Near the turn of the last century, a brilliant young theorist suggested that time should be understood as the fourth dimension of physical reality. He pointed out that an object that has only length, breadth and width is merely a geometrical abstraction unless it also has duration. He wrote, "There is no difference between Time and any of the three dimensions of Space except that our consciousness moves along it." He hypothesized that since it was possible to move about in any direction in three-dimensional space, one should also be able to do the same with time, making time travel a theoretical possibility.
As it happens, Albert Einstein was not the one who first popularized this notion, although he incorporated four-dimensional space-time into his special theory of relativity in 1905. The brilliant young theorist in question was H. G. Wells, who published his ideas a decade earlier in the science fiction classic, The Time Machine. Wells' Time Traveller built a machine that propelled him into a distant future in which the ruling class and the working class had evolved into separate and warring species, the gentle Eloi and the brutish Morlocks. The Time Traveller never ventured into the past, thereby sidestepping the paradoxes that have bedeviled any serious discussion of time travel -- and that have fueled the imagination of science fiction writers and even some physicists since Wells' day.
The chief paradox is this: If you travel backward in time, you will inevitably change the past and set in motion events that would alter your circumstances in the present -- or what had been your present when you began your journey in time. The world you returned to would not be same one you left, raising the question of whether there were now parallel realities. And what if your time machine landed on your grandfather when you first ventured into the past, thereby nullifying your own existence? Would there be alternate versions of reality in which you existed and you did not, like Schrödinger's cat?* Or would you find yourself trapped in a closed time loop in which you bounced like some cosmic commuter between existence and nonexistence?
Such conundrums have led many physicists to reject the possibility of time-travel, even if the mathematics of special relatively theory would seem to allow it. Some have argued that even if time-travel into the future were feasible, revisiting the past would violate the second law of thermodynamics (entropy), which requires that the arrow of time always point the other way. There may be an even more fundamental problem. Although physicists routinely factor it into their equations, they don't actually know what time is or whether it has any physical properties. Time-travel may prove to be impossible because time as we usually think of it may not exist.
A century before Aristotle, the philosopher Zeno of Elea posed a series of paradoxes intended to show that motion -- and therefore time -- were illusory. He argued, for example, that an arrow in flight does not really move, because at any given instant it can only occupy a space equal to itself; it cannot be in two places at the same time. Since arrows somehow find their targets anyway, one is left to wonder whether the problem lies in trying to slice time into frozen instants.
Another brilliant young theorist, Peter Lynds, has picked up on Zeno's arguments to suggest that, in fact, there are no instants of time and therefore no succession of instants corresponding to a flow of time. Lynds asserts that our concept of time does not originate in the external world at all but in neurobiological processes, principally a "persistence of vision" that causes the mind to string together a series of impressions to create the perception of movement. St. Augustine arrived at a similar conclusion in the 4th century. "I say that I measure time in my mind," he wrote. "For everything which happens leaves an impression on it, and this impression remains after the thing itself has ceased to be....When I measure time it is this impression that I measure."
Time as we usually think of it may be just that: a thought. What we experience as duration is the recollection of a prior event in relation to the present moment. We can measure the interval between the two with great precision using a clock, but in reality we are measuring nothing more tangible than memory. The present moment surely exists, but the prior event does not, at least not now. In reality, right now is all there is.
Is time travel possible? When Wells' Time Traveller was challenged on this point by skeptical dinner guests, he reminded his companions that people time-travel constantly. "For instance," he said, "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 fact, we might argue that this sort of time travel is the only way we can perceive time at all, since there must be some thought of the past or future to create a sense of duration. In other words, we are all cosmic commuters caught in a closed time loop, bouncing from past to future and back again. And yet only the thinnest veneer of thought separates us from our starting point, which is eternity.
*Erwin Schrödinger's famous illustration of quantum mechanics in which a cat in a sealed box was shown to be both alive and dead at the same time.
H. G. Wells, The Time Machine
Peter Lynds (www.peterlynds.net.nz)
Augustine of Hippo, Confessions