For explaining the double slit experiment, many such as Jim Al Khalili (https://www.youtube.com/watch?v=A9tKncAdlHQ) start with an example of having one slit - the pattern on the screen is that which you expect from particle-like behaviour. When there are two slits the probability waves of two electrons interfere to create an interference pattern arising from their wave-like behaviour. Since particle like behaviour when observed is still a wave, just concentrated at one point probabilistically, since the electron is unobserved, why does a single-slit experiment not also show wave-like behaviour and spread out (like light would)?
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$\begingroup$ Please see my answer and links here for single electron at at a time double slits physics.stackexchange.com/questions/720089/… . single electrons have a dot (point like, particle like) footprint . see also en.wikipedia.org/wiki/… $\endgroup$– anna vCommented Aug 6, 2022 at 18:28
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$\begingroup$ This can still all be explained on a particle basis. When electrons are accelerated, especially through a slit they emit billions of coherent photons. The photon spread out from those points creating a pattern that directly affects the Trajectories of the electrons. The interference comes from the two edges of the slit $\endgroup$– Bill AlseptCommented Aug 6, 2022 at 18:31
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2 Answers
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An electron beam passing through a single slit does spread out and forms diffraction patterns.
answered Aug 6, 2022 at 13:13
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$\begingroup$ Thank you very much for your answer. Does it still do so for an electron? (I am asking as the Wikipedia page talks about light, and I could not understand quite a bit of it) $\endgroup$ Commented Aug 6, 2022 at 13:17
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1$\begingroup$ @Jonathan AN electron can only ever be located at one place at any given moment in time. What "spreads out" is the probability distribution function that tells you where to go looking for the electron. If you send thousands of electrons through the same slit with the same energy, the pattern becomes apparent as each of those different electrons ends up in a different place. $\endgroup$ Commented Aug 6, 2022 at 13:30
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Because the video is wrong, and it's a huge problem on YouTube videos even by reputable people. You need 2 overlapping (sum-of-squares) diffraction patterns (hint: wave) when you have which-way information. When you lack that information, an interference pattern (square-of-sums) modulates the overlapping diffraction patterns.
If the single pattern is an isolated stripe, opening the second slit will also produce an isolated stripe, not because there is no wave behavior, but because the wavelength is too short to observe it.
In an MIT video, they actually do the experiment:
