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I Concerning the Nature of Quantum Measurement...

  1. Jul 3, 2017 #1
    I'm well aware of the common adage and quantum fact that, until a particle is measured by some sort of instrument, it exists in a state of superposition, can interfere with itself, etc. My questions pertains to the definition of "measurement".

    In order for something to qualify as a measurement and collapse the wave function, does a human being actually have to observe the results? For instance, let's say you had a Mach–Zehnder interferometer at your disposal, and placed detectors along the photon trajectories that told you down which path the photon traveled. Abiding by the Mach–Zehnder thought experiment, this would collapse the wave function, and you'd have 50-50 odds of seeing the photon at either detector at the end of the experiment. However, what if nobody actually looked at the results? What if the devices made the measurement, but - say - the monitor displaying the results was turned off?

    The experimenter had no knowledge of what either sensor had actually measured. Will such measurement still collapse the wave function, or will the photons continue on, as though nothing ever happened, and interfere with each other as they would had no measurement taken place? Or will the detectors somehow be included in an even larger wave function, altering the results but not into the 50-50 odds we see when someone makes a direct observation?
  2. jcsd
  3. Jul 3, 2017 #2


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    The quick answer is "no".

    The idea that a conscious observer has anything to do with measurement was abandoned more than eighty years ago and forms no part of the modern theory of quantum mechanics. Unfortunately, it leaked out in the popular imagination and lives on as an urban legend - one of those things that everybody "just knows" but isn't true. We have a bunch of threads outlining the history of how this idea came to be briefly considered and why it was then rejected, and you might also give David Lindley's layman-friendly book "Where did the weirdness go?" a try.
  4. Jul 4, 2017 #3


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    To see how difficult question this is, consider the following: Would a chimpanzee watching the apparatus count as a conscious observer? What about a cat? A mosquito? We don't even have a definition of consciousness that sets a clear enough limit on what counts as "conscious" to use it as a criterion in a law of physics.

    Also, if there's only one person monitoring the experiment and his colleague is waiting in another room, would the collapse initially happen only from the viewpoint of the person who's present and the situation in the lab still be in a "superposition" of many results having happened from the viewpoint of the person outside (before he goes in and asks what happened)?

    This kind of questions are not really physics questions as they can't be tested experimentally.
  5. Jul 4, 2017 #4


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    The question whether a certain physical process "qualifies as a measurement" is ill-posed.

    The term "measurement" doesn't refer to a physical process alone but includes the intention of someone to take data. The Copenhagen interpretation of QM says that if someone is going to take data use this set of rules (including the collapse of the wavefunction) to predict the outcomes. It allows only limited statements about what happens if nobody takes data.
    Last edited: Jul 4, 2017
  6. Jul 4, 2017 #5


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    So the meaningful question is "What can we say about the physical processes during a measurement?". The answer to this question is complicated.

    If we include the measurement apparatus in the quantum description, certain parts of the Copenhagen rules can be derived but other parts can not. The key notion here is called "decoherence". Unfortunately, I don't have an idea how to explain it concisely. One example to read about it would be "Decoherence, the measurement problem, and interpretations of quantum mechanics" by Maximilian Schlosshauer (https://arxiv.org/abs/quant-ph/0312059).
    Last edited: Jul 4, 2017
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