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As far as I can tell Eberhard and Ross didn't "prove" quantum entanglement can't implement FTL communication in "QUANTUM FIELD THEORY CANNOT PROVIDE FASTER-THAN-LIGHT COMMUNICATION", 1988 (http://link.springer.com/article/10.1007/BF00696109). But it's often cited as such a proof, in threads on PF and elsewhere.
E & R describe a standard gedanken with sender and receiver (today we would say "Alice and Bob") outside each other's light cones, performing measurements on a pair of entangled particles.
Section 2.2.2 says:
"According to Hypothesis 2, the field operators phi-sub-j(xs,ts) at points (xs,ts) in [sender's light cone] commute (or anticommute for Fermion fields) with field operators phi-sub-j(xr,tr) at points (x,t) in [receiver's light cone], when ts < tr. This follows from the fact that two such points are outside each other's light cone. ... It follows that the measurement operators M and Ms in the Heisenberg representation commute."
Here M and Ms refer to the two measurements of sender and receiver, which may be of observables which would be non-commuting if not space-like separated; such as Z and X axes' spin, or position and momentum. And hypothesis 2 is simply the statement, from standard QFT, that measurements made outside each other's light cones commute (or for fermions anti-commute). That's true - IF you assume FTL communication is impossible (which QFT, being relativistic, does). Thus they appear to assume what they're trying to prove!
They go on to show that, given that M and Ms commute, probabilities of Bob's eigenvalues can't be affected by Alice's measurement. Then in Section 3 they show the same is true of the joint probability distribution of M and Ms (here called M1 and M2). It's inevitable given their assumptions, but as I say they appear to commit the fallacy of petitio principii.
But finally they conclude, in 4.3.2:
"... the possibility of faster-than-light communication is not unthinkable. [But] It is in contradiction with quantum field theory, which is the only known relativistic quantum theory. [Therefore] Justification for any effect providing faster-than-light communication should not be looked for in theories that abide with orthodox quantum field theory but in theories that allow some deviations from it."
So the point of the paper seems to be: if FTL communication can, in fact, be achieved via entanglement, any relativistic version of QM - namely, QFT - must be wrong. In fact that's what the title of the paper indicates. But this is no proof against FTL communication - as claimed by many writers, including posts below. E & R merely say that if you assume (following SR) that FTL communication is impossible, then FTL communication is impossible.
Since experts often cite the paper as "proof" against FTL communication via entanglement, there must be something wrong with my reasoning. What is it?
E & R describe a standard gedanken with sender and receiver (today we would say "Alice and Bob") outside each other's light cones, performing measurements on a pair of entangled particles.
Section 2.2.2 says:
"According to Hypothesis 2, the field operators phi-sub-j(xs,ts) at points (xs,ts) in [sender's light cone] commute (or anticommute for Fermion fields) with field operators phi-sub-j(xr,tr) at points (x,t) in [receiver's light cone], when ts < tr. This follows from the fact that two such points are outside each other's light cone. ... It follows that the measurement operators M and Ms in the Heisenberg representation commute."
Here M and Ms refer to the two measurements of sender and receiver, which may be of observables which would be non-commuting if not space-like separated; such as Z and X axes' spin, or position and momentum. And hypothesis 2 is simply the statement, from standard QFT, that measurements made outside each other's light cones commute (or for fermions anti-commute). That's true - IF you assume FTL communication is impossible (which QFT, being relativistic, does). Thus they appear to assume what they're trying to prove!
They go on to show that, given that M and Ms commute, probabilities of Bob's eigenvalues can't be affected by Alice's measurement. Then in Section 3 they show the same is true of the joint probability distribution of M and Ms (here called M1 and M2). It's inevitable given their assumptions, but as I say they appear to commit the fallacy of petitio principii.
But finally they conclude, in 4.3.2:
"... the possibility of faster-than-light communication is not unthinkable. [But] It is in contradiction with quantum field theory, which is the only known relativistic quantum theory. [Therefore] Justification for any effect providing faster-than-light communication should not be looked for in theories that abide with orthodox quantum field theory but in theories that allow some deviations from it."
So the point of the paper seems to be: if FTL communication can, in fact, be achieved via entanglement, any relativistic version of QM - namely, QFT - must be wrong. In fact that's what the title of the paper indicates. But this is no proof against FTL communication - as claimed by many writers, including posts below. E & R merely say that if you assume (following SR) that FTL communication is impossible, then FTL communication is impossible.
Since experts often cite the paper as "proof" against FTL communication via entanglement, there must be something wrong with my reasoning. What is it?