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Double Slit Experiment and Measurements

  1. Jan 31, 2010 #1


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    When a photon encounters a beam splitter, say a half-silvered mirror, there is a 50-50 chance it will be reflected or passed (tunneling). Does being reflected/passed constitute a QM "measurement"?

    Thanks in advance.
  2. jcsd
  3. Jan 31, 2010 #2


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    It could and generally would, but there are also cases where the results of the measurement are "erased". So you need to consider the entire context of the experiment.
  4. Feb 1, 2010 #3
    No, it's not a "measurement" in the sense that it doesn't collapse the wave function. The two alternate paths both have to be followed to account for the future behavior of the system. That's why you can later recombine the two beams and get interference.
  5. Feb 1, 2010 #4
    It depends on the interpretation you choose to work in. Any interpretation that might include considering just being reflected or passed as a measurement, however, has to introduce very careful provisos that take into account what is done with the beams subsequently. In other words, the entire context of the experiment is ultimately important.

    Personally, I prefer interpretations that take a measurement to have occurred only when a number has been written in a computer memory. Then a paper can be submitted to Physics Review Letters that says, "the raw measurement data (100MB) is available on a CD on request," and goes on to describe the statistical computations that were done using that data to show how well the data matches up with a proposed quantum mechanical model for the experiment. That sets the standard for measurement as "a PRL paper", in contrast to setting the standard for measurement as something like "it's in my head", or "it's in the head of someone who has a Ph.D" (which is a John Bell joke). I might be OK with a PRD paper as an arbiter of whether a measurement happened, for example, but perhaps not with a JMathPhys paper. Endless fun can be had deciding which journals' imprimatur is OK.
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