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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 eigenfunc

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.
    - edna314 7y ago
      > What causes it to occur or not occur? Measurements cause it to occur. What causes entanglement to occur? > quantum eraser Not sure how it works, but Wikipedia says that it can be explained using Copenhagen carefully. > (1/sqrt(2))(|up>|you after observing up> + |down>|you after observing down>) After the measurement I’m not in a superposition of the two entangled states but in one of the two, because I do observe the photon in a particular spin state. Which one is it? And how is it decided?
      - lmm 7y ago
        > Measurements cause it to occur. Ok, but what's the objective physical distinction between a measurement and a non-measurement? > What causes entanglement to occur? Various specific kinds of normal physical interactions, the details of which are an established part of QM. > After the measurement I’m not in a superposition of the two entangled states but in one of the two What would you expect to be different if you were in a superposition of two states? > I do observe the photon in a particular spin state Sure, because your state is entangled with its state. That's the normal, established behaviour of entangled states.
        edna314 7y ago
        > Ok, but what's the objective physical distinction between a measurement and a non-measurement? Various specific kinds of measuring devices the details of which are an established part of QM. > Various specific kinds of normal physical interactions, the details of which are an established part of QM. So, which is the interaction that causes entanglement? I was asking for this before. > What would you expect to be different if you were in a superposition of two states? Well, I would expect my measurement to also have all possible outcomes given by the superposition. > Sure, because your state is entangled with its state. That's the normal, established behaviour of entangled states. Only after a one of the entangled particles is measured. Either my brain must be measured or the result must be measured. In this sense entanglement doesn’t work without an ill defined concept of measurement either.

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.
  - edna314 7y ago
    > What causes it to occur or not occur? Measurements cause it to occur. What causes entanglement to occur? > quantum eraser Not sure how it works, but Wikipedia says that it can be explained using Copenhagen carefully. > (1/sqrt(2))(|up>|you after observing up> + |down>|you after observing down>) After the measurement I’m not in a superposition of the two entangled states but in one of the two, because I do observe the photon in a particular spin state. Which one is it? And how is it decided?
    - lmm 7y ago
      > Measurements cause it to occur. Ok, but what's the objective physical distinction between a measurement and a non-measurement? > What causes entanglement to occur? Various specific kinds of normal physical interactions, the details of which are an established part of QM. > After the measurement I’m not in a superposition of the two entangled states but in one of the two What would you expect to be different if you were in a superposition of two states? > I do observe the photon in a particular spin state Sure, because your state is entangled with its state. That's the normal, established behaviour of entangled states.
      - edna314 7y ago
        > Ok, but what's the objective physical distinction between a measurement and a non-measurement? Various specific kinds of measuring devices the details of which are an established part of QM. > Various specific kinds of normal physical interactions, the details of which are an established part of QM. So, which is the interaction that causes entanglement? I was asking for this before. > What would you expect to be different if you were in a superposition of two states? Well, I would expect my measurement to also have all possible outcomes given by the superposition. > Sure, because your state is entangled with its state. That's the normal, established behaviour of entangled states. Only after a one of the entangled particles is measured. Either my brain must be measured or the result must be measured. In this sense entanglement doesn’t work without an ill defined concept of measurement either.