I always wondered if the electrons superposition is so fragile, why doesn't it interact with the wall or plate with the double slits? Do the slits have to be a certain size?

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    $\begingroup$ Is your question: why is there no wave function collapse upon interaction of the electron with the slit system? If so, please edit. $\endgroup$
    – my2cts
    Feb 11 at 10:24
  • $\begingroup$ physics.stackexchange.com/questions/588887/… $\endgroup$ Feb 11 at 15:13
  • $\begingroup$ The EM field controls it all. It's not fragile .... even single electrons eventually make the pattern. $\endgroup$ Feb 11 at 15:14

2 Answers 2


Even with light, you will see the photons interacting with the slit(s) / apparatus:

enter image description here

Note the green laser everywhere that is not a screen. Those photons have left the experiment.

Looking at one slit, for simplicity, with the slit being defined as only the unblocked path, the entire diffraction pattern is cause by everything but the slit. E.g., in Huygens principle, the lack of emitters outside of the slit is what causes the plane wave to spread out--same for Feynman's path integral, but Huygens principle is easier to visualize.

Diffraction determines slit requirements--if the diffraction patterns of the two slits don't overlap, it will be hard to see the interference pattern. (Note: many sources show the particles moving straight through the slits when the path is known: enter image description here


A diffraction pattern for a $2d$ width slit is the interference pattern for two slits of width $d$ with zero separation: it's always a wave.


There was an experiment in which the particles (idk what they were, but not light) interacted with some atom at the slit (maybe), and when it did, the coherence was lost with obvious results. It's on PSE, somewhere.


I don't care for quantum woo, esp in pop culture. It's misleading. Quantum Mechanics is an approximation, irl, there are no particles. They are quanta of a quantum field (which has no need to be localized), and out best computational versions of that has a known initial state (a plane wave beam), and a known final state (outgoing wave, hitting a detector), and all we can do is calculate the scattering amplitude. No particles. No woo. No fuss (renormalization notwithstanding).

  • $\begingroup$ a down vote should really come with a comment, otherwise it's useless, since I don't care about points, but I would like to know the complaint, whether noob or expert (I'll learn something from either). $\endgroup$
    – JEB
    Feb 11 at 19:01
  • $\begingroup$ What I still never understand is how a single wave passes through two slits and remains collected. Such a description makes it sound like a wave is made from smaller components which can recombine after passing through the slit. If that isn't the case (and I'm sure no one agrees it is) then why is the double-slit a factor at all? If a non-localised quanta is just a type of exchange and the centre of the double slit is an impenetrable barrier then why isn't the quanta affected so that it goes through one side or the other? I mean, are we saying the whole quanta gets beyond--I don't get it : ) $\endgroup$
    – Wookie
    Apr 3 at 6:01
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    $\begingroup$ @Wookie that's a legit concern, but it's best not to force quantum systems into our classical thinking. We often think of a wave function as a probability density of a point particle, which is just wrong. It's a complex probability amplitude (which admits no intuition not based on the math). QM is an approximation to QFT, where an electron is a minimum excitation of the field, not a point particle, and that excitation goes through both slits. $\endgroup$
    – JEB
    Apr 3 at 11:02
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    $\begingroup$ @Wookie Since we're trained on the EM field, where excitations come and go (a photon is its own antiparticle), it can give us the wrong idea. The electron excitation has conserved quantum numbers, and cannot just disappear, and it has mass: so it looks like a particle, but it's a field doing field things. The only way to get ride of it is with a positron excitation (W-bosons notwithstanding). $\endgroup$
    – JEB
    Apr 3 at 11:07

In this answer I'm going to assume quantum mechanical equations of motion actually describe reality.

If a quantum system is evolving without interacting with other systems to a good approximation, then it may exhibit a process called quantum interference. This means that in general the system acts as if exists in multiple versions each one corresponding to different possible values of some measurable quantity and those versions interact so that all the versions affect the outcome, see "The Fabric of Reality" by David Deutsch, Chapter 2. Saying those versions actually exist is called the many worlds interpretation of quantum theory but it is just an implication of quantum theory:


When a system $S_1$ interacts with another system $S_2$, then information about some measurable quantity $X$ may be copied from $S_1$ to $S_2$. In general, this process suppresses interference between the different versions: a process called decoherence:



After such an interaction, the state of the system is reset to a relative state corresponding to whatever version of the system you happen to see. Since interference isn't allowed between the different relative states you have no way to see a different relative state.

When an electron interacts with a screen with slits in it, some of the versions of the electron are absorbed by the plate. Other versions of the electron pass through the slits and aren't absorbed. This effectively copies information about the measurable quantity that answers the question: "was this electron absorbed by the screen or did it pass through the slits?" The versions of the electron that are absorbed don't interfere with those that pass through the slits. But two or more different versions of the electron that did pass through the slits can undergo interference.

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    $\begingroup$ we need to ban Many Worlds, imho. $\endgroup$
    – JEB
    Feb 11 at 14:30
  • $\begingroup$ @JEB Agreed. This answer is all about MWI and has absolutely no relevance to the question itself - which is about the slit. One of my biggest gripes is that answers like this are not removed and/or authors are not warned. to stop this nonsense. Flagging does not seem to accomplish anything. At a minimum, anyone presenting an interpretation as an answer should be required to disclose it as such. $\endgroup$
    – DrChinese
    Feb 11 at 15:01
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    $\begingroup$ @JEB If you have a counterargument against the MWI then why not explain that it and convince MWI advocates to discard the theory? $\endgroup$
    – alanf
    Feb 11 at 17:45
  • $\begingroup$ Well my counter argument to MWI is strictly phenomenological: No one has ever gotten tenure pushing MWI, including Hugh Everett and Sean Carroll. $\endgroup$
    – JEB
    Feb 17 at 15:14
  • $\begingroup$ @JEB That assertion, if true, is irrelevant to the truth or lack thereof of the MWI and has nothing to do with physics. I don't understand its relevance or why it would change anybody's mind. $\endgroup$
    – alanf
    Feb 19 at 10:46

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