After so many deep theological posts, I decided to post something a little lighter. This essay was written for the final exam of my college composition class. I may the only student in the history of my college to argue against time travel in his final essay. As this is a college essay, works cited are available HERE.
The steely-eyed traveler in his old fashioned overcoat slowly eased the leaver into position. The room began to whirl, the world outside his study widow changing from night to day at an exponential rate. The sky gradually took on a dull yellow hue as he traveled at fantastic speeds. Slowly, he pulled the lever back to the starting position. He glanced down at the dial on his machine, noting that his machine had traveled two years into the past, before he had begun construction on his magnificent time machine. Since H. G. Well’s groundbreaking novel The Time Machine first hit store shelves, the world of science fiction has become filled with stories of travelers in the fourth dimension like this one. From movies like Back to the Future to TV shows like Doctor Who and the little remembered Time Tunnel, sci-fi enthusiasts have become enchanted with tales of eccentric temporal explorers gallivanting through the past. But should we travel in time? Should the technology become available, would backwards time travel be worth the risk? Sadly, the answer is, for the moment, no. For a variety of reasons rooted in physics, backwards time travel is not only too risky to attempt but may be impossible.
It may be helpful to first explore one of the most basic objections to backwards time travel. In our story from before, our brave inventor went backwards in time two years, before he had built a time machine. But this raises a basic problem, how can our traveler be in two places at the same time? Presuming for a moment that the time traveler from two years earlier is in his study pouring over a novel or Jules Verne, it seems impossible that he can both be in his armchair and in his study and climbing out of his time machine in the lab. This is knows as the double-occupancy problem. In order for a traveler to go back in time, he must be in two places at once. This seems to contradict the conventional rules governing physics and matter. While some physicists have theorized various ways to overcome this apparent paradox, no simple solution has presented itself.
Even if a solution to the double occupancy problem could be conceived, making time travel possible, time travel comes with a host of other potential paradoxes, arguably the most famous of these being the grandfather paradox. In his essay “Time Travel, Coincidences and Counterfactuals”, Theodore Sider describes a variation on this classic dilemma:
Imagine you had a time machine. Nothing would stop you from going back in time and killing yourself as an infant, before you ever entered the time machine. But then a contradiction would ensue: you would never have entered any time machine since you were killed before doing…, and yet you would have entered a time machine, in order to travel back in time to kill yourself.
In this paradox, the time traveler is both dead and alive and both killed himself and did not. The contradiction in this scenario is apparent and troubling for backwards time travel.
Like the double occupancy problem, the grandfather paradox has drawn criticism and potential solutions. One such solution is the use of coincidences. Again, Sider explains, “Repeated attempts at autoinfanticide [killing oneself as a child] would lead to repeated “coincidental” failures – slips on banana peels, failures of nerve, etc.” In other words, the universe would conspire to stop a paradox before it occurred. However, Sider quickly refutes this. “But we have empirical reason to think that such repeated coincidences do not occur. We do notice the odd slip on a banana peel en route to a murder, but such slips are rare indeed.” While it is possible the coincidences would occur to stop a paradox, there is little evidence to back this claim up. In his article, “Protecting the Past”, renown theoretical physicist Stephen Hawking addressed the possibility that wormholes could work with the laws of physics to make such coincidences possible, “This [his theories using worm holes] supports what I have called the Chronology Protection Conjecture: that the laws of physics conspire to prevent time travel by macroscopic objects. Although time loops are allowed by the sum over histories, the probabilities are extremely small. Based on the duality arguments I mentioned earlier, I estimate the probability that Kip Thorne [a colleague] could go back and kill his grandfather as less than one in ten with a trillion trillion trillion trillion trillion zeroes after it.” So, while it is possible that coincidences could occur to prevent catastrophic paradoxes, the likelihood is very small. Because of the possibility of causing a paradox of this magnitude, backwards time travel should not be attempted.
The biggest danger of backwards time travel is not, however, from a vindictive scientist killing his grandfather, but from small changes to the time stream through seemingly insignificant interactions with a past world. In his novel A Sound of Thunder, science fiction author Ray Bradbury presented a tale in which a time traveler stepped on a butterfly in the prehistoric past, unleashing a series of changes that rendered his future unrecognizable. This story is based on a mathematical chaos theory in which it was demonstrated that a butterfly flapping its wings could affect weather patterns across the world. Any time traveler venturing into the past could potentially unleash a casual chain resulting in drastic changes to the future.
From being in two places at the same time, to causing a temporal loop in which one both does and does not exist, to unleashing doom upon the future because of squishing a member of the family Lepidoptera, backwards time travel is wrought with danger and challenges. For the time being, backwards time travel should not be attempted. However, should theoretical physicists finally crack the potential paradoxes, it would certainly be understandable if some young man in a Victorian outfit would care to venture back to the future.