# I Is it Impossible for Entanglement > FTL Communication?

1. Mar 3, 2017

Lots of people have wondered about whether we can get FTL communication from entanglement.

My question is slightly different.

Is it impossible to get FTL communication out of entanglement? If it has been shown to be impossible, then we can shut off the entanglement-FTL avenue altogether and focus on other areas of research.

2. Mar 3, 2017

### rootone

Unless you can show what is wrong with general relativity, it's not possible,

3. Mar 4, 2017

### Demystifier

What's the difference between FTL communication from entanglement and FTL communication out of entanglement?

4. Mar 4, 2017

Using entanglement to communicate FTL

5. Mar 6, 2017

### Demystifier

And which of the two expressions is supposed to mean that?

BTW, I like your avatar picture. Can I buy a t-shirt with that?

Last edited: Mar 7, 2017
6. Mar 7, 2017

### David Neves

Let's say you have particle-antiparticle creation of a electron-positron. If one is spin up, the other has to be spin down. You could say that since if one is spin up, then the other must be spin down, therefore, one instantly affects the other. However, this does not allow faster than light communication since you have no control over whether the first particle is spin up or spin down. Therefore, even if you have "spooky action at a distance" you still do not have have faster than light communication.

The fact is that faster than light communication can be easily disproved as physically impossible, because if it were possible, you could have time travel, and the grandfather paradox. You can read more about it here.

The original poster wrote "If it has been shown to be impossible, then we can shut off the entanglement-FTL avenue altogether and focus on other areas of research."

There is no such thing as an "entanglement-FTL avenue". It is not an "area of research". We are not "focusing" on FTL at the expense of "other areas of research". The way the original poster phrased it, it's as if they imagine that physics departments at universities devote resources to a FTL research program. Of course, in reality, the fact that faster than light communication is impossible can be easily proved in five minutes by any physics undergraduate with a pencil and single sheet of paper. No actual physicist would waste one second considering such a thing. I strongly suspect that the original poster read some crackpot claptrap on the Internet pushing various crank "alternative" theories, all sorts of silly nonsense, while cultivating the false impression that it's all quite reasonable.

7. Mar 10, 2017

### newjerseyrunner

The easiest way I've found to think about entanglement is to remove the idea of particles or QM or anything and think about it purely in terms of math. If you have two particles entangled, then they are described by any equation: f(x). Now if you pull out one of the particles into it's own equation: g(x), then the other particle can have no value other than f-g(x). There is no delay, the instant you define one particle, you define the other.

8. Mar 15, 2017

### maline

Have you never read any sci-fi? The "grandfather paradox" is problematic, but not necessarily a game-ender. Not by a long shot.

Besides, we can't prove that there is no preferred set of coordinate frames (spacetime foliation). The fact that the laws we know of have Lorentz symmetry is very important and has lead to most of the Standard Model, but there can still be a more fundamental theory that does not have this symmetry, as long as it explains where the symmetry comes from on the scales of QFT and above. I believe several of the models in LQG are like this. In that case, it could be that FTL communication would work only in a preferred frame and so would not allow back-in-time communication.
Well, I think there are a few legitimate researchers who do spend time on such things. Google "ER=EPR". Definitely not "mainstream" though.

The bottom line is that "no FTL causality", a.k.a. the "clustering principle", is one of the Wightman Axioms underlying QFT. Assuming QFT is a consistent theory- which hasn't been proven- we can be sure that no correct calculation using "our" quantum theory will ever predict FTL communication. But if and when we move on to Quantum Gravity- the bets are off.

9. Mar 15, 2017

### maline

Okay, that was an exaggeration. Given that all known phenomena- except gravity- seem to fit perfectly with QFT, and that we haven't yet come up with any way at all to do an experiment that would involve Quantum Gravity effects, it's a pretty good bet that whatever theory we finally come up with will be make predictions so similar to what we already know, that the theory will be irrelevant for all or nearly all practical purposes (Sorry, Interstellar...).
But of course, you never know...

10. May 13, 2017

### Denis

You cannot use entanglement to make FTL communication, and this is quite easy to prove. The point is that you can explain the QT correlations in two ways, first by some hidden causal influence $A \to B$, and, second, by some hidden causal influence $B \to A$ (this is, essentially, the consequence that Bell's theorem excludes a common cause explanation $C\to A, C\to B$. Above explanations can be given in quite explicit formulas, say, using dBB theory.

But for an effect which could be used to communicate $A \to B$, an explanation by some hidden $B\to A$ signal would be impossible. So, the theorem which proves that there has to be some FTL effect to explain the correlation, also shows that this effect cannot be used for communication.

11. May 30, 2017

### nikkkom

What about CTCs in Kerr metric then?