When a bicycle maker from Dayton, Ohio wrote a letter to the Smithsonian Institution in 1899 asking for information to help him build a flying machine, he felt compelled to assure them he was not a “crank.” The scientific establishment of the day generally regarded powered flight by heavier-than-air machines to be impossible. Fortunately, the Smithsonian’s secretary, Charles Langley, was not among the skeptics and had already experimented with steam-powered flying machines. Wilbur Wright’s letter to the Smithsonian – surely one of the most consequential in history – produced the information he sought. Four years later, he and his brother Orville flew their airplane at Kitty Hawk in North Carolina.
The Wright Brothers example might serve as a cautionary note for anyone who is too quick to dismiss as impossible some potential development on the frontiers of science. Time travel comes to mind. Many in today’s scientific establishment regard time travel as a practical, if not theoretical, impossibility. There is, to begin with, the Second Law of Thermodynamics, which effectively points the “arrow of time” in one direction only, from past to future. So even if you could build a machine that traveled to the future, it would be a one-way trip, because you couldn’t get back again. Einstein’s general theory of relativity does seem to permit time travel under certain circumstances – something that Einstein himself found disconcerting when the mathematician Kurt Gödel pointed it out to him. But just because you can do the math doesn’t mean you can build an actual time machine. And there remains the problem of causality, which goes back to that one-way arrow of time. In terms of time travel, the issue is sometimes referred to as the “grandfather paradox.” Simply stated, what happens if you go back in time and murder your own grandpa and thereby ensure that you were never born? It makes for an intriguing plot device in science fiction movies like Back to the Future and The Terminator; it is less clear how this might play out in reality.
If a would-be Wilbur Wright were to send a letter to the Smithsonian asking for information about building a time machine, he would at least have the general theory of relativity to back him up. But he might yet prove to be a crank for reasons that have nothing to do with causality or the arrow of time. Philosophers, mystics and even a growing number of otherwise reputable physicists believe that time travel would be impossible because time as we usually understand it doesn’t exist. Time as a measure of change certainly exists, but it is less clear that our sense of duration, or time passing, is anything more than a “stubbornly persistent illusion,” as Einstein once characterized it. If there is no duration, then there would be nowhere for a time machine travel to, because “right now” is the only moment that actually exists.
Einstein spent the last 30 years of his life in a futile attempt to come up with a “theory of everything” that would unify the fundamental forces of nature into a single theoretical framework. What stymied him and those who came after him is that general relativity works well with larger bodies, but the subatomic realm is governed by an entirely different set of rules known as quantum mechanics. In trying to develop a single framework for both, physicists John Wheeler and Bryce DeWitt put forward an equation that might eventually prove to be key in tying everything together. However, time is notably absent from their equation, which suggests that the universe may indeed be timeless.
We don’t need to do the math to come to this conclusion. But first we must grapple with the sense we all have that time passes, even if we can never quite put our finger on what it is that is passing. We can feel it in our bones, even if none of our physical senses actually comes into play – not sight, not sound, not touch, not smell, not taste. “What then is time?” St. Augustine wondered long ago “If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.” We resort to metaphors like the arrow of time, even though time does not actually move in any physical direction; in fact, it may not move at all, although things certainly appear to move in time. We think of time unspooling like film through a projector, with each moment illuminated one frame at a time. Except that in reality, there is no wind-up reel. The past has vanished, and the future has not yet arrived. The present moment is all that tangibly exists. But a moment by definition has no duration, and the instant we try to lay hold of it, it too has disappeared down the same rabbit hole as all the moments before it.
So what then is time? If I may steal a phrase from Marcel Proust, it may be nothing more complicated than the remembrance of things past. As I sit here in my study, hunched over my laptop, I reach for my cup of coffee and bring it to my lips. I remember picking it up, so I do not need to wonder how it got to my lips. I remember brewing the coffee before that and pouring it into the cup that I then brought into the study and set down at my place – an entire sequence of events that tells me something called “time” has elapsed. But is time anything more than the persistence of memory? To go back to the metaphor of time unspooling like film through a projector: when we see a movie on a screen, it appears that everything is moving, but what we are actually seeing is a sequence of still images running through the projector at 24 frames per second. By the same token, our sense of time passing may be nothing more than individual moments threading through our mind in rapid sequence. "I say that I measure time in my mind," St. Augustine concluded. "For everything which happens leaves an impression on it, and this impression remains after the thing itself has ceased to be.” He added, “When I measure time it is this impression that I measure."
What would happen if those individual moments left no impression on our minds? In fact, there are a number of well-documented cases in the medical literature of patients who have suffered severe damage to the hippocampus in the brain and consequently have lost the ability to form new memories. The result is that for these patients “right now” is truly all there is, with no connection to the moment before or the moment to come. Time, for all practical purposes comes to a stop. As it happens, my mother suffered from vascular dementia, which destroys short-term memory, so I have first-hand knowledge of the temporal dislocations that can ensue. As her disease progressed, her life degenerated into a succession of disconnected instants that were not anchored in memory. If I tried to take her out for a drive, she would often want to turn around right away, because she couldn’t tell whether we had been gone for five minutes or five hours. Without memory, there can be no duration.
H.G. Wells coined the term “time machine” in his novel of the same name, published in 1895. When the protagonist’s dinner guests scoffed at the notion of time travel in the opening chapter, he reminded them that people travel in time contantly in their thoughts. “We are always getting away from the present moment,” he told them, adding, “if I am recalling an incident very vividly I go back to the instant of its occurrence: I become absent-minded, as you say.” In truth, we do not need a time machine to travel to the past or the future, since we spend most of our time there already. So there’s no need for a machine to get us there; if anything, we need a machine to get us here.