Comparing to sonar isn't valid since objects at subsonic and supersonic speeds still obey the rule that 1 + 1 = 2. For high velocities, this rule is no longer t

Comparing to sonar isn't valid since objects at subsonic and supersonic speeds still obey the rule that 1 + 1 = 2. For high velocities, this rule is no longer true and you need to resort to using a spacetime diagram to compute velocities. If you draw a tachyon on a spacetime diagram going faster than the speed of light, you can create situations where the tachyon travels back in time for certain observers. > AFAICS if information can travel at >c, then physical objects traveling at >c doesn't violate causality If information can travel faster than c, then you can create situations where it's also violating causality.