The question is - is this a technology which can be operated at will. That would be different from the issue of whether nature might generate something like this, outside of human intentions. If I have a friend 4 light years away, and one day I've got a black box I invented which can send him a letter in what he and I both agree is 3 years, then we are in causality violation territory. Say we've been sending chess moves back and forth at the speed of light then one day I have a black box (containing Alcubierre drive with warp bubble) and I put my chess move in it and send it to him in 3 years. Then he sends the object back to me in 3 years with his move.