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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

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?
    - mitthrowaway2 2y ago
      > Presumably some state is changed somewhere in the system to indicate a detection. Yes, but "state change" happens all the time! When the photon bounces off the mirror, even if 100% of its energy is reflected, it imparts momentum to the mirror. When the photon propagates through free space, it "changes state". All state changes occur exactly in accordance with the time-evolution of the Schrodinger equation. > Do we know that branching is unbounded when we're not looking? This is a basic principle of QM, which would have pretty measurable macroscopic impact if it were invalid. "Branches" are not even finite in number, even for a single particle in free space; its position and momentum distributions are both continuous, with a minimum possible area guaranteed by the Heisenberg uncertainty principle, and even so much as precisely localizing its momentum already creates an infinity of equally-likely possible positions.

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?
  - mitthrowaway2 2y ago
    > Presumably some state is changed somewhere in the system to indicate a detection. Yes, but "state change" happens all the time! When the photon bounces off the mirror, even if 100% of its energy is reflected, it imparts momentum to the mirror. When the photon propagates through free space, it "changes state". All state changes occur exactly in accordance with the time-evolution of the Schrodinger equation. > Do we know that branching is unbounded when we're not looking? This is a basic principle of QM, which would have pretty measurable macroscopic impact if it were invalid. "Branches" are not even finite in number, even for a single particle in free space; its position and momentum distributions are both continuous, with a minimum possible area guaranteed by the Heisenberg uncertainty principle, and even so much as precisely localizing its momentum already creates an infinity of equally-likely possible positions.