[MattPalmer1086]

In general relativity, FTL can be used as a time machine.

[withinboredom]

Then you should be able violate casualty with simple water, otherwise FTL cannot be a Time Machine because it is quite simple to go FTL.

[MattPalmer1086]

Faster than light in a vacuum. Not faster than light in other mediums. See the many other explanations on this thread.

[withinboredom]

It's literally the same thing. The medium doesn't matter, see Einstein's equations. It all boils down to "relative to what". Light moves slower around a black hole, and that is in a vacuum. None of the arguments make any actual sense.

[MattPalmer1086]

There is a maximum speed, "c" that anything can move relative to something else in this universe.

If light happens to move slower than c under some conditions, that is irrelevant. It isn't the speed of light we care about, it is c.

Essentially, when we say FTL, it means "faster than c", not "faster than light".

[withinboredom]

Maybe we should have started with c. Could have avoided this whole discussion.