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Let me say that my previous comment was unfair. It's unfair to say that no one sane would consider quantum measurement related to consciousness. Rather its an e

Let me say that my previous comment was unfair. It's unfair to say that no one sane would consider quantum measurement related to consciousness. Rather its an extremely fringe position at this point. It's certainly a deep mystery of nature and I don't claim to have an answer. Quantum erasure experiments strongly suggest to me that measurement has to do with information about the system that entails constraints on the evolution of the system. Nature seems to abhor inconsistency. If the state of the system is such that one can in principle derive constraints on its evolution up to this point, then nature ensures its evolution up to this point is consistent with that information. A measurement then is just any interaction with the property of imparting a constraint on the evolution of the system up to now.

mitthrowaway2 2y ago
But that seems circular; what determines whether an interaction imparts a constraint, or doesn't? Let's say that I send a photon through a beam-splitter which will deflect its momentum either right or left. QM predicts that the photon at the output of the beamsplitter will be in a superposition of both trajectories, and we can prove this by adding mirrors that combine both possible output beams back together with another and generate an interference pattern. The interference pattern will be detected by a sensitive photodiode array, capable of detecting individual photons. Would you say the interaction with the photodiode array counts as a measurement? And the interaction with the mirror does not? How does QM know whether a photon interaction with a bunch of atoms in a mirror coating, vs in a silicon diode, creates a constraint on the evolution of the system? Why is one interaction measurement-like in nature, but not the other? Is there some property of silicon diodes that make them immune to being put into quantum superpositions? I would have said that quantum erasure experiments make by far the most sense if you consider measurement to never happen, and the result is merely a superposition of both outcomes. The photon is put into a superposition of both states when it reaches the beamsplitter, the detector screen is put into a superposition of both states when the superposition-photon reaches it, and so are you in turn when you observe the detector screen.
- hackinthebochs 2y ago
  > Why is one interaction measurement-like in nature, but not the other? The mirror reflects all the energy of the photon whereas the photodiode absorbs the energy and its state is altered by it. How nature knows the difference is a deep question. Presumably the rules governing nature ensure this consistency. What that looks like is anyone's guess. >I would have said that quantum erasure experiments make by far the most sense if you consider measurement to never happen I agree, but then this interpretation comes with the exponentially growing cost of actual branching. In my view, this rationally must be the interpretation of last resort, for reasons stated in the other threads.
  - mitthrowaway2 2y ago
    Counterintuitively, the best sensors actually behave just like mirrors themselves, re-emitting a reflected photon rather than absorbing any of the energy of the incoming signal. This increases the signal-to-noise ratio, because the E-field magnitude at the sensor is doubled, as it combines the constructive interference of the incoming and reflected photon. And as mentioned elsewhere, all QM theories do have to deal with exponentially increasing branching. A cost that apparently increases if we grind up photodiodes and turn them into mirrors. If wavefunction branching is a problem, how does QM get away with it when nobody is looking? Deep questions usually have simple answers! The simplicity is what makes the answer difficult to accept.
    - hackinthebochs 2y ago
      > the best sensors actually behave just like mirrors themselves, re-emitting a reflected photon rather than absorbing any of the energy of the incoming signal. Presumably some state is changed somewhere in the system to indicate a detection. >If wavefunction branching is a problem, how does QM get away with it when nobody is looking? Do we know that branching is unbounded when we're not looking? Is there a detectable difference in a large, presumably entangled system and its classical counterpart?
      - 2y ago

mitthrowaway2 2y ago

But that seems circular; what determines whether an interaction imparts a constraint, or doesn't? Let's say that I send a photon through a beam-splitter which will deflect its momentum either right or left. QM predicts that the photon at the output of the beamsplitter will be in a superposition of both trajectories, and we can prove this by adding mirrors that combine both possible output beams back together with another and generate an interference pattern. The interference pattern will be detected by a sensitive photodiode array, capable of detecting individual photons. Would you say the interaction with the photodiode array counts as a measurement? And the interaction with the mirror does not? How does QM know whether a photon interaction with a bunch of atoms in a mirror coating, vs in a silicon diode, creates a constraint on the evolution of the system? Why is one interaction measurement-like in nature, but not the other? Is there some property of silicon diodes that make them immune to being put into quantum superpositions? I would have said that quantum erasure experiments make by far the most sense if you consider measurement to never happen, and the result is merely a superposition of both outcomes. The photon is put into a superposition of both states when it reaches the beamsplitter, the detector screen is put into a superposition of both states when the superposition-photon reaches it, and so are you in turn when you observe the detector screen.

hackinthebochs 2y ago
> Why is one interaction measurement-like in nature, but not the other? The mirror reflects all the energy of the photon whereas the photodiode absorbs the energy and its state is altered by it. How nature knows the difference is a deep question. Presumably the rules governing nature ensure this consistency. What that looks like is anyone's guess. >I would have said that quantum erasure experiments make by far the most sense if you consider measurement to never happen I agree, but then this interpretation comes with the exponentially growing cost of actual branching. In my view, this rationally must be the interpretation of last resort, for reasons stated in the other threads.
- mitthrowaway2 2y ago
  Counterintuitively, the best sensors actually behave just like mirrors themselves, re-emitting a reflected photon rather than absorbing any of the energy of the incoming signal. This increases the signal-to-noise ratio, because the E-field magnitude at the sensor is doubled, as it combines the constructive interference of the incoming and reflected photon. And as mentioned elsewhere, all QM theories do have to deal with exponentially increasing branching. A cost that apparently increases if we grind up photodiodes and turn them into mirrors. If wavefunction branching is a problem, how does QM get away with it when nobody is looking? Deep questions usually have simple answers! The simplicity is what makes the answer difficult to accept.
  - hackinthebochs 2y ago
    > the best sensors actually behave just like mirrors themselves, re-emitting a reflected photon rather than absorbing any of the energy of the incoming signal. Presumably some state is changed somewhere in the system to indicate a detection. >If wavefunction branching is a problem, how does QM get away with it when nobody is looking? Do we know that branching is unbounded when we're not looking? Is there a detectable difference in a large, presumably entangled system and its classical counterpart?
    - 2y ago