FTL: Faster Than Light Communications Don’t Violate Causality

Many physicists take for granted that if Faster Than Light (FTL) communications were possible some signals could be received earlier than they have been emitted. That’s not correct. “Proofs” that Faster Than Light would demolish causality rest on a confusion between causality and temporality. Time is affected by motion, causality isn’t.



Let’s call the Fast Frame FF and the Inertial Frame IF. The great result of Poincaré is that if a clock in Inertial Frame is accelerated to Fast Frame, once it sits still in FF, it will run slow (it’s called the “Twin Paradox”). 

Henri Poincaré pointed out as early as 1898 (Einstein was 19), events which are VIEWED AS simultaneous in the Fast Frame FF may not be VIEWED AS so in the Inertial Frame IF  depending on the exact motion of FF. (If light behaved as sound, it would be obvious why: sometimes sound takes longer to catch up. It’s basically the same mathematical idea.)

The fact events look simultaneous, or not, depends upon defining simultaneity with light speed, and that c is finite. The sketch above is ROUGHLY correct… But not really, because it contains implicitly a subtlety which needs to come to the fore to be fully correct: the LOCAL time inside the train is NOT the same as the LOCAL time on the platform watching the train… so the detail of the drawing is not as simple as the first sketch… But the basic concept remains: light will be seen as hitting the left side of the carriage first if the carriage is moving towards the right…

It’s very simple. Let’s visualize IF and FF the former as a train station, the second as a train, rushing by. Consider the two extremities of a carriage, A and B, call M the middle of the carriage. Then shine a light at M. From inside the carriage, one can see (say by reflecting the light on mirrors), that the light arrived at A and B simultaneously. 

However, from the outside, as seen from the Inertial Frame, the train station, as the carriage is going towards M, the light is going to hit A first, and B, later. 

The proof rests only on the speed of light being finite (something we have known since studying the moons of Jupiter in the Seventeenth Century. 

So simultaneousness of events having to do with light as defined in the Inertial Frame will be different from simultaneousness of events having to do with light in the Fast Frame… As seen with light!  The order of some events may appear one way in IF, and the opposite in FF. If one defines causality as temporal ordering, one is lost. All of this was discovered by Henri Poincaré… and probably why Henri proposed to call this “the theory of relativity”… Relativity of light events as seen by light. All very enlightening, but all too light.

Now here is a philosophical question: suppose we have an instantaneous way to communicate when things are happening. Would that redefine simultaneity? Of course. now in the real world, spacetime is curved, and light goes neither straight, nor constantly, and may even not go anywhere at all:

Spacetime is curved, and in other words, light is neither straight nor constant. More precisely this means that parallel transport along a ray of light brings a rotation of frames. Also shrinkage in various dimensions.

At first sight, physics locked the notion of time down. Indeed, at first sight…. we have only two long range interactions: gravity and electromagnetism (light). Moreover because of Energy = Mass (Henri Poincaré, 1899, La Sorbonne, Paris), they go at the same speed.

At second sight, we have Quantum Entanglement… Yes, I know, many physicists say Quantum Entanglement can’t be an interaction, because it would go faster than light, and that would violate causality. They may as well quote the Bible… It’s faith on their part, and dogma, not science…

Moreover, time, as defined by light clocks is only local, a notion living in the tangent space to spacetime. An explicit illustration of that is local time in a GPS satellite is not Earth time, and modifications to clocks have to be made constantly.

In this real world, defining time only with light is not the only problem. Wait, some will say, do you have any better than light and gravitation to tell us of distant events? Of course, we do: Quantum Entanglement. In my vision of the matter, matter, and space are literally created by Quantum Entanglement. And that goes at a faster clip than electromagnetism and gravity, or things would fall apart.

Patrice Ayme

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2 Responses to “FTL: Faster Than Light Communications Don’t Violate Causality”

  1. Gmax Says:

    Very nice and clear… Except for the causality thing which I didn’t get. Can you explain so simple minds get it?


    • Patrice Ayme Says:

      Time is local, transmission at a distance, instantaneous or not, is not. So (Special) Relativity is local, and can’t be used to disprove something nonlocal.


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