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B Measuring velocity and interference?

  1. Mar 24, 2017 #1
    If I measured the velocity of an electron what happens to its position? Does it collapse into a specific position or is it still uncertain?
    Here's a thought experiment... we measure the velocity of an electron and then send it through the double slit at the same time. Does the electron still create an interference pattern?
  2. jcsd
  3. Mar 25, 2017 #2


    Staff: Mentor

    The way you are asking the question presupposes a collapse interpretation. That's not a good idea. It's much better to focus on the things that are not dependent on your choice of interpretation.

    Also, velocity and position are observables. It's a good idea to keep observables in QM conceptually distinct from the wave function/state of the system (in this case, the electron).

    All that said, before your question can be answered, you need to specify how you are measuring the electron's velocity. There are ways of doing it that involve measuring the electron's position, and there are other ways that don't. The answer to your question depends on which way you pick.
  4. Mar 25, 2017 #3
    What about in the penning trap, does it collapse the position in order to find the velocity? the experiment does effectively determine
    the particle's velocity, since the measured image current is proportional
    to this parameter.
  5. Mar 25, 2017 #4
    The penning trap shoots radio frequencies at the particle in so it can create side bands to the particles harmonic motion
  6. Mar 25, 2017 #5


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    Staff Emeritus
    Science Advisor
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    Yes, it will still create an interference pattern. Measuring where the electron is BEFORE it enters a double slit doesn't remove the superposition of paths that the electron can take at the slit.

    So I can use something such as a loop of wire, and wait for a signal in that loop, because that tells me that an electron has passed through it (thus, giving me a position of the electron. After that, it can do whatever it wants, such as going into a double slit and doing its thing.

    In particle accelerators, we do this all the time where we make a "non-destructive" determination of electron position in the accelerator beam pipe, either by using BPMs (beam positioning monitors), or ICTs (integrated charge transformers).

  7. Mar 25, 2017 #6


    Staff: Mentor

    Once again, you should not be asking questions in a way that presupposes a collapse interpretation.

    It's not proportional to any velocity observable, as far as I can tell, so no, the experiment does not "determine the particle's velocity". In other words, the operator that the Penning trap realizes is not, as far as I can tell, a velocity or momentum operator, or anything equivalent to it.

    The term "velocity" is used in some discussions of the trap (or related terms like "classical trajectory"), but it seems to refer to an internal parameter of the wave function, not to an observable.
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