That's always struck me as sort of inconsequential, even though light is the apparent "speed limit" of the universe.

The same thing happens with sound regularly. A thing happens, and we receive light before we receive sound. Some third faster thing that gets to us before light would just be some other thing.

The distance of travel requires time relative to both reference frames.

If a star explodes at A and I travel to B at 2x the speed of light to tell folks at B, so what? Some time has elapsed and those two frames are still chugging along with causality.

Maybe there's some physics I've not learned that makes this obvious, but I've never grasped exactly why violating light speed prevents entropy from continuing. Even if at two different rates.

If we perfectly timestamped everything happening on two sides of the world and sent a telegram at twice lightspeed from A to B, it would still arrive after it is sent.

Let’s say you can travel at 10c. At Point A, the Arlington Photons win the Galaxy Cup 1-0. You zip to Point B and place a bet on that game, borrowing the money to place that bet from the Baltimore branch of the Arlington Legbreaker Gang. Wait for the photons, collect your winnings at Point B, then zip back to Point A ahead of your loan. The loan info arrives at Point A, and you pay it off a microsecond later, and pocket the difference.

Congrats, you have just traveled “back and forth” through time. To people in both the Point A and Point B reference frames, you have “seen the future” and acted on it.

Now, the Legbreakers aren’t stupid. Once they know someone is screwing around with FTL they’ll just hold bets and payoffs until light can travel. Yes, but… how do they synchronize their clocks between A and B? And how can they distinguish between a lucky person and a time traveler? Etc etc, ad regressum.

"Now" and "Here" as fundamental concepts only make sense inside a single reference frame. If you break the idea that information has a speed limit, then both of those ideas shatter into tiny little pieces.

More precisely, how do they synchronize their clocks between A and B when the maximum speed of light/information is no longer a known quantity?

The whole point of relativity is that there are no absolute time stamps. There are only causal relationships which are observer dependent.

If you start being able to travel >c those relationships immediately get impossibly and paradoxically twisted.

See e.g.

http://www.theculture.org/rich/sharpblue/archives/000089.htm...

But you can’t go back to A and change those events before they happen.

That’s not time travel. You could get the same effect by just blocking the photons before they make it to B, and then traveling at conventional speed.

Yes however if you were to pop back to A you wouldn't have any future knowledge at A (minus the 'future' knowledge of B).