3 ms·

I thought you were referring to the relativity of simultaneity. I'm glad you insist on the FTL communication issue, maybe now I will be able to convince you tha

by kgwgk 8y ago

I thought you were referring to the relativity of simultaneity. I'm glad you insist on the FTL communication issue, maybe now I will be able to convince you that there is no issue at all... > The answer to 2 and 3 must be "yes" if you want to avoid FTL (because otherwise Bob could determine whether or not Alice has performed measurements). The answer to 2 is "no". If Alice has not performed any measurement, the N "B" particles that Bob has are not in a pure state. They are a subsystem of the N (indentical) entangled pairs formed by the N "A" particles that Alice has plus the corresponding N "B" particles that Bob has (let's call that state E). So, depending on whether Alice has done the measurement or not, what we have is (I undertstand you don't really accept this description, it's for the sake of the argument where you will try to show how this description is unacceptable): Measurement not done Pairs remain entangled The N particles held by Bob are not in a pure state (The A/B pairs are in a pure state E) Measurement done Pairs are no longer entangled The N particles held by Bob are each in one definite state P or P' or P'' or P''' (Alice knows the precise state of Bob's particles, she knows how to rotate each particle to get the state P) You say that, before any message from Alice can reach Bob, he can determine wheter or not she has performed the measurement. Can you explain how? (Hint: no, you cannot.)

lisper 8y ago
> I thought you were referring to the relativity of simultaneity. Yes, I was. > I'm glad you insist on the FTL communication issue, It amounts to the same thing. > maybe now I will be able to convince you that there is no issue at all... Maybe, except that I'm not sure you're clear on what we're actually disagreeing about. > You say that, before any message from Alice can reach Bob, he can determine wheter or not she has performed the measurement. Only in a hypothetical case, not realizable in practice (probably -- see below), where Bob has chosen N/4 particles at random and those just happen to coincide with the particles that Alice measured and found to require no rotation. In that case, Bob will, on your view, observe first-order interference if Alice has measured, and not otherwise. Maybe it would help if I describe it as a detailed thought experiment. The experiment takes place in four phases. Phase 1: Alice and Bob choose a basis for a pure state P which they will use for the remaining two phases. Phase 2: Alice teleports N copies of state P to Bob using the normal protocol, including the transmission of the results of her measurements. Bob takes the N/4 particles that require no rotation and tests them to see if they produce first-order interference relative to the basis of P. I trust you and I agree that he will observe interference in this case. Phase 3: This is the part that can't be realized in practice, and I have to number the steps here because this phase runs in a loop. Phase 3 is designed to answer the question: what would have happened in phase 2 if Bob had chosen the same N/4 particles by pure chance? So... 1. N EPR pairs are distributed. 2. Alice performs her part of the teleportation protocol, but does NOT tell Bob the results of her measurements. 3. Bob selects N/4 particles at random and tests for first-order interference relative to P. (The vast majority of the time the result will be negative.) 4. Bob and Alice compare Bob's random selections in step 3 with the results of Alice's measurements in step 2. If they happen to coincide, i.e. Bob just happened to pick the correct N/4 particles that did not require rotation, the experiment halts, with the result being whatever Bob observed on the last iteration of step 3. I trust that you and I will agree that Bob will also observe interference in this case. Phase 4 (this is the interesting one): This is identical to phase 3, except that steps 2 and 3 are reversed, i.e. Bob makes his random selection and interference measurement BEFORE (in a fully relativistic sense) Alice makes her measurements. i.e.: 1. N EPR pairs are distributed. 2. Bob selects N/4 particles at random and tests for first-order interference relative to P. 3. Bob communicates to Alice that he is done. 4. AFTER Alice receives Bob's signal from step 3, she performs her side of the teleportation protocol for N copies of P. 5. They compare the results of Alice's measurements in step 4 with Bob's random choices made in step 2. If they coincide (i.e. Bob just happened to choose the correct N/4 particles that did not require rotation) the experiment concludes, with the result being whatever Bob observed on the final iteration of step 2. Otherwise, they go to step 1 and try again. On your view, the predicted result of phase 4 will be negative. So if Alice cheats and performs her measurements before receiving Bob's signal, Bob can tell. Postscript: now that I think about it, it might actually be possible to perform this experiment. A mach-zehnder interferometer can be pretty sensitive, so it might be possible to get N down low enough that the experiment could terminate in a reasonable amount of time.
- kgwgk 8y ago
  > I'm not sure you're clear on what we're actually disagreeing about. To be clear, I disagree with your claim that the QM description is not correct because it leads to FTL communication. If you stand by that, you're wrong. If that's not what you meant, then there is no disagreement. Now, if I understand your latest comment, you say that it's "Only in a hypothetical case [...] where Bob has chosen N/4 particles at random and those just happen to coincide with the particles that Alice measured." This is like saying that two standard 52-card decks can be used for FTL communication, in the hypothetical case where Bob picks the same card that Aliced picked. If you think that is FTL communication I cannot agree with that. This kind of "FTL communication" doesn't present any problem at all for the QM description. Maybe I'm missing something, but I don't see in your description of the experiment any trace of potential FTL communication. You're not even trying! I would like to see something like the following (but I understand you cannot provide it, because it's impossible): 1) N EPR pairs are distributed 2) Bob does something ("selects N/4 particles at random and tests for first-order interference relative to P" or whatever) and concludes that "Alice has done something to her particles" or "Alice has not done anything to her particles" depending on the result of his measurement. [The prediction is done at this point, the verification step follows.] 3) Alice tells Bob if she did or not measure her particles before he did. Or Bob tells Alice his prediction. Or Alice and Bob tell Charlie and he performs the check. In any case, this requires slower-than-light communication. This experiment can be repeated several times to see if Bob's prediction is right every time or if at least there is some correlation between what Alice did and what Bob guessed. Your description has additional steps involving communication between Alice and Bob before there is any result or prediction. It is not in any way incompatible with the standard QM formulation that you reject, as far as I can see. It looks like if, in the card-picking example, Bob picks a card and then compares his card with Alice's card. If the card is the same they announce that FTL communication had been achieved. If the card is not the same, they try again. If you think this is not a fair characterisation, please make clear what prediction is Bob doing before communicating with Alice. To make my point as clear as I can once again: according to QM, Bob cannot tell if Alice has performed or not a measurement on her side. For any measurement that Bob can do on his particle, the predicted distribution of outcomes conditional on "the entanglement is broken, the particle is now in an unknown pure state" is exactly the same as the predicted distribution of outcomes conditional on "the entanglement is intact, the particle is not in a pure state (the pair A/B is still in the pure state E)". Whatever the outcome he gets out of his measurement, it cannot be evidence for one or the other.

by kgwgk 8y ago

lisper 8y ago

> I thought you were referring to the relativity of simultaneity. Yes, I was. > I'm glad you insist on the FTL communication issue, It amounts to the same thing. > maybe now I will be able to convince you that there is no issue at all... Maybe, except that I'm not sure you're clear on what we're actually disagreeing about. > You say that, before any message from Alice can reach Bob, he can determine wheter or not she has performed the measurement. Only in a hypothetical case, not realizable in practice (probably -- see below), where Bob has chosen N/4 particles at random and those just happen to coincide with the particles that Alice measured and found to require no rotation. In that case, Bob will, on your view, observe first-order interference if Alice has measured, and not otherwise. Maybe it would help if I describe it as a detailed thought experiment. The experiment takes place in four phases. Phase 1: Alice and Bob choose a basis for a pure state P which they will use for the remaining two phases. Phase 2: Alice teleports N copies of state P to Bob using the normal protocol, including the transmission of the results of her measurements. Bob takes the N/4 particles that require no rotation and tests them to see if they produce first-order interference relative to the basis of P. I trust you and I agree that he will observe interference in this case. Phase 3: This is the part that can't be realized in practice, and I have to number the steps here because this phase runs in a loop. Phase 3 is designed to answer the question: what would have happened in phase 2 if Bob had chosen the same N/4 particles by pure chance? So... 1. N EPR pairs are distributed. 2. Alice performs her part of the teleportation protocol, but does NOT tell Bob the results of her measurements. 3. Bob selects N/4 particles at random and tests for first-order interference relative to P. (The vast majority of the time the result will be negative.) 4. Bob and Alice compare Bob's random selections in step 3 with the results of Alice's measurements in step 2. If they happen to coincide, i.e. Bob just happened to pick the correct N/4 particles that did not require rotation, the experiment halts, with the result being whatever Bob observed on the last iteration of step 3. I trust that you and I will agree that Bob will also observe interference in this case. Phase 4 (this is the interesting one): This is identical to phase 3, except that steps 2 and 3 are reversed, i.e. Bob makes his random selection and interference measurement BEFORE (in a fully relativistic sense) Alice makes her measurements. i.e.: 1. N EPR pairs are distributed. 2. Bob selects N/4 particles at random and tests for first-order interference relative to P. 3. Bob communicates to Alice that he is done. 4. AFTER Alice receives Bob's signal from step 3, she performs her side of the teleportation protocol for N copies of P. 5. They compare the results of Alice's measurements in step 4 with Bob's random choices made in step 2. If they coincide (i.e. Bob just happened to choose the correct N/4 particles that did not require rotation) the experiment concludes, with the result being whatever Bob observed on the final iteration of step 2. Otherwise, they go to step 1 and try again. On your view, the predicted result of phase 4 will be negative. So if Alice cheats and performs her measurements before receiving Bob's signal, Bob can tell. Postscript: now that I think about it, it might actually be possible to perform this experiment. A mach-zehnder interferometer can be pretty sensitive, so it might be possible to get N down low enough that the experiment could terminate in a reasonable amount of time.

kgwgk 8y ago
> I'm not sure you're clear on what we're actually disagreeing about. To be clear, I disagree with your claim that the QM description is not correct because it leads to FTL communication. If you stand by that, you're wrong. If that's not what you meant, then there is no disagreement. Now, if I understand your latest comment, you say that it's "Only in a hypothetical case [...] where Bob has chosen N/4 particles at random and those just happen to coincide with the particles that Alice measured." This is like saying that two standard 52-card decks can be used for FTL communication, in the hypothetical case where Bob picks the same card that Aliced picked. If you think that is FTL communication I cannot agree with that. This kind of "FTL communication" doesn't present any problem at all for the QM description. Maybe I'm missing something, but I don't see in your description of the experiment any trace of potential FTL communication. You're not even trying! I would like to see something like the following (but I understand you cannot provide it, because it's impossible): 1) N EPR pairs are distributed 2) Bob does something ("selects N/4 particles at random and tests for first-order interference relative to P" or whatever) and concludes that "Alice has done something to her particles" or "Alice has not done anything to her particles" depending on the result of his measurement. [The prediction is done at this point, the verification step follows.] 3) Alice tells Bob if she did or not measure her particles before he did. Or Bob tells Alice his prediction. Or Alice and Bob tell Charlie and he performs the check. In any case, this requires slower-than-light communication. This experiment can be repeated several times to see if Bob's prediction is right every time or if at least there is some correlation between what Alice did and what Bob guessed. Your description has additional steps involving communication between Alice and Bob before there is any result or prediction. It is not in any way incompatible with the standard QM formulation that you reject, as far as I can see. It looks like if, in the card-picking example, Bob picks a card and then compares his card with Alice's card. If the card is the same they announce that FTL communication had been achieved. If the card is not the same, they try again. If you think this is not a fair characterisation, please make clear what prediction is Bob doing before communicating with Alice. To make my point as clear as I can once again: according to QM, Bob cannot tell if Alice has performed or not a measurement on her side. For any measurement that Bob can do on his particle, the predicted distribution of outcomes conditional on "the entanglement is broken, the particle is now in an unknown pure state" is exactly the same as the predicted distribution of outcomes conditional on "the entanglement is intact, the particle is not in a pure state (the pair A/B is still in the pure state E)". Whatever the outcome he gets out of his measurement, it cannot be evidence for one or the other.