The science behind time travel is iffy, but say it were a flawless operation. Say you traveled back in time and killed your grandfather, before he conceived your father. Would you still be born? And if not, how then did you manage to travel back in the first place?
Known as the "Grandfather Paradox," this is one of many mind-bending scenarios that physicists have imagined as they've considered the implications of time travel. Although in reality, making a time-machine (by stabilizing a wormhole, for instance, or by successfully traveling faster than the speed of light) seems physically near impossible, the hypothetical issues that arise in contemplation are valuable for their insight into time and causality.
Consider traveling back to yesterday. Would you be able to alter previously recorded events? Or perhaps create new events, which seemingly have no beginning? Further insight into these questions is made possible with the many-worlds interpretation. A minority of physicists, the radicals, you could say, believe the universe doesn't have one world history, but an infinite number of parallel histories: stemming from quantum mechanics, the many-worlds interpretation postulates that, for every observation or decision on earth, a new world history is created. In other words, you could kill your grandfather, and alter history, making it so that you were never born, but in doing so, the universe would split into separate branches; a new universe, with a different future, would branch into creation where you've killed your grandfather, while the universe from where you travelled would exist as it did before. Travelling in time would then seem like hopping from one train track to another, with new universes generated for every action you took.
But the many-worlds interpretation isn't considered to be as well supported as another, more conservative, theoretical approach, which stands by the principle of self-consistency. Developed by Russian physicist Igor Novikov, the principle of self-consistency maintains a time traveler cannot change the past because he or she was always a part of it. You simply couldn't kill you grandfather, because you're alive to do it. Your own existence refutes such an event ever happening.
Say I set up two small corresponding time machines, A and B, and I position them so that when I roll a billiard ball into B, the ball travels back in time, and reappears in the past through A, with a trajectory aimed to disrupt the ball's original path into B. The implication is that, as soon as I roll the ball towards B, the same ball will fly out of A and knock it off its path! A paradox is created: if the ball is knocked away from entering B, how could it then come out of A?
The solution to this paradox must be that the billiard ball is only lightly tapped by itself: it still enters and exists the time machine, but at a slightly different angle. This explanation was discovered and supported through mathematical calculation by physicist Kip Thorne. How does it translate to human time travelers? Well, quite simply, you can go back in time and punch, kick, mace, give a noogie to and/or trip your grandfather, but you can't kill him, otherwise you wouldn't exist to kill him in the first place. In other words, Bill and Ted's Excellent Adventure is, technically, scientifically accurate.
Novikov wrote of the principle of self consistency:
This means our free will must be constrained. If I meet a younger version of myself and wish to kill that younger version, then the laws of physics will prevent me from doing so. Such a constraint on our free will is unusual and mysterious but not completely without parallel. For example, it can be my will to walk on the ceiling without the aid of any special equipment. The law of gravity prevents me from doing so; I will fall down if I try, so my free will is constricted. Of course, in the case of a time machine, the nature of the restriction on free will is different, but not essentially different.
But it's not something we should lose sleep over. In the words of Arthur C. Clarke, "The most convincing argument against time travel is the remarkable scarcity of time travelers," And he's right: where are the people from the future with their fantastical stories about how great time travel is?
By Michael Baptist