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entropy1
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If A gets a hit, we know its angle. But how does B know if A got a hit?Mentz114 said:Isn't it sufficient to project the second measured photon ( say B) into the angle of A's polarizer if A gets a hit ?
If A gets a hit, we know its angle. But how does B know if A got a hit?Mentz114 said:Isn't it sufficient to project the second measured photon ( say B) into the angle of A's polarizer if A gets a hit ?
B does not need to know that. The paired photon may have changed state after the first measurement. That is what entanglement is supposed to mean.entropy1 said:If A gets a hit, we know its angle. But how does B know if A got a hit?
vanhees71 said:Of course, you have to precisly simulate the probabilities given for the assumed experimental setup. Of course, I have to use the relative angle of the detectors to calculate these probabilities, and thus this information goes into the code.
I don't know if this is addressed to me but - I'm not worried nor ever have been by any of the great 'problems' in QT !vanhees71 said:Well, in the usual setup the measurement of B doesn't do anything on A. To ensure this you make the measurement acts spacelike separated. That's what's so mind boggling about these stronger-than-classically-possible correlations described by an almost trivial math. As easy is the math as mind boggling are the implications, particularly if you stick to traditional collapse assumptions of the early days! The worrying immediately stops if you simply accept that nature is inherently probabilistic and that there is a very successful formalism called QT that tells you probabilities and only probabilities.
Sure, I'm aware of this, but no matter when you choose the orientation (even in the last femtosecond before the envisaged photon hits the detector) you know the probabilities you have to simulate, when using this setup to calculate the corresponding probabilities (or expectation values). The probabilities are known of course; it's what's predicted by QT, and that's what's simulated with a correspondingly programmed MC simulator.stevendaryl said:But the relative angle is not known until the last minute. The situation is the following:
View attachment 217546
You have three devices: C, which is a source of message pairs, simulating photon pair production, and A and B, which simulate the measurement events.
Bell's inequality implies that [itex]|\langle R_A(\theta_A) R_B(\theta_B) \rangle| \leq 2[/itex], no matter what algorithms are used by A, B, and C, as long as
- Every "round", C sends out a pair of messages, [itex]m_A[/itex] to A and [itex]m_B[/itex] to B.
- After the messages are sent, but before they are read, settings for A and B are chosen, independently. The settings are two angles, [itex]\theta_A[/itex] and [itex]\theta_B[/itex].
- Device A determines an output, [itex]R_A(\theta_A)[/itex], which is either +1 or -1, based on the message received from C and the setting [itex]\theta_A[/itex].
- Similarly, device B determines an output, [itex]R_B(\theta_B)[/itex] based on its message and setting.
- Over many, many rounds, we can gather statistics for the correlation: [itex]\langle R_A(\theta_A) R_B(\theta_B) \rangle[/itex] as a function of the pair of settings, [itex]\theta_A, \theta_B[/itex].
On the other hand, if instead of C sending messages, it generates a pair of entangled photons, and sends one to A and one to B, then you can violate the inequality. (Inside A and B, instead of a computer algorithm, you have polarizing filters and photon detectors, and each sends out +1 if the photon passes through the filter at the orientation specified by [itex]\theta_A[/itex] or [itex]\theta_B[/itex].
- There are no communications among A, B, C other than those specified.
- The settings [itex]\theta_A[/itex] and [itex]\theta_B[/itex] for each round are unpredictable by C.
vanhees71 said:Sure, I'm aware of this, but no matter when you choose the orientation (even in the last femtosecond before the envisaged photon hits the detector) you know the probabilities you have to simulate,
https://en.wikipedia.org/wiki/Monte_Carlo_methodstevendaryl said:I don't really know what you (@vanhees71) mean by a "Monte Carlo" simulation for this experiment, but the only way you can reproduce the predictions of quantum mechanics for this case is if the settings for the detectors are known by the simulation code. In other words, by cheating (according to the rules laid out).
Mentz114 said:
vanhees71 said:Of course, I have to use the relative angle of the detectors to calculate these probabilities, and thus this information goes into the code.
Isn't it enough if "Charlie" (or "C") knows both measurement settings as said by @stevendaryl in post #43 ?DrChinese said:As mentioned in post #25: you cannot simulate entanglement if Alice doesn't know Bob's choice of measurement setting, and Bob doesn't know Alice's choice of measurement setting. So yes, if you circumvent that, you "cheat". That is to say that using the relative angle in the code to generate the answer is a cheat. If you don't cheat, you can't even hand pick data sets that match the probability predictions of QM.
So hopefully we are all in agreement on this point, which was what was in error in the OP's original code - which made use of the relative angle.
The measurement settings are not fixed in principle; they can be changed at the last moment.forcefield said:Isn't it enough if "Charlie" (or "C") knows both measurement settings as said by @stevendaryl in post #43 ?
I know that - are you saying that I can't simulate entanglement without changing measurement settings at the last moment ?entropy1 said:The measurement settings are not fixed in principle; they can be changed at the last moment.
The principle is that the settings are unknown (till the last moment). If they are fixed and conveyed (to C) we have a special case IMHO.forcefield said:I know that - are you saying that I can't simulate entanglement without changing measurement settings at the last moment ?
This is hard to explain because I cannot see the problem. We know that A and B will have definate settings before they project their photon. It does not matter when they get them as long as the selection is random (independent).stevendaryl said:But in a Monte Carlo simulation, the inputs are generated, as well as the outputs, which means that the inputs (the detector settings, in this case) are known in advance.
forcefield said:I know that - are you saying that I can't simulate entanglement without changing measurement settings at the last moment ?
Mentz114 said:This is hard to explain because I cannot see the problem. We know that A and B will have definate settings before they project their photon. It does not matter when they get them as long as the selection is random (independent).
The simulation works stepwise. Produce a random orientation ##\theta_0## ( 1 random used ). Now assume that A's photon reaches the polarizer set to ##\theta_A##. The probability of passing we know is ##\cos(\theta_A-\theta_0)^2##. Now draw another RN to see if it passes.
If passes we can say the alignment of photon B is ##\theta_A## and now we can calculate if it will pass B's polarizer.
It works.
Yep. It is possible to simulate entanglement. Only the (simulated) projection of both photons is required to do this. A's setting is not 'known' by B - it is carried by the photon and affects B's result.DrChinese said:This would simulate QM entanglement, because of your usage of ##\theta_A## to calculate B's outcome. Without that information, you can't get agreement with the predictions of QM. So if the A and B algorithms are separate, per Bell you can't get that agreement.
If the wavefunction (which is what you mean, I think) is transporting information from A to B, wouldn't we have manifest non-locality?Mentz114 said:Yep. It is possible to simulate entanglement. Only the (simulated) projection of both photons is required to do this. A's setting is not 'known' by B - it is carried by the photon and affects B's result.
When either of the photons is projected into a definate polarization state the other must also be in that state. Experiments seem to show that the separation is irrelevant. So information has gone from the first projected photon to the other ( it is said ) but I think they are just always are in the same state - a shared field.entropy1 said:If the wavefunction (what is what you mean, I think) is transporting information from A to B, wouldn't we have manifest non-locality?
I am not so sure myself; there is no temporal ordening of the detections; A is not before B, nor B before A in a spacelike separated setting. So there is no 'transporting' in any definite direction.Mentz114 said:When either of the photons is projected into a definate polarization state the other must also be in that state. Experiments seem to show that the separation is irrelevant. So infoemation has gone from the first projected photon to the other.
entropy1 said:I am not so sure myself; there is no temporal ordening of the detections; A is not before B, nor B before A in a spacelike separated setting. So there is no 'transporting' in any definite direction.
Mentz114 said:This is hard to explain because I cannot see the problem. We know that A and B will have definate settings before they project their photon. It does not matter when they get them as long as the selection is random (independent).
The simulation works stepwise. Produce a random orientation ##\theta_0## ( 1 random used ). Now assume that A's photon reaches the polarizer set to ##\theta_A##. The probability of passing we know is ##\cos(\theta_A-\theta_0)^2##. Now draw another RN to see if it passes.
If it passes we can say the alignment of photon B is ##\theta_A## and now we can calculate if it will pass B's polarizer which depends on B's setting.
It works. Statistically the results are obviously violating the expectations.
DrChinese said:If your algorithm for determining the result of A's measurement requires knowledge of B's setting, or if you need to know A's setting to determine B's results: then you are not using a separable algorithm. Thus the "cheat" and it is not simulating local realism. If you allow the "cheat", you CAN simulate QM/entanglement. But that is the only way.
This statementstevendaryl said:I don't know exactly what it is that you are describing here. This is the answer to what question?
I take to be an objection of some kind. But it does not make sense in the context of a MC simulation. I respectfully suggest that you try to understand how the MC works.stevendaryl said:But in a Monte Carlo simulation, the inputs are generated, as well as the outputs, which means that the inputs (the detector settings, in this case) are known in advance.
Mentz114 said:I take to be an objection of some kind. But it does not make sense in the context of a MC simulation.
The first two things you've listed belong in space-time and dynamic simulations. The third is a about probability and is respected in the simulation.stevendaryl said:I guess I don't understand why Monte Carlo simulations are relevant. Of course, we can simulate the probabilistic predictions of quantum mechanics. What we can't do, as implied by Bell's inequality, is simulate it in a way that respects the causal relationships between the parts, namely,
- there are no signals propagating between the two detectors
- there are no signals propagating back from the detectors to the source
- the detector settings are unpredictable
Of course, the settings of the detectors have to be known by the simulation code and also in the real experiment to be able to analyze it. That's in the very foundations of QT, and that's at the heart of all these interpretation issues: You need to know both the prepared quantum state and the setup of the experiment (i.e., knowledge about what's measured) to get the probabilities according to Born's rule, no matter when you choose the setup of the measurement devices (often also using a random choice in post-selection mode, but of course, to test QT you need to know which choice has been made in the coincidence measurements to be able to analyze the experiment in comparison to QT, i.e., the measurement protocol must contain for each event the randomly chosen orientation of the polarizers and the like).stevendaryl said:I don't really know what you (@vanhees71) mean by a "Monte Carlo" simulation for this experiment, but the only way you can reproduce the predictions of quantum mechanics for this case is if the settings for the detectors are known by the simulation code. In other words, by cheating (according to the rules laid out).
vanhees71 said:Of course, the settings of the detectors have to be known by the simulation code and also in the real experiment to be able to analyze it. That's in the very foundations of QT, and that's at the heart of all these interpretation issues: You need to know both the prepared quantum state and the setup of the experiment (i.e., knowledge about what's measured) to get the probabilities according to Born's rule, no matter when you choose the setup of the measurement devices (often also using a random choice in post-selection mode, but of course, to test QT you need to know which choice has been made in the coincidence measurements to be able to analyze the experiment in comparison to QT, i.e., the measurement protocol must contain for each event the randomly chosen orientation of the polarizers and the like).
entropy1 said:Do you have locality then?
Sorry, I mixed your name up with Mentz's.stevendaryl said:You mean in an actual QM EPR experiment? I don't know. There is certainly no possibility of FTL communication, so by that definition, it's local.
This business of the settings being 'known' or not is irrelevant. When a photon interacts with a polarizer the only things that matter are the 1) photons polarization and 2) the polarizer angle at that time. To do the simulation only those quantities can and must be used. In a real experiment it is the same. The photon knows nothing, we know nothing but those things have a value.stevendaryl said:I really don't understand the point that is being made. In an actual EPR experiment, it is not necessary to know the two settings of the detectors ahead of time. The setting choices can be made at the last moment, using independent means. For comparison with QM, it's only necessary to record the settings afterward.
Mentz114 said:This business of the settings being 'known' or not is irrelevant. When a photon interacts with a polarizer the only things that matter are the 1) photons polarization and 2) the polarizer angle at that time. To do the simulation only those quantities can and must be used. In a real experiment it is the same. The photon knows nothing, we know nothing but those things have a value.