A recent article in The Huffington Post repeated the claim made by some physicists that time-travel is possible only into the future. Travelling into the future is obvious in one sense–we’re doing it right now. But what we typically mean is motion in time that doesn’t match what we’d expect. A jump of decades, for example.
Albert Einstein told us one way to go about leaping ahead into the future. All you have to do is get close to the speed of light. Or you can pass through an intense gravitational field. In both of those cases, time for you will slow down in relation to what those outside of the relativistic effects are experiencing.
But there’s nothing particularly challenging about the idea of travelling into the future. As I said, we’re doing it right now, just at a familiar rate. We understand speeding up, though, and getting to tomorrow in a hurry seems natural to those of us who live busy lives. It’s going back to the past that throws logic into a loop and threatens our understanding of who we are.
One reason for this is the grandfather paradox. It’s pleasing to note that this term comes from a science fiction writer, one René Barjavel, but we’ll get back to that genre in a bit. The paradox goes as follows:
You go back into the past and kill your grandfather before he meets your grandmother. But since your grandfather isn’t available to be your ancestor, how did you come into being?
This paradox assumes that the universe operates according to logic, but that’s a safe assumption, given the predictive power of science. But the universe doesn’t have a law book. In physical terms, what would enforce this?
Travel to the past would require some kind of gateway between that moment and the present. Stephen Hawking offers one possible answer to what would make this impossible. This gateway–often a wormhole is suggested here–would not just allow the time traveller to go through, but would also permit natural radiation such as decaying atoms, light, and so forth to pass. But that radiation would then move forward into the future, creating a feedback loop that would rapidly collapse the wormhole. Rapidly here means too fast for someone to use it. Now since according to the First Law of Thermodynamics, energy can neither be created nor destroyed, this might itself be a violation, though energy can be transformed. A wormhole, for example, might cause energy sent into it to blueshift, and this higher frequency could be what would cause the collapse. We also have to recall that the Second Law tells us that energy can be changed into a form that cannot be used to do work. Instead of causing feedback, energy sent into the past might cease to be useful by the time it reached the present again.
Here we’re at risk of getting into deep mathematics. Sticking with the notion of causality–namely the idea that someone has to be my grandfather’s killer and I can’t be that someone if I didn’t have a grandfather–we still have the intuitive sense that going into the past should be impossible.
The way around this, one that we science fiction writers have been tossing about for decades, involves parallel realities. An excellent illustration of this is Jorge Luis Borges’s “The Garden of Forking Paths.” In it, the narrator confronts the nature of choice. At each moment, he can pick from many options what he will do. What if each one of those is actually the one he selects? In other words, imagine that at each moment all possibilities fork off into separate universes–separate expressions of each possible quantum state.
This is how I answer the grandfather paradox. I’ll throw in some dates and names to make this easier. Henry Dowland, a nineteenth century gunfighter, decides that his life in 1870-A is intolerable and travels back to 1800-A, a time before his father was born, to kill his paternal grandfather. Upon doing so, Dowland returns to 1870. But the 1870 that he arrives in is not 1870-A, but 1870-B. The act of killing his grandfather was done in a different timeline, the B line, but he brings along with him all of his personal history that he accumulated in the A line. Thereby, he can still exist, since he didn’t kill the A grandfather. He may or may not go on to live a wonderful life, but the past that he remembers won’t be what those around him recall.
What we see here is that while time travel to the past might be possible, travel to our own past may not work. As distressing as jumping around to alternative realities would be, at least this does relieve us of the burden of logical paradoxes.