DavidB66
Senior Member.
On the subject of FTL and causality violation, consider a simple thought experiment, assuming that FTL exists.
There are two participants, A and B, both of whom have FTL transmitters. Participant A sends an FTL signal to B, recording the time of transmission as shown on a clock in his own reference frame. By a pre-arranged plan, as soon as B receives the signal he sends an FTL signal back to A to confirm receipt of A's signal. A records the time at which he receives B's signal, as shown on A's clock. Does anyone dispute that the time recorded by A for his receipt of B's return signal will be later than the time he records for his original signal to B? If so, then there is no violation of causality in A's reference frame. If there is an apparent violation of causality in some other reference frame, I would argue that either the reference frame or the method of calculating time is inappropriate for recording FTL events. It would be like using a method based on the speed of sound, assumed to be the highest possible speed, for recording signals transmitted by light.
I am not for a moment denying the vast amount of evidence that the speed of light is constant, and is the highest physically possible speed, in all inertial reference frames, and that FTL signals or travel are therefore physically impossible. I am just questioning that argument that FTL is impossible because it would violate causality. I think that argument only works in the conceptual framework of Special Relativity, which assumes as a postulate that the speed of light is a constant and limiting velocity. If that assumption is empirically false, all bets are off.
There are two participants, A and B, both of whom have FTL transmitters. Participant A sends an FTL signal to B, recording the time of transmission as shown on a clock in his own reference frame. By a pre-arranged plan, as soon as B receives the signal he sends an FTL signal back to A to confirm receipt of A's signal. A records the time at which he receives B's signal, as shown on A's clock. Does anyone dispute that the time recorded by A for his receipt of B's return signal will be later than the time he records for his original signal to B? If so, then there is no violation of causality in A's reference frame. If there is an apparent violation of causality in some other reference frame, I would argue that either the reference frame or the method of calculating time is inappropriate for recording FTL events. It would be like using a method based on the speed of sound, assumed to be the highest possible speed, for recording signals transmitted by light.
I am not for a moment denying the vast amount of evidence that the speed of light is constant, and is the highest physically possible speed, in all inertial reference frames, and that FTL signals or travel are therefore physically impossible. I am just questioning that argument that FTL is impossible because it would violate causality. I think that argument only works in the conceptual framework of Special Relativity, which assumes as a postulate that the speed of light is a constant and limiting velocity. If that assumption is empirically false, all bets are off.