FTL travel - the ability to travel from one solar system to another solar system faster than the speed of light can travel the same distance. Doesn’t matter how you do it - warp drive, alternative space, magic, I don’t care, as long as you can do it reliably and repeatedly.
Time travel - the ability to move in time in a manner analogous to the way we already move in space, backwards, forwards, whatever. Perhaps like H G Wells device, or more like Dr. Who. There may be restrictions (there may be Time Lords, for that matter) but basically you can go whenever you want.
I voted for “the same thing, and impossible,” but it does occur to me that the concept of a Tipler Machine – a rotating cylinder of incredible mass – is just on the vague edge of possible, so I probably should have voted that time travel is slightly more likely than FTL.
I voted “FTL travel and Time travel are basically the same thing and may be possible”; but that’s with the qualification that I think that they probably are indeed impossible.
I just don’t think we know enough of physics to know for* certain* that there’s no loopholes allowing them.
They’re both impossible, so that was how I answered. I figured that was the important part. They’re different things, sure, but since neither of them will ever happen, the difference is vacuous.
Why is time travel impossible? Because it would violate causality. But so would FTL. If you can travel FTL you could violate causality. Yes, only in specific convoluted circumstances, but if you can violate causality even a little bit then it’s like you’re a little bit pregnant.
Maybe we’ll find that causality isn’t all that. But it sure doesn’t look that way as of Jan 28 2016. So both are impossible, for the same reasons. Or to put it another way, if one is possible then the reasons we think the other would be impossible become very hard to justify. If you have a time machine you can easily travel faster than light–just travel to your destination slower than light, then get in your time machine and travel back in time such that you arrive faster than light could have arrived.
Time travel is impossible; it’s not a “dimension” like the three dimensions (that we know of) in space. Even if it was, you can’t travel from one time to another instantaneously any more than you can teleport; you have to pass through all of the time between the starting and ending point, which is impossible going backwards as you will “run into the version of yourself that was already there” when you start traveling.
I am not 100% convinced that FTL is impossible. It probably will never happen, but that doesn’t mean that it’s impossible.
Except it turns out that particles do indeed move from one place to another without traveling all the distances between them.
Let’s remember that common sense is a very poor guide to physics when dealing with very small or very large distances, velocities, or energies. Our brains evolved to solve problems on a mesoscopic scale. On a microscopic or macroscopic scale it turns out that things don’t work in the way we would expect them to work.
It’s really easy to see why time travel implies FTL travel: If you can travel in time, then you can just go wherever you want to go, then travel back in time so that your net travel time is less than it would take light to get there. Ta-da, FTL.
And, as Half Man Half Wit points out, FTL travel is time travel according to relativity.
If you’re taking “frames of reference” into account, then FTL travel is already possible.
At some point in time (call it “0 seconds”), something located 3 light-seconds away from you starts traveling directly towards you at 2/3 the speed of light for 3 seconds. At 3 seconds, it reaches the point 1 light-second away from you and stops.
From your frame of reference, at 3 seconds, you see it start to move from the point 3 light-seconds away, and at 4 seconds, you see it reach the point 1 light-second away, so it appears to have traveled 2 light-seconds in 1 second, at a velocity of twice the speed of light.
“Same thing” is perhaps putting it too strongly, but “strongly related” is the way I’d phrase it. If you have one, then you have at least a limited form of the other.
I’m comfortable answering that they’re impossible because they are impossible based on what we know now. Anything that allowed one or the other would be a substantial change in how we understand the universe. I wouldn’t rule out that kind of substantial change - quantum mechanics and relativity both qualify, and we have yet to figure out how to reconcile them to each other.
I guess Star Trek has affected my rational brain, because I chose FTL.
I don’t think either is likely, but there might be some sort of ‘cheat’ in the offing that allows technology to change space itself, so getting from point A to point B is not the same thing as traveling the shortest 3-dimensional distance from point A to point B. And I think that is more likely than traveling through time, even though unlikely.