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Double slit electron experiment

  1. May 30, 2008 #1
    It seems physicists have come to the conclusion that a single electron travels through both slits at the same time, otherwise there wouldn't be an interference pattern when only one electron is being sent through the apparatus. Apparently this experiment has been done in real life. You can't send only a single electron though the apparatus at a time, can you? You have to send a bunch of them at a time, I would think. If so, how do we know that the interference wasn't caused by two or more electrons entering the slits and causing the interference pattern?
  2. jcsd
  3. May 30, 2008 #2
    this is what I meant
  4. May 31, 2008 #3


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    There will be an interference pattern even if you send one photon (or electron) through at a time. (I think it's easier to do it with photons). I don't think it's hard to get one photon at a time to go through. You should just have to lower the intensity of the light source until it emits one photon at a time.

    Feynman's book "QED: The strange theory of light and matter" explains this stuff very well. Everyone who's interested in physics should read it.

    I would say that the claim that the "photon takes both paths" is misleading. Given an emission event, we can calculate the probability of each possible detection event. We can, in principle, do this by associating a probability amplitude (a complex number) which each path from the emission event to the detection event, and add them up to get a final amplitude, which then tells us the probability.

    Quantum mechanics isn't very confusing as long as you think about it as an algorithm that lets you calculate probabilities of possibilities. It's when you start to think about what "really happens" that it gets confusing. This might mean that it's simply wrong to think that quantum mechanics describes the universe. A theory doesn't have to describe anything to be scientific. It just have to be able to predict the outcomes of experiments, and quantum mechanics does that better than any other theory.
    Last edited: May 31, 2008
  5. May 31, 2008 #4


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    Both have their problems. From an experimental point of view, creating electron beams is a bit more complicated than creating photon beams, just as a crt is a bit more complicated than a flashlight, but when it gets to single electrons versus single photons electrons are usually better as they are fermions and therefore the the Pauli exclusion principle works for them and they have the tendency to show antibunching anyway. I think there was even a report on a coherent single electron source somewhere in Science last year.

    Photons are bosons and have the tendency to bunch. Lowering the intensity of the light source leads to a thermal or coherent state with low average photon number, which is still a superposition of several Fock states (photon number states). If you really want to have just one photon at a time, you need to get the field to the n=1 Fock state, which is possible by using single atoms or single quantum dots as a light source, but complicated.
  6. Jun 4, 2008 #5
    From the moment you start to think of a particle as a probability wave, it gets straight forward - it is easy to imagine a wave that goes through both slits. It only stops being a probability wave when it appears at one point according to the probability distribution (wavefunction collapse). Which typically happens in an interaction with another particle.
  7. Jun 10, 2008 #6
    subtle for quantum mechanics.
  8. Jun 10, 2008 #7
    In other words?
  9. Jun 10, 2008 #8
    If two slits are open and we see an interference pattern, I agree that it is straight forward. But if there is only one slit open ...? In this case I usually imagine something analogous to the typical water wave after throwing a stone into the pond. The screen recording the photons would be analogous to a cross cut through the water wave at some point in time. But then I don't understand why there is no wavy pattern on the screen when only one slit is open. It would be more regular than the interference pattern, but still not just a single-maximum curve?

    What is wrong with my analogy of the water wave here?

  10. Jun 10, 2008 #9
    In 1980, experiments were done by Japanese scientists at Hitachi laboratories to determine the nature of electrons and photons being fired through double-slits. In one such experiment, electrons were fired, one at a time, through a double-slit. The electrons were detected by a big (TV like) screen on the other side of the slits, with each electron making a spot of light as it arrived and hit the screen. This confirmed that each electron acts as a particle (because it only made one spot...then two...then three....etc.)

    However after thousands of electrons were fired, one at a time, the pattern spots of the screen looked like a classic interference pattern for waves. It was as if each electron "knew" where all it's predecessors had gone, where all electrons coming behind it would go, and where it was supposed to be in the pattern.

    It was very weird....yet it is true.
  11. Jun 10, 2008 #10
    This is just the nature of light and it's particle-wave duality. Think of it as 10,000 surfers all riding one surf wave at the same time. The surfers are all moving together in a wave like pattern, however each individual surfer can still be considered a "photon".
  12. Jun 12, 2008 #11
    This analogy would include that if one of the servers is swallowed by a shark and leaves a red patch of blood on the surface, all other surfers disappear at the same instant without a trace:biggrin: Not each one represents a photon, but collectively they represent a single photon.

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