1

Any demonstration I've seen of using FTL signals to violate causality seemed to involve an exchange of FTL signals. Would FTL travel be consistent with causality if it always involved motion in one general direction?

For example, call the direction toward the Andromeda Galaxy from here (and continuing beyond in the same direction, of course) "universal west." You could go as fast as you want to if you're going west, northwest, or southwest, but not an inch east.

I think this gives up "Lorentz invariance," but that's OK. I can't remember getting any Christmas cards or birthday gifts from Lorentz invariance in, like, ever.

EDIT: I'm not worried about things appearing out-of-order to an external observer, but really only worried about preventing FTL signals from being able to directly or (as in every case I've seen) indirectly send a signal into one's own past. The "universal west" direction is arbitrary but would be the same for all observers.

Darth Wedgius
  • 158
  • As long as you can hide your FTL existence completely, yes. That, of course, assumes that you are completely shielded from the rest of the universe wherever you are, in which case it doesn't matter where you are, at all, since you can't see anything. That kind of FTL travel is quite easy. Just close your eyes, wish yourself upon a star and DO NOT OPEN your eyes, ever! Done. – CuriousOne Apr 27 '15 at 03:56
  • @CuriousOne Either you misunderstood my question, or I'm too dim to see the connection. A possibility I'm certainly not discounting. I did not intend any "hiding" to occur -- Spaceman James T. Spiff would be able to plant the flag and even radio Earth that he'd arrived. He just wouldn't be able to radio them back FTL. – Darth Wedgius Apr 27 '15 at 04:03
  • My point was that once you are FTL, you are FTL and you would violate causality in some inertial system. I don't see how not not going back will cure that. Maybe I am missing something myself. As for not radioing back... that's hard to avoid without complete EM shielding (which violates thermodynamics). – CuriousOne Apr 27 '15 at 04:22
  • 1
  • If you are traveling faster than light, there is always some observer who observes that you are infinitely long and exist only for an instant. 2) If you are traveling faster than light and (according to you) fall off your ladder onto the floor, there is always some observer who sees you fall off the floor onto your ladder. 3) If you are traveling faster than light (relative to me), I can observe a third party fall off his ladder to the floor while you observe the same third party fall off his floor to his ladder. Does any of this bother you?
    • – WillO Apr 27 '15 at 04:42
  • @WillO Basically, I'm fine as long as you cannot in any way use this to send a signal into your past, even with an unlimited number of signallers. – Darth Wedgius Apr 27 '15 at 06:08
  • But now you are back to my first suggestion: you have to be completely shielded from the rest of the universe. – CuriousOne Apr 27 '15 at 07:23
  • @CuriousOne Could you describe a scenario where one-way FTL travel allows one to send information into one's own past? – Darth Wedgius Apr 27 '15 at 08:20
  • Based on real physics? Based on real physics I can't even describe FTL because all equations predict that you would decay like tachyons do. The problem with causality (should you magically survive your tachyonic phase) is explained here: http://physics.stackexchange.com/questions/52249/how-does-faster-than-light-travel-violate-causality – CuriousOne Apr 27 '15 at 08:40
  • @DarthWedgius: What do you mean by not being able to send a signal "into your past"? Your world line does not extend into your past light cone in either direction. The two directions along your world line are perfectly symmetric. Some observers will call one of them your past and the other your future; others will call the first one your future and the other your past. Do you mean to say that you can't send signals in either direction along your world line, or do you mean to say that there's a distinguished direction in which you can't send signals? If so, what distinguishes it? – WillO Apr 27 '15 at 14:44
  • @CuriousOne The scenario you linked to requires two-way FTL travel, and is precisely what I am trying to avoid with the "one-way" limitation. – Darth Wedgius Apr 27 '15 at 15:42
  • @WillO Every scenario I've seen where FTL communication/travel allows one to send a signal into one's own past involves an exchange of signals between two sender/receivers. I'm not talking about me sending a signal directly into my own past, but a scenario such as the one CuriousOne linked to. And I'd be able to receive and send signals in any direction just fine, but only one direction would allow FTL signalling; the rest would be light-speed at most. The reason that direction is preferred is arbitrary on my part, but would be the same for all observers. – Darth Wedgius Apr 27 '15 at 15:47
  • @DarthWedgius: Okay, I see now that this should have been clear from your original post. Sorry to have made you spell it out again. – WillO Apr 27 '15 at 15:59
  • @WillO I'll edit the post to make it clearer. CuriousOne seems to be having a lot of trouble understanding it too. – Darth Wedgius Apr 27 '15 at 16:02
  • Physics doesn't allow you NOT to send an electromagnetic signal about your existence back at the speed of light, so the combined speed of both "travels" will always be FTL. It's not a great mystery. Anyway, the real difficulty would be to survive the first FTL travel... – CuriousOne Apr 27 '15 at 19:38
  • @CuriousOne In what scenario could this be used to send a signal to one's own past? – Darth Wedgius Apr 27 '15 at 20:20