In Search of Lost Time Machines
It seems to be true that humanity has no reliable evidence of visitors from the future, or the past for that matter, or from universes orthogonal to our own. Many of the time travel stories we have examined explain this absence variously: you can go, but you can’t come back; paradoxes will prevent you leaving in the first place, or destroy your machine and you with it even before you get started; you can only go into diverged, parallel histories and once there you are stuck. Or perhaps you create the universe that you end up in, and while you can leave it you cannot return to your origin, only to a different alternative world. Or the Time Patrol will nab you, sequester your machine (or induct you into their ranks) and make certain you don’t try to kill Hitler because that never works out well.