two.

Has anyone done an experiment where they send just one particle? (And end the experiment) Do you observe two points on the screen in that case or just one?

  • $\begingroup$ The conservation of energy still counts, so it would still show up at only one position. Only the probability of the 'particle' will be determined by the double-slit-interference-pattern. $\endgroup$
    – fibonatic
    Jul 12, 2013 at 12:20
  • $\begingroup$ BTW, The "one at a time" condition is not hard to achieve. You say "Well, the optical path is about 3 meters long so if we turn the source down until the rate is $10^{5}\,\mathrm{Bq}$ then the average occupancy of the device is $10^{-3}$." Or if that doesn't make you happy turn the rate down to 10,000 per second to get a mean occupancy of one in ten thousand and so on. $\endgroup$ Jul 12, 2013 at 12:47
  • $\begingroup$ The same Dr Quantum video in other Phys.SE posts: physics.stackexchange.com/search?q=url%3A%22DfPeprQ7oGc%22 $\endgroup$
    – Qmechanic
    Jul 12, 2013 at 13:29

2 Answers 2


Has anyone done an experiment where they send just one particle? (And end the experiment) Do you observe two points on the screen in that case or just one?

You don't observe two points - you get a probability. This is what the video was trying to indicate by showing dots on the screen. Those dots are the reason for postulating the role of probabilities in quantum mechanics.

The idea is that you can't predict where a single electron will hit the screen, the only thing you know is that some places are more probable than others. The probability is dictated by the probability density function which ostensibly can come from the quantum mechanical wave equation of the electron.

The probability distribution function for a single particle is the same function (aside from units/magnitude) as the intensity you see for a large number of particles. After all, that is always what we see when we observe the interference pattern. You just get enough particles that you can't distinguish single hits anymore.

  • $\begingroup$ Thanks. I did not have enough rep to upvote this answer or fibonatic's comment, but yes, very useful. I was put off a bit by the idea of the particle splitting. I will post a separate question about that. $\endgroup$
    – mehfoos
    Jul 12, 2013 at 12:54
  • $\begingroup$ @user1218748 I would suggest doing just that. If a new curiosity stems off of a prior question then it makes sense to ask a new question. The wording of the video (or any double-slit intro) has lots of references to higher physics. For instance, saying that the electron travels every possible path references path integrals. I never understood those very well myself. $\endgroup$ Jul 12, 2013 at 13:00

Yes, several people have done the experiment, and the photons/electrons are not split, you can get one hit at a time. Here is a second video .

The wikipedia two slit article has a build up of the electron interference pattern one at a time.


It is a probability pattern that shows the interference. The electrons and photons either go through one of the slits or the other, whole. Nothing mysterious is happening.

There have been experiments recently where one can detect which the slit the particle went through and still the interference pattern appears.

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    $\begingroup$ Thanks for the images, but isn't an experiment where one can detect the slit the particle went through and observe the interference pattern a violation of complementary principle?! I have heard of the controversial Afshar experiment, but I was under the impression that experiments such as Wheeler's delayed choice experiment pretty much conclusively proved that observing the particle can influence the experiment. (Not merely because the observation leads to a causal effect) $\endgroup$
    – mehfoos
    Jul 12, 2013 at 14:23
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    $\begingroup$ What experiments were recently made in which you know which way the particle went but you still get the inteference? $\endgroup$
    – alex
    Nov 20, 2013 at 19:00
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    $\begingroup$ @alex there is a link above to the wiki article, but the reference is behind a paywall. If you have access to a library you could see it. $\endgroup$
    – anna v
    Nov 20, 2013 at 19:35
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    $\begingroup$ If I send one particle now,another particle 6 months later and so on ,will I get interference pattern after few years? $\endgroup$
    – Paul
    Mar 25, 2015 at 2:43
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    $\begingroup$ Thanks #annav ! QM implies that which way information directly impacts the interference pattern. The article is fully consistent with this. Unfortunately the authors talk about wave function collapse, which is an unphysical concept . $\endgroup$
    – my2cts
    Apr 13, 2020 at 10:55

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