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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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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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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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