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> I am on board with the "measurement is just entanglement" solution Then what is "entanglement"? I sure would agree that 'measurement' and 'entanglement' have

> I am on board with the "measurement is just entanglement" solution Then what is "entanglement"? I sure would agree that 'measurement' and 'entanglement' have a similar nature. I just think this argument doesn't answer the original question, but it is trying to hide it.

lmm 7y ago
Entanglement is a well-understood part of QM, and even Copenhagenists agree on how it applies to elemental particles. (They just think something strange and different happens to macroscopic objects).
- edna314 7y ago
  If it is well understood, could you tell me how it works? I know the mathematics of entanglement just as I know the the mathematics of a collapse of a wave function, so I wouldn't take any mathematical argument as a valid answer.
  - gliese1337 7y ago
    The math is the description of how it works. If you are not willing to accept any mathematical argument as a valid answer, you better explain what kind of answer you would accept as an alternative, 'cause it's not at all obvious that any other explanation could even exist.
    - edna314 7y ago
      I can argue in the same way in favor of Copenhagen. The math of Copenhagen is clearly defined, as the collapse of a wave function is the projection on eigenfunctions of the measuring device.
      - gliese1337 7y ago
        No, it isn't. The mathematics that describe the results of measurement are clearly defined, but what constitutes a measurement--i.e., when you need to apply that process--is not. There is no mathematics to back it up--it's purely ad-hoc. That is what "measurement is just entanglement" provides.
        edna314 7y ago
        I’m sorry, but I disagree. A measurement process is mathematically speaking an operator which acts on the wave function in Copenhagen interpretation and the collapse of the wave function is a projection onto the eigenfunctions of this operator. What would you be missing here? Instead of “measurement is just entanglement” I would claim “measurement is just an operator”. Since entanglement and operators can only be defined mathematically, I don’t see the point choosing one over the other. Besides that, there is one point I don’t get about many worlds and entanglement theories: assuming that “measurement is just entanglement”, there are many possibilities how the observer gets entangled with the wave function it observes. How is it decided which entanglement (or which world) is the one we actually observe?
        lmm 7y ago
        > A measurement process is mathematically speaking an operator which acts on the wave function in Copenhagen interpretation and the collapse of the wave function is a projection onto the eigenfunctions of this operator. What would you be missing here? When and how does that projection (which is non-unitary and otherwise ill-behaved) happen? What causes it to occur or not occur? How do you explain the results of the quantum eraser experiment, where the exact same physical process retroactively turns out to constitute a measurement or not depending on what happens in the future? > Besides that, there is one point I don’t get about many worlds and entanglement theories: assuming that “measurement is just entanglement”, there are many possibilities how the observer gets entangled with the wave function it observes. How is it decided which entanglement (or which world) is the one we actually observe? No, there is only one possible way to get entangled with what you observe. If you observe a spin-half particle that's in state (1/sqrt(2))(|up> + |down>), then afterwards the you-photon system is in state (1/sqrt(2))(|up>|you after observing up> + |down>|you after observing down>), exactly as if you were a normal physical object obeying the normal laws of physics. The question you can and should still ask is "what should we expect the subjective experience of being in this state to be like". And to a certain extent, why the Born rule applies is still something to be explained (though I'm not aware of any alternative possibilities that would be well-behaved). But that's a much smaller and simpler question.

lmm 7y ago

Entanglement is a well-understood part of QM, and even Copenhagenists agree on how it applies to elemental particles. (They just think something strange and different happens to macroscopic objects).

edna314 7y ago
If it is well understood, could you tell me how it works? I know the mathematics of entanglement just as I know the the mathematics of a collapse of a wave function, so I wouldn't take any mathematical argument as a valid answer.
- gliese1337 7y ago
  The math is the description of how it works. If you are not willing to accept any mathematical argument as a valid answer, you better explain what kind of answer you would accept as an alternative, 'cause it's not at all obvious that any other explanation could even exist.
  - edna314 7y ago
    I can argue in the same way in favor of Copenhagen. The math of Copenhagen is clearly defined, as the collapse of a wave function is the projection on eigenfunctions of the measuring device.
    - gliese1337 7y ago
      No, it isn't. The mathematics that describe the results of measurement are clearly defined, but what constitutes a measurement--i.e., when you need to apply that process--is not. There is no mathematics to back it up--it's purely ad-hoc. That is what "measurement is just entanglement" provides.
      - edna314 7y ago
        I’m sorry, but I disagree. A measurement process is mathematically speaking an operator which acts on the wave function in Copenhagen interpretation and the collapse of the wave function is a projection onto the eigenfunctions of this operator. What would you be missing here? Instead of “measurement is just entanglement” I would claim “measurement is just an operator”. Since entanglement and operators can only be defined mathematically, I don’t see the point choosing one over the other. Besides that, there is one point I don’t get about many worlds and entanglement theories: assuming that “measurement is just entanglement”, there are many possibilities how the observer gets entangled with the wave function it observes. How is it decided which entanglement (or which world) is the one we actually observe?
        lmm 7y ago
        > A measurement process is mathematically speaking an operator which acts on the wave function in Copenhagen interpretation and the collapse of the wave function is a projection onto the eigenfunctions of this operator. What would you be missing here? When and how does that projection (which is non-unitary and otherwise ill-behaved) happen? What causes it to occur or not occur? How do you explain the results of the quantum eraser experiment, where the exact same physical process retroactively turns out to constitute a measurement or not depending on what happens in the future? > Besides that, there is one point I don’t get about many worlds and entanglement theories: assuming that “measurement is just entanglement”, there are many possibilities how the observer gets entangled with the wave function it observes. How is it decided which entanglement (or which world) is the one we actually observe? No, there is only one possible way to get entangled with what you observe. If you observe a spin-half particle that's in state (1/sqrt(2))(|up> + |down>), then afterwards the you-photon system is in state (1/sqrt(2))(|up>|you after observing up> + |down>|you after observing down>), exactly as if you were a normal physical object obeying the normal laws of physics. The question you can and should still ask is "what should we expect the subjective experience of being in this state to be like". And to a certain extent, why the Born rule applies is still something to be explained (though I'm not aware of any alternative possibilities that would be well-behaved). But that's a much smaller and simpler question.