Fermi famously argued that either aliens aren’t out there, or they can’t or don’t want to get here, since we see none around us. An undated Stephen Hawking lecture makes a similar argument about time travel:
If sometime in the future, we learn to travel in time, why hasn’t someone come back from the future, to tell us how to do it. Even if there were sound reasons for keeping us in ignorance, human nature being what it is, it is difficult to believe that someone wouldn’t show off, and tell us poor benighted peasants, the secret of time travel. … If governments were hiding something, they are doing a pretty poor job of extracting useful information from the aliens. … Once you admit that some are mistakes, or hallucinations, isn’t it more probable that they all are, than that we are being visited by people from the future, or the other side of the galaxy? If they really want to colonize the Earth, or warn us of some danger, they are being pretty ineffective.
Many people seem quite resistant to the idea that fundamental limits might apply to our descendants, limits that continue even after trillions of years of advancement. But if we have vast hordes of descendants over trillions of years, almost none of them have the ability and inclination to travel back in time to visit us now. Because almost none are visiting. Some things really are impossible.