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I am not sure that paper knows it, but it is essentially showing that many worlds is correct-- that the wavefunction never collapses: > the implications on con

by tbabb 8y ago

I am not sure that paper knows it, but it is essentially showing that many worlds is correct-- that the wavefunction never collapses: > the implications on conditional probabilities hold for other measurements throughout the entire spacetime, present and past. [emphasis theirs] And the math in the paper which supports that statement works by keeping all the superpositions around and allowing us to project to them at any time. This is the picture of many worlds! Copenhagen says the opposite: When you make a measurement, the unmeasured superpositions go away. The paper confirms, that's nonsense! If you shade any region of the spacetime diagram where the superposition is gone ("collapsed"), you'll be wrong!

kgwgk 8y ago
The author says: "Note: in this work, we will not make any reference to why this (apparent) collapse occurs. Not only is this a much harder issue, it is simply not relevant to discussion we will present." Also: "It is important to note that arriving at our conclusions did not require introducing new physics. We only relied on elementary quantum mechanics: not on novel ‘backwards time’ concepts, nor on any particular interpretation: we only used the Born rule ‘as is’. [...] With the remarks and intuition presented here, there really is no mystery whatsoever in any of the discussed experiments." This experiment is not more mysterious than the rest of QM, but of course if you find the whole theory unsatisfactory you won't be satistified with this explanation either.
- tbabb 8y ago
  That language is why I think the authors aren't aware that they are advocating many worlds. Also, this is a fantastic example of the "don't ask questions" attitude I think is so shameful. If they had bothered to take the small step of asking, they'd have come to a clear, evidence-based refutation of Copenhagen! There are regions of spacetime after the measurement, where the wavefunction is not collapsed, which the authors explicitly point to. That's in direct contradiction to the premise of Copenhagen.
  - kgwgk 8y ago
    The point is that the order of the measurements is irrelevant: QM predicts the same probability distribution of outcomes. So the problem can be solved using either order and the response is the same. I am not sure what premise of Copenhagen is being contradicted. Edit: solving the problem in the "natural" order would give the same probabilities but is much more difficult to handle. You need to get a probabilistic outcome for the position of each single photon on the screen, which "collapses" the state and determines the wave function of the other photon. At this point, the first photon has been detected somewhere but it can only be labeled as "interference" or "not interference" later after the detection of the second photon. The probability of being labeled as "interference" or "not interference" does depend on the position (because the collapsed wave function depends on what the outcome of the previous measurement was). When everything is said and done, looking at the subset of events labeled "interference" there is an interference pattern and looking at the subset of events labeled "not interference" there is not an interference pattern. There is no mystery.
    - tbabb 8y ago
      > I am not sure what premise of Copenhagen is being contradicted. Before measurement, the state of the particle system is Σ α_i|i>. Copenhagen says, "after you measure the system, all but one of the α_i go to zero". The authors don't do that, and in fact say that you can't. Unless I am misunderstanding something, they keep all the α_i around at all times and project the measurement for each particle separately, regardless of whether it has been (or will be) measured somewhere else. There is nothing philosophically or physically wrong with this, as they point out, but it is different than what Copenhagen says you should do when you measure something. And if you look at what it means (which they refuse to do), you'll see that after Alice makes her measurement, Bob's quantum state is still explainable in terms of a superposition. When is the measurement (joint or individual) "finished"? Answer, per the authors: Never. The superposition permeates spacetime. That's how we escape the need for a causal connection between Alice and Bob's measurements, and, naturally, that's how many worlds does it. (And if we take it further and ask, when Bob makes his measurement, "What is the state of Alice's particles?" we'll see that she is in a superposition of being entangled with each of the superpositions of the particle, which remains in superposition before, throughout, and after our ~measurement of~ entanglement with it).

by tbabb 8y ago

kgwgk 8y ago

The author says: "Note: in this work, we will not make any reference to why this (apparent) collapse occurs. Not only is this a much harder issue, it is simply not relevant to discussion we will present." Also: "It is important to note that arriving at our conclusions did not require introducing new physics. We only relied on elementary quantum mechanics: not on novel ‘backwards time’ concepts, nor on any particular interpretation: we only used the Born rule ‘as is’. [...] With the remarks and intuition presented here, there really is no mystery whatsoever in any of the discussed experiments." This experiment is not more mysterious than the rest of QM, but of course if you find the whole theory unsatisfactory you won't be satistified with this explanation either.

tbabb 8y ago
That language is why I think the authors aren't aware that they are advocating many worlds. Also, this is a fantastic example of the "don't ask questions" attitude I think is so shameful. If they had bothered to take the small step of asking, they'd have come to a clear, evidence-based refutation of Copenhagen! There are regions of spacetime after the measurement, where the wavefunction is not collapsed, which the authors explicitly point to. That's in direct contradiction to the premise of Copenhagen.
- kgwgk 8y ago
  The point is that the order of the measurements is irrelevant: QM predicts the same probability distribution of outcomes. So the problem can be solved using either order and the response is the same. I am not sure what premise of Copenhagen is being contradicted. Edit: solving the problem in the "natural" order would give the same probabilities but is much more difficult to handle. You need to get a probabilistic outcome for the position of each single photon on the screen, which "collapses" the state and determines the wave function of the other photon. At this point, the first photon has been detected somewhere but it can only be labeled as "interference" or "not interference" later after the detection of the second photon. The probability of being labeled as "interference" or "not interference" does depend on the position (because the collapsed wave function depends on what the outcome of the previous measurement was). When everything is said and done, looking at the subset of events labeled "interference" there is an interference pattern and looking at the subset of events labeled "not interference" there is not an interference pattern. There is no mystery.
  - tbabb 8y ago
    > I am not sure what premise of Copenhagen is being contradicted. Before measurement, the state of the particle system is Σ α_i|i>. Copenhagen says, "after you measure the system, all but one of the α_i go to zero". The authors don't do that, and in fact say that you can't. Unless I am misunderstanding something, they keep all the α_i around at all times and project the measurement for each particle separately, regardless of whether it has been (or will be) measured somewhere else. There is nothing philosophically or physically wrong with this, as they point out, but it is different than what Copenhagen says you should do when you measure something. And if you look at what it means (which they refuse to do), you'll see that after Alice makes her measurement, Bob's quantum state is still explainable in terms of a superposition. When is the measurement (joint or individual) "finished"? Answer, per the authors: Never. The superposition permeates spacetime. That's how we escape the need for a causal connection between Alice and Bob's measurements, and, naturally, that's how many worlds does it. (And if we take it further and ask, when Bob makes his measurement, "What is the state of Alice's particles?" we'll see that she is in a superposition of being entangled with each of the superpositions of the particle, which remains in superposition before, throughout, and after our ~measurement of~ entanglement with it).