The possibility of time travel

Vishaka asked:

Is travelling back in time really possible? do time machines really work?

Answer by Geoffrey Klempner

A few years ago, I had the opportunity to research this question when I was asked to write an Afterword to a new edition of David Gerrold’s sci-fi classic The Man Who Folded Himself published by Benbella Books. (See http://klempner.freeshell.org/articles/afterword.html.)

I started by reading H.G. Wells The Time Machine which gives the traditional view of time travel that generates the so-called ‘time travel paradoxes’. For example, if travelling back in time was really possible you could accidentally kill your grandfather in which case, logically, you ought to cease to exist.

H.G. Wells chooses simply to ignore the paradoxes. His novel is no less entertaining for that. Other science fiction writers have come up with dubious fixes. For example, as my jeep runs over a boy who turns out to be my grandfather, I gradually fade away into nothing as the ‘time lines adjust themselves’. Or my grandfather miraculously comes back to life. Or, supposing that I deliberately tried to kill my grandfather, every attempt fails for one obscure reason or another.

Time travel is illogical. The impossibility of a time machine has nothing to do with technology. It requires contradictory propositions to be true (for example, ‘My grandfather lived a long and uneventful life,’ ‘My grandfather was run over and killed by a jeep when he was 9 years old’).

One of the things that makes Gerrold’s novel so interesting is his solution to the paradoxes. Time travel isn’t really time travel at all, because every time you ‘hop’ to a time in the future or the past what you are really doing is creating an alternative ‘future’ or ‘past’. In that way, you can meet alternative versions of you, change the history of ‘the world’ to meet your requirements or aesthetic taste. The one thing you can’t do is go back to where you started, as H.G. Wells’ time traveller did, because ‘where you started’ is also a different world.

One criticism that could be made of my essay is that I don’t consider the possibility that reality itself might be malleable, that what makes a ‘world’ isn’t just a hard set of ‘facts in logical space’. Although my grandfather lived to old age, isn’t it still possible to bring about his death without simultaneously committing suicide? Can’t we just change the past a bit, while keeping other subsequent events the same?

For example, someone who believes in God might pray that they have passed the exam, while they actually hold the results envelope in their hand. This implies that whatever did happen in the past can be altered now, if only by God’s will. As an atheist, that’s not something I believe, but I can imagine someone believing it. It creates a problem for logic, to be sure, but we can still describe what such a person is thinking without obvious self-contradiction.

Just to think what God would have to do to change my ‘fail’ to a ‘pass’, or what would have to happen in order that I live even though my grandfather died at the age of 9, is mind-boggling. The world would have to be a very strange place indeed, if that could happen, more like a fluid dream than reality. But then, maybe that’s all it is?