So, this is the first post that we will dedicate to the question of time-travel in physics. We’ll start easy, but things may get pretty confusing soon, so behold!
Of course, we’re all time traveling, right now. We’re traveling towards the future, at a rate of one second per second. Strange speeds in our time travel appear as early as the Mahabharata, when king Kakudmi visits lord Brahma for some chat and, when he returns, many years have gone by. Yet, travel to the past appears later in stories, and mostly for the pleasure of anachronism. The time machine appears by the end of the XIX century in a short story from a Spanish writer, Enrique Gaspar y Rimbau, el “Anacronópete”, where the theory is exposed that it is the atmosphere causing the flow of time, as can be checked by the conservation of food in hermetic cans… His machine travels to the past much like Superman, flying against the rotation of the Earth.
The first story to deal with the problems of time travel to the past seems to be
Tourmalin’s Time Cheques, by Thomas A. Guthrie, under pseudonym in 1891, which I can’t discuss yet… (it’s in my reading list).
To the best of my knowledge, the first story which shows the problems and paradoxes of time travel to the past in its full glory is By his own bootstraps, by Robert A. Heinlein, in 1941. If you enjoy discussion about these topics, you really should read that short story.
The first and foremost paradox of time-travel is the grandfather murder case. I travel 50 years back in time and kill my grandfather before he meets my grandmother… so I can’t be born, and can’t kill my grandfather. So, if I do A, I force not-A, which forces A… what is the way out? Somehow, something should prevent you from killing your grandfather, so that history remains coherent.
We physicists love to give a name to everything, so we’ve baptized it as the Novikov principle. History should be coherent. Perhaps, after all, I do not have free will, I can’t choose to kill my grandpa… You see, the paradox with people gets somehow out of focus. Let us put it up simply will balls. This way, we call it Polchinski’s paradox:
We have a time-machine which has an input slot, an output slot and one dial. If you put something in the input slot, it will come out of the output slot some time before given by the mark in the dial. OK. Now, we put the dial to “1 second” and throw a ball towards the input slot. The same ball will come out of the output slot 1 second before the original one hits the input slot, OK? Now we can fix the geometry so that the second ball hits the first and puts it out of the way. So the output ball will prevent the input ball from entering the machine and, therefore… where did the second ball come from?
There are ways to overcome this paradox. Can you think of any?