In Search of Lost Time Machines

Abstract

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.

Cosmic string spacetime contains matter that has positive energy density and is physically reasonable. However, the warping that produces the closed timelike curves extends all the way out to infinity and back to the infinite past. Thus these spacetimes were created with time travel in them. We have no reason to believe that our own universe was created in such a warped fashion, and we have no reliable evidence of visitors from the future. (I’m discounting the conspiracy theory, that UFOs are from the future, and that the government knows and is covering it up. Their record of coverups is not that good!)

Stephen Hawking , “Chronology Protection,” 2002, 91–92