3 ms·

I don't understand why "faster than light" automatically implies "go back in time". If it takes me 10 seconds to travel 10 lightyears, then that means I should

by cygx 6y ago

I don't understand why "faster than light" automatically implies "go back in time". If it takes me 10 seconds to travel 10 lightyears, then that means I should still be traveling forward in time (specifically by 10 seconds), no? Say you had a faster than light transmitter that can send signals with (from your perspective) twice the speed of light (2c). If the receiver has no relative motion, everything seems to work fine. But if the receiver moves away from you fast enough (I think 0.5c should do it, but don't quote me on that one), then, from the receiver's perspective, the transmission will happen instantaneously. If the receiver moved even faster than that, the signal would seem to arrive before it had been sent(!), and if they had their own faster-than-light transmitter, they could relay the message back into the emitter's past.

AnIdiotOnTheNet 6y ago
For more information, see: https://en.wikipedia.org/wiki/Tachyonic_antitelephone https://en.wikipedia.org/wiki/Tachyonic_antitelephone
yellowapple 6y ago
> But if the receiver moves away from you fast enough (I think 0.5c should do it, but don't quote me on that one), then, from the receiver's perspective, the transmission will happen instantaneously. Why? I mean, I get that time is progressing more slowly for the receiver because it's traveling at relativistic speeds, but instantaneous arrival of anything doesn't seem to follow from that, and therefore... > If the receiver moved even faster than that, the signal would seem to arrive before it had been sent(!) ...neither does this. Further, the whole point of wormholes (in the context of superluminal travel/communication) is that neither the signal nor the receiver is actually moving at relativistic speeds, but rather that the signal is instead taking a shorter path between two points in spacetime specifically to avoid having to move at such speeds from a local perspective.

by cygx 6y ago

AnIdiotOnTheNet 6y ago

For more information, see: https://en.wikipedia.org/wiki/Tachyonic_antitelephone https://en.wikipedia.org/wiki/Tachyonic_antitelephone

yellowapple 6y ago

> But if the receiver moves away from you fast enough (I think 0.5c should do it, but don't quote me on that one), then, from the receiver's perspective, the transmission will happen instantaneously. Why? I mean, I get that time is progressing more slowly for the receiver because it's traveling at relativistic speeds, but instantaneous arrival of anything doesn't seem to follow from that, and therefore... > If the receiver moved even faster than that, the signal would seem to arrive before it had been sent(!) ...neither does this. Further, the whole point of wormholes (in the context of superluminal travel/communication) is that neither the signal nor the receiver is actually moving at relativistic speeds, but rather that the signal is instead taking a shorter path between two points in spacetime specifically to avoid having to move at such speeds from a local perspective.