Just when I think I have a relatively (I know, bad
pun) good grasp of
Relativity, I get
confused by an issue like this one. Specifically, there are two points I'm not clear on: (And I'll mention in advance that I'm
not a physicist. =)
First, Is it possible to transmit information faster than light? I don't mean with current technology, of course, but does the Theory of Relativity specifically forbid this? It's often said that transmitting information FTL would cause or allow time travel-like paradoxes (i.e.- If you transmit a signal which arrives at its destination before it is sent, you could decide not to send it after it arrives, creating a paradox). But (in my understanding) when people say this, they seem to be confusing light speed with infinite speed. (correct me if I'm wrong, but...) Something travelling at infinite velocity leaves its origin and reaches its destination at precisely the same time (zero travel time). To arrive before its departure (negative travel time), it would have to travel at greater than infinite velocity. This brings me to my second point.
Would an object travelling faster than light travel back in time? Light speed, c, is finite. Twice the speed of light, or 2c, is also finite. However, according to Relativity, as one approaches light speed, subjective time slows, and ultimately stops. Here's how I understand it--A spaceship travelling 2 lightyears at the speed of light (ignoring acceleration and deceleration) would take 2 years to complete the trip, but no time would pass for the passenger(s), because of the time dilation effects. Based on this, a ship travelling at twice the speed of light would travel the same distance in half the time, but would still arrive one year after it left. So, (according to my possibly flawed logic) an object does not travel back in time by travelling FTL. I mentioned subjective time because that was the idea on which, I think, many people based the entire FTL time travel concept. To speculate along those lines, time (from my perspective) would slow down as I accelerate, until it finally stops when I reach c. To an outside observer (assuming they could somehow observe me going that fast), my watch would stop, and I would stop aging. If I were to exceed light speed, would subjective time reverse? This would not seem to have any effect on the outside universe, but would seem rather confusing a hypothetical traveller who suddenly realizes he's already on the vacation to Alpha Centauri he had been planning to go on several months (or years) later. He would have to reset his watch and calendar, and be reminded of all the things he forgot, since during his flight, he would have gotten younger, and lost memories of things that happened before his departure. That gets rather mind-boggling...and I've gotten off my original topic. Hopefully the actual experts can clear some of this up a little. ;)