• Question: If you do an interference experiment with electrons (the double slit experiment, if you wish), the curve you get is similar to that of wave interference. However, if you try to watch said electrons, to see how this works (since it is rather puzzling, and doesn't quite make sense, because even if you fire off the electrons one by one you still get this interference pattern, EVEN THOUGH THEY ARE NOT INTERFERING) by by using a light source to see where the electron scatters the light, the interference pattern becomes like that of a particle, or bullets. So it's impossible to actually see what's happening to the electron to produce the first interference pattern - this is the uncertainty principle at work. Heisenberg states that if it were possible to measure the momentum and position of a electron simultaneously with greater accuracy, the quantum mechanics would collapse. Two parts to this question; 1) Why do you think the electron produces an interference pattern like that of a wave, even when you fire the electrons off one by one, and thus they cannot interfere with each other? 2) Do you ever believe that there will be a way to measure the precise position and momentum of an electron with the quantum theory collapsing, as Heisenberg states?

    Asked by fauxvive to Arttu, Ceri, James_M, Monica, Philip on 14 Jun 2011.
    • Photo: Arttu Rajantie

      Arttu Rajantie answered on 14 Jun 2011:

      1) The electron is not really a particle in the classical sense. You can understand some of its properties by thinking of it as a wave, and this is one of them.
      2) It is certainly true that in quantum mechanics a particle cannot have definite position and momentum at the same time, so if you could measure them, it would show that the quantum theory is wrong. Quantum mechanics works incredibly well, so I think this is unlikely, but in principle it is possible.

    • Photo: Philip Dolan

      Philip Dolan answered on 14 Jun 2011:

      I don’t know if it’s of interest to you, but position and momentum are only one combination of variables that you can’t simultaneously measure. Any pair of what are called ‘canonical variables’ cannot be simultaneously known have this property including energy and time (it is this energy-time uncertainty that give us zero point energy, which doesn’t quite explain dark energy). In fact this property was known before quantum mechanics came on the scene, but it was Heisenberg that used it to obtain his uncertainty relation.

      Uncertainty is one of the cornerstones of quantum mechanics, a similarly fundamental one is Schrödinger’s idea of a wavefunction (as Arttu said) it is best to think of particles only as wavefuntions, and depending on the circumstances that the wavefunction is presented with (i.e. interference slits, or a detector) it will respond to look like either a wave or a particle. But everything is, at it’s core, just a wavefunction.

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