[Yashcheritsiy]

We can assume that they are. But we cannot prove that they are, because we cannot transmit the results of our measurements to each other in any Lorentz frame faster than the speed of light, and until we communicate our results to each other, neither of us actually has experimental evidence of what the other already knows.

Well, the obvious answer is to build a subspace ansible. What is this, the 21st century or something?

[FredZarguna]

Ursula K. Le Guin's grasp of science -- like her understanding of human nature -- was not particularly good. To be fair to her, she came out of a ψφ culture that took the admonition against superluminal travel seriously without understanding what it actually meant. The existence of the "ansible" implies time travel just as surely as physically sending a person into the past does, but that idea was one that 1960's science fiction writers [and those who followed after] never seemed to grasp.