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I don't really know if my question even makes sense but, we know that for the double slit experiment, there is no way possible of telling which slit the electron will pass, prior to observing. Does that mean that if we saw the electron passing from slit 1 for example, if we were to travel back in time and observe the same electron again, would there still be a probability that the electron could pass from slit 2?

user279163
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    Time travel doesn't exist. I suggest you to change your example with a realistic one – Nicolas Schmid Nov 16 '23 at 15:09
  • I was asking if it existed @NicolasSchmid. – user279163 Nov 16 '23 at 15:16
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    @user279163 time travel doesn’t exist so we can’t speculate on results of experiments involving time travel without knowing the rules of this fictional time travel. – Jagerber48 Nov 16 '23 at 15:51
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    @Jagerber48, ok suppose the rule is you unwind everything, and come back in the same position you were prior to the result of the experiment. – user279163 Nov 16 '23 at 16:02
  • @user279163 You don't need to "rewind"; if you prepare an electron that has the same properties as the first one, then it's the same exact experiment. In your hypothetical time travel case there would be no way to differentiate the two electrons – BioPhysicist Nov 16 '23 at 16:13
  • Related/possible duplicate: https://physics.stackexchange.com/q/566573/50583 – ACuriousMind Nov 16 '23 at 16:18
  • The title of your question seems to have nothing to do with the body of your question. – hft Nov 16 '23 at 16:33

3 Answers3

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I don't really know if my question even makes sense but, we know that for the double slit experiment, there is no way possible of telling which slit the electron will pass, prior to observing.

The statement is incorrect: we do not know through which slit electron passed, because we do not try to observe it passing through a slit. If were trying to observe through which slit it passes (as the following sentence in the OP suggests), there would be no ambiguity.

What seems behind the confusion is the two ways the term observe is used in discussion of the two-slit experiment:

  1. we observe the pattern on the screen behind the slits.
  2. we may try to observe through which slit the electrons pass, e.g., by illuminating it with a stream of photons, and detecting, if any of them a scattered (from the electron)

The pattern observed on the screen can be of two types:

  • A. an interference pattern, as if through a wave diffracting from the two slits
  • B. a localized pattern, as if from a particles passing through slits.

If we try to observe through which slit electron has passed, we get pattern B - the electron behaves as a particle. If we do not try to measure its path - we get pattern A. If our observation of the path is not perfect, we can have some mixture of the both - we are not 100% sure, through which slit it passed, and some interference is still preserved.

Roger V.
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  • i was talking about 2nd option that you said. We wont be able to know which slit it is going to pass before observing it passing trough a slit using illumination for example. Just forget about the screen. – user279163 Nov 16 '23 at 18:29
  • @user279163 but in this second option there's no ambiguity - it passes through one slit. – Roger V. Nov 16 '23 at 19:02
  • yeah but which slit? – user279163 Nov 17 '23 at 14:31
  • @user279163 the one through which you saw it pass. We wont be able to know which slit it is going to pass before observing it passing trough a slit using illumination for example. - before doesn't make sense here. We observe the interference pattern on the screen, which is present or not, depending on whether we try to observe through which slit electron passed. There is no a "passing process" developing in time here. – Roger V. Nov 17 '23 at 14:37
  • i am not talkin about the screen, suppose that we can observe 100 percent of the electrons which slit they passed from. there wont be any interference on the screen but who cares about the screen? I am asking for an indivual electron, which slit it is going to pass? you said the one trough which you saw it pass. I am asking is there any way knowing this apriory. The answer should be no here because before measurement, the wave eq. was not collapsed, so it would be again random. So if we travel back in time and look at the same electron, it can pass from any slit. I hope it makes sense now. – user279163 Nov 17 '23 at 16:38
  • @user279163 a simplified model is to consider electron in a superposition of states corresponding to each slit: $|\psi\rangle =a_1|1\rangle +a_2|2\rangle$. Then you find it in state $i$ with probability $ |a_i|^2$. But there's nothing specifically related to two-slit experiment in this - take a look at Feynman's discussion of Stern-Gerlach. – Roger V. Nov 17 '23 at 19:56
  • yes it was just a example. I was just giving an example about inherit randomness of quantum particles. Can you answer my question now? – user279163 Nov 18 '23 at 06:50
  • If you observed an electron in a state (as passing through a certain slit), you collapsed the wave function, and now it is in this state with 100% certainty. But the next electron might collapse into the other state. In the canonical interpretation of QM, all the experiments are performed repeatedly on a very large number of particles. – Roger V. Nov 18 '23 at 09:35
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A measurement is something special in quantum mechanics, which changes in a non reversible way the state of your system. Your example with time travel does therefore not make sense, because unlike in classical mechanics, you can't take the physics equations to go back in time before a measurement in quantum mechanics.

But let's say that you don't go back in time but you have several electrons which are exactly in the same state. In this case, yes, it is possible that you will sometimes measure an electron at slit 1 and sometimes at slit 2.

Remark: I think that you wan't to use "the same electron" for both experiments, but actually, there is no such thing as "the same electron". Electrons are undistinguishable from each other, and there is no way to tell if we sapped an electron with another. So two electrons prepared in the same state are really 100% identical, no need for time travel in your though experiment.

Nicolas Schmid
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If the double slit shows a diffraction pattern for electrons, it is because the electron went through both slits.

It is common to say that a photon or an electron is sometimes a particle and sometimes a wave. This isn't really right. They are quantum mechanical things something like a classical particle and something like a classical wave, but really neither one.

They are like a wave in that they can go through both slits. They are like particles in that they can hit one atom on the screen and miss all the others.

Quantum randomness comes in when you look how often each atom gets hit. It is predictable to a degree, but only to a degree. Any atom on the screen can be hit. It is impossible to say ahead of time which one will. Ones where the diffraction pattern is most intense are more likely to be hit.

mmesser314
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  • this doesn't really answer my question. But I can ask a similar one based on your answer. if the electron E hits the atom A on the screen, if I went back in time and do the experiment again, will electron E again hit the atom A or does it have a chance of hitting B or any other atom? In my opinion, since we go back in time, the wave equation was not collapsed yet so we again wouldn't have any possible way of determining which atom it will hit, so the answer would be any atom not only A. – user279163 Nov 16 '23 at 15:11
  • You can't go back in time, but you can send another electron F just like it through. It has the same chance of hitting A or B or any other atom that E had. – mmesser314 Nov 16 '23 at 15:14
  • I was asking about the same electron, I guess my question doesn't really make sense. – user279163 Nov 16 '23 at 15:15
  • It does. It just isn't possible to go back in time to do what you want. But electron E cannot be set up so that it goes through both slits and then hits a predictable atom. Both theory and experiment confirm this. Sending electrons through over and over is just like sending the same electron through over and over. – mmesser314 Nov 16 '23 at 15:17
  • @user279163 Using a new electron is similar to your "going back in time"; there wouldn't be anything different between the two electrons at all. No way to tell the difference – BioPhysicist Nov 16 '23 at 15:35