We, as humans might not know but the universe will. FTL is not decided by humans, the universe is, by every sane theory in existence, is structured for FTL not to happen. If we, by some voodoo eventually make it happen, I expect the universe to be very upset, unhappy until we make an FTL bomb (bombs are easy, space-ships are hard) and destroy humanity before the bombs are launched.If something changes in the past the ever present now will adjust to match it. And we would never know. Time as we understand it in the first person may just be an illusion. Time travel may yet indeed be possible, but the consequences for us would be catastrophic. Again, we would never know what could have been.
To use the term rewrite concerning time travel in faster than light travel, is probably an error because our language can't handle the concepts. There may indeed just be one now, and no other.
Since the dawn of spaceflight, mankind has been stalled by the limiting factor that is the speed of light. But no longer–with the advent of Faster-Than-Light (FTL) travel, the entire galaxy is now our backyard. The idea of visiting other worlds, perhaps to seek out new life and new civilizations, going boldly where no one has gone before… it’s very appealing, isn’t it? And it’s finally a reality, so you might be tempted; Earth is boring and well explored, after all, and no new intelligent species are likely to be popping up any time soon. But before you sign up to join the space exploration agency of your choice, consider all the ways this FTL trip could go horribly, horribly wrong. Warp drives, Jump drives, or Hyperspace, they all have their problems, and here are 4 of the biggest dangers of FTL travel.