Causality and FTL Signals...

[Sanders]

This is a pretty basic concept of relativity that used to make sense to me but I have since become a little unclear on the issue.

The question is how can one violate the principle of causality simply by broadcasting information that travels faster-than-light?

[Hurkyl]

Because a FTL signal going forward in time in one reference frame will go backwards in time in some other reference frame.

[Sanders]

Originally posted by Hurkyl
Because a FTL signal going forward in time in one reference frame will go backwards in time in some other reference frame.

How would you describe this with a high-speed (.95c) spacecraft leaving Earth?

The spaceship, experiencing time dilation, would be sending messages into the past while Earth would be sending messages into the future, correct?

[Hurkyl]

There is no such thing as "experiencing time dilation"...

Anyways, that aside, (assuming it is possible to send FTL signals) it is possible both for the earth to send an FTL signal which the spaceship perceives as going backwards in time, and it is possible for the spaceship to send a FTL signal which earth would perceive as going back in time.