ok... so in other words, it's not something that someone has said is impossible because of relativity (specifically the impossibility of ftl travel).The way I would answer this is that the information encoded in entanglement is only extractable when you look at correlations between measurements on both the entangled systems. So to access that correlation information, you would need communication anyway, and that communication could not be FTL. If you only look at either system, but not the other, then you need no such communication, but you also can extract no information from the entanglement. This is actually a good thing, because much of science is done by ignoring entanglements, and the reason we get away with that is the information we are ignoring cannot interfere with our interpretation of the results of our experiment. In other words, aliens aren't talking to us in our laboratory experiments, because if they were, we could not disentangle their messages from the laws of physics themselves.
Relativity is a set of axioms. We should therefore never say something is impossible "because of relativity", we should say "if it is possible, then something in relativity is wrong". We have many tests of relativity, which gives us some confidence it is not wrong, but it does not mean it could not be wrong, i.e., it does not mean we can use relativity to make statements about how reality must behave.ok... so in other words, it's not something that someone has said is impossible because of relativity (specifically the impossibility of ftl travel).
It's not meant to be confusing-- the point is, we have to start our experiments somewhere, and we need to assume our particles are coming into our experiments with no hidden FTL entanglements that could mess up our experiment. If we could extract information from them in a acausal way (i.e., FTL), then we have lost control of our experiment, because it would require that two experiments prepared in identical ways could yield statistically different results (due to the acausal information arriving). So my point is simply, one man's "FTL communication" is another's "unreliable experiment". We can't have it both ways.I must say your aliens analogy is rather confusing, and hence i've ignored it... like any good quantum scientest ignores entanglements;)
Yes, they could for example agree on the axis of a spin measurement. Then each would know, as soon as they measure their own qubit, what the other got/will get. So they gain information about distant places instantly, but no information is going FTL, it only travels around in their brains, in a causal way (presumably).However, I'm also confused about this restriction. If Alice needs to communicate to Bob how to measure his qubit to get the same outcome as her, can't they agree when they generate the state to perform a measurement to get this result at a v.similar time when separated to transfer the info faster than light?
quite simply the above question.
Why does quantum entanglement not allow for faster than light communication?
I'm afraid I don't understand the point you are making there. Are you saying that information is sent and special relativity is violated, that information is sent but special relativity is not violated, or that information is not sent?
I think it's been definitively proven that if the accepted equations of quantum theory are correct, then no possible experiment can be used for FTL communication--this source says it's ruled out by "Eberhard's theorem" in section 2.3, and this article by Cramer says:There are those (myself among them) who believe that there might be a way to use complementarity to send a signal by altering the behavior of entangled quanta depending on how they're measured. There have been experiments showing that an interference pattern in photons is created if and only if the entangled twins are detected with absolutely ambiguous position information. So far, these experiments all require correlations because only a subset of photons is able to be detected at a given time. However, if ALL of the photons can be detected ambiguosly, then, in theory, the entangled twins should create a visible interference pattern and, perhaps, allow for signaling. John Cramer, a physicst at Wash U. is working on such an experiment now.
So, it seems Cramer is pinning his hopes on a nonlinear modification to the accepted equations of QM. I think this is the paper by Eberhard they're referring to, by the way.At the AQRTP Workshop we considered the question of whether quantum nonlocality was a possible medium for FTL communication. In the context of standard quantum mechanics there is good reason for believing that it is not. Eberhard has proved a theorem demonstrating that the outcomes of separated measurements of the same quantum system, correlated by nonlocality though they are, cannot be used for FTL observer-to-observer communication. A possible loophole in Eberhard's theorem could arise if, following the work of Nobel Laureate Steven Weinberg, one modifies conventional quantum mechanics by introducing a small non-linear element into the standard QM formalism. It has been shown that in slightly non-linear quantum mechanics, the observable nonlinear effects that would arise would make possible FTL communication through nonlocality.
But the reason no information is sent is not because the information looks random, it is because the information is not sent.If the random outcome of an entanglement experiment is communicated at
superluminal speed then the claim is:
"Special Relativity is not violated because no information is send"
But you still haven't explained why you think that information is ever sent in an entanglement experiment. I don't see it going anywhere.For example: One of the outputs of the detector may be configured to trigger the
"mother of all bombs" which "blows up earth". One can hardly say that the random
bit transmitted at superluminal speed has "no information", is not physically relevant,
and therefor doesn't violate special relativity......
Yes, it seems that. My "horse sense" tells me that if quantum mechanics with Weinberg's nonlinearity worked in the real world, there would also appear some other aspect of the correction that still makes FTL communication impossible. In other words, there does not seem to be any principle of a corrected quantum mechanics that would be more fundamental than the principle of causality. Of course, experiment may prove me wrong, I'm just saying where'd I'd put my money if anyone gave me the chance.So, it seems Cramer is pinning his hopes on a nonlinear modification to the accepted equations of QM.
But the reason no information is sent is not because the information looks random, it is because the information is not sent. But you still haven't explained why you think that information is ever sent in an entanglement experiment. I don't see it going anywhere.
The "teleportation" process requires waiting for information to be transported across classical channels. (Are you unfamiliar with this?)The process goes by the name "quantum teleportation". This literally says that something (the quantum state) is being transported over far.
Read more carefully.The "teleportation" process requires waiting for information to be transported across classical channels. (Are you unfamiliar with this?)
Quantum teleportation is not FTL. As I said, I agree that transporting "random" information FTL would still violate SR, the point is, that's the incorrect reason that entanglement doesn't produce FTL communication. The correct reason is simply that it doesn't "transport" anything, random or otherwise, FTL. Perhaps you are not disagreeing with that.The process goes by the name "quantum teleportation".
No, This is the reasoning of the EPR crowd:Quantum teleportation is not FTL. As I said, I agree that transporting "random" information FTL would still violate SR, the point is, that's the incorrect reason that entanglement doesn't produce FTL communication. The correct reason is simply that it doesn't "transport" anything, random or otherwise, FTL. Perhaps you are not disagreeing with that.
Are you sure about the claim that the "quantum state is teleported instantaneously"? Do you have a reference? It seems to me that if that were the case, one could still gain probabilistic information about the original, distant state that was teleported by looking at the outcome when the teleported state was measured.No, This is the reasoning of the EPR crowd:
1) The quantum state is teleported instantaneous.
2) We can not control the collapse of the wave-function.
3) Therefor we can not use it to communicate data.
4) Therefor no information is send.
There is no meaning to "instantaneous" except for the person doing the original experiment. Nothing is "teleported" unless there is classical slower-than-light communication, so it is not instantaneous.No, This is the reasoning of the EPR crowd:
1) The quantum state is teleported instantaneous.
That's not the correct reason why we can't communicate instantaneously, the correct reason is that nothing is transported instantaneously in the first place. I cannot speak for whoever you mean by "the EPR crowd"-- I agree that argument would be spurious, but it's not the right argument anyway.3) Therefor we can not use it to communicate data.
But it's all a strawman, that's my point. If the "EPR crowd" think they require that explanation, they don't understand information theory, but since a lot of people do, I don't see that as likely. There may be a difficulty in finding people interested in philosophy who are also versed in physics.Claim 4) violates Shannon's information theorem. that's my point
I don't know what they believe, but I don't think personal beliefs are terribly relevant either.Of coarse, the majority of the EPR crowd doesn't believe this to be fundamental.
I get the impression that many of them are really chasing their Science Fiction
dreams and that statements like: Special Relativity is not really violated are
more to appease peer reviewers than that they themself believe in it.