One of the oldest themes in science fiction is time travel, and one of the oldest story lines in the time travel genre deals with the paradoxes of going back in time. What if you went back into time and, like Marty McFly, did something that changed the future course of events so dramatically that you never actually came into being?
It’s called the Grandfather Paradox. Specifically, what if you went back in time and killed your own grandfather before he had the chance to father your mother or your father? And if you did, and your parents and, ultimately, you never existed as a result, then how could you have been here to go back into time and kill dear old Granddad in the first place?
Science fiction deals with this in all kinds of interesting ways — postulating, for example, the creation of parallel universes every time a back-in-time traveler messes with the existing continuum of events and offs an ancestor — but science isn’t so easily satisfied. It’s clear that forward time travel actually can occur under Einstein’s theory of relativity and concepts of time dilation; tests have proven that as a spacecraft’s speed increases, a clock on board the ship runs more slowly than a clock back on Earth. In short, blast off and travel fast and far enough, and you’ll return to a world where your children are older than you are.
Einstein’s theories also suggest that travel back in time is theoretically possible, because the interaction between gravity and spacetime means that if a sufficient gravitational field existed, a closed timelike curve could be created and the time traveler could travel along that curve to the past. Some scientists, like Stephen Hawking, argue that the Grandfather Paradox means that backward time travel and therefore closed timelike curves cannot exist, and they puckishly argue that the fact that we aren’t currently besieged by future beings who’ve figured out how to journey back in time means such travel is not possible.
Other scientists, however, accept the possibility of moving along a closed timelike curve and have been testing theories that would prevent Grandpa’s untimely demise. One theory focuses on consistency, and another on correlation. The “consistency” theory argues that any object that enters a closed timelike curve must exit the curve with the same properties — which evidently means that, thanks to your self-directed consistency, you couldn’t go back and kill your grandfather and prevent your own existence. Scientists have actually tried to test this theory, using polarized photons launched through a time loop simulator, and the tests showed that the simulated time-traveling photons had the same properties the theory would predict. Another theory contemplates a kind of “post-selection” concept that (I think) means that you couldn’t go back into time unless you had already gone back into time and were therefore part of the causal chain that created the world in which you live. The time loop is closed, and whatever you would do on your backward trip would inevitably be what you had already done.
Like everything in quantum physics, it’s all very weird and confusing, and of course the theoretical physicists don’t explain why anyone would want to go back into time and murder their own grandfather, anyway. But the upshot of the theories and testing seems to be that, even if backward time travel could occur, Grandpa apparently is safe. All grandfathers and potential future grandfathers can now breathe a sigh of relief.