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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" atti

by 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).
  - kgwgk 8y ago
    Please see what I added to my previous comment. You could in principle do it in the "right" order and you would get the same result. They routinely solve quantum optics problems using standard QM and the experimental results match the predictions. What he shows in that paper is that the order of the measurments is irrelevant [1]. So he does solve the problem in the reverse order where it can be done easily writing a few quantum states [2]. Note that QM is not about causal connections, is about correlation. Once a pair is correlated, the correlation may appear when measurements are done. But it's not that observing one outcome here and now "causes" a particular outcome there and later (or before). One doesn't need to keep superpositions to think it "correlation" terms (instead of "causal" terms). [1] Using the projection postulate in the derivation: "Say we have indeed measured on B and got OB = bJ . The state then collapses onto ..." He concludes: "None of this looks very surprising, but we want to stress that the total probability to find OA = aI and OB = bJ does not depend on the place or time at which the measurements occur." [2] He also uses the projection postulate here in the usual way: "So the experimental outcome (encoded in the combined measurement outcomes) is bound to be the same even if we would measure the idler photon earlier, i.e. before the signal photon by shortening the optical path length of the downwards configuration. Then, if the idler is detected at D4 for example, the above state ‘collapses’ onto ..."
    - tbabb 8y ago
      Yep, I'm with you that we don't need to bring causality into it. And yes, you don't need to ask "when does the wavefunction collapse" to manipulate joint probabilities of measurements that happen at disparate locations and times. In fact, that's my objection: If you do ask, you find that there is no consistent answer! (And I suggest it's because wavefunction collapse is not a thing the universe does). Re: "interference tagging"-- do you have a link to some material? (I'd love to understand something specific before commenting). EDIT: Also, I didn't see Appendix B at first-- The authors do understand and even advocate the Everettian view! Though I still don't quite understand/agree with their earlier timidity about finding and interpreting conflicts with Copenhagen.
      - 8y ago

by tbabb 8y ago

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).
- kgwgk 8y ago
  Please see what I added to my previous comment. You could in principle do it in the "right" order and you would get the same result. They routinely solve quantum optics problems using standard QM and the experimental results match the predictions. What he shows in that paper is that the order of the measurments is irrelevant [1]. So he does solve the problem in the reverse order where it can be done easily writing a few quantum states [2]. Note that QM is not about causal connections, is about correlation. Once a pair is correlated, the correlation may appear when measurements are done. But it's not that observing one outcome here and now "causes" a particular outcome there and later (or before). One doesn't need to keep superpositions to think it "correlation" terms (instead of "causal" terms). [1] Using the projection postulate in the derivation: "Say we have indeed measured on B and got OB = bJ . The state then collapses onto ..." He concludes: "None of this looks very surprising, but we want to stress that the total probability to find OA = aI and OB = bJ does not depend on the place or time at which the measurements occur." [2] He also uses the projection postulate here in the usual way: "So the experimental outcome (encoded in the combined measurement outcomes) is bound to be the same even if we would measure the idler photon earlier, i.e. before the signal photon by shortening the optical path length of the downwards configuration. Then, if the idler is detected at D4 for example, the above state ‘collapses’ onto ..."
  - tbabb 8y ago
    Yep, I'm with you that we don't need to bring causality into it. And yes, you don't need to ask "when does the wavefunction collapse" to manipulate joint probabilities of measurements that happen at disparate locations and times. In fact, that's my objection: If you do ask, you find that there is no consistent answer! (And I suggest it's because wavefunction collapse is not a thing the universe does). Re: "interference tagging"-- do you have a link to some material? (I'd love to understand something specific before commenting). EDIT: Also, I didn't see Appendix B at first-- The authors do understand and even advocate the Everettian view! Though I still don't quite understand/agree with their earlier timidity about finding and interpreting conflicts with Copenhagen.
    - 8y ago