… Is not possible. Let me establish that right away. I don’t say this as a physicist or philosopher, but as a lay practitioner. It would be really neat if time travel were possible, but my instincts and my sense of everything I read in the serious scientific literature say not.
There are some tantalizing developments in theoretical physics that suggest time travel might be possible—by traveling through wormholes, or shooting through the axis of a spinning black hole, or riding faster-than-light tachyons—but these remain intellectual concepts only. So much of modern physics seems to be intellectual play, a kind of fun with mathematics and numerical coincidences, that theoretical physics seems to be less and less a science, a way of actually knowing the world.
Take wormholes, a staple of science fiction, for example, as such a theoretical construct. If the universe as we know it were crumpled up like a sheet of paper, then it should be possible by the application of some exotic energy to go from one surface to the next, like punching through the layers of a flaky croissant. This assumes that the three dimensions we can detect with our senses—x, y, and z,, or left-and-right, up-and-down, and outward-and-inward, all according to a fixed point of view—are merely a naïve understanding of what space actually is. In this interpretation, space is supposed to have more dimensions, hidden from ordinary eyes, where the crumpling occurs. Uh-huh …
The notion of time travel comes from a similar confusion about dimensions. Time is thought to be a dimension of the universe similar to x, y, and z. But where those three dimensions describe a coordinate in terms of distance, time describes a coordinate in terms of persistence. During t time, the object occupies x, y, and z space. Before or after t time, it is somehwere else. Once you make this correlation, you can write interesting equations about spacetime and come up with all sorts of sophisticated propositions about the universe.
And, if you accept that time is a dimension similar to the dimensions of space, it’s not too far a step to imagine that, as you can travel left or right along the x axis, so you can travel forward and back along the t axis. Time becomes a road or a river. The river has a flow, the road has a direction, and we travel that road in the preferred direction effortlessly.
(This is unlike the other dimensions, where you have to expend energy to make a change in location. Indeed energy is defined by the the equation f=ma, or force equals mass times acceleration. Acceleration is a change in velocity over time, and velocity is a change in location over time. So acceleration is change in location over time squared.)
Presumably, with the application of enough energy, you can change your direction of travel along time’s path. Reverse the flow and go backwards in time. Increase the flow and go forwards in time. Usually, there is thought to be some machine that will apply this energy, as a car applies energy to the physical, x-dimensional road through its wheels.
All of this, of course, ignores another aspect of time: cause and effect, and the application of probability to effects that are not under the control of some superior cause. (That is, on a quiet summer day a leaf falling from the tree may sail to the left or right depending on the vagaries of air currents, but during a hurricane there’s not much question about where the leaf goes.) Time might be a straight and definable road for any single object, but for a complex collection of objects time is a web of interactions, of choices and probabilities. And what is a human being except a collection of atoms, of molecular bonds being made and broken, of nucleotide and protein interactions? Just sitting there, you are changing from instant to instant depending on this internal dance.
But all of this does not mean that I don’t enjoy reading—and occasionally telling—stories about time travel. They provide puzzles and paradoxes that give the intellect a workout. In the same way, murder mysteries give the reader a chance to exercise insight, imagination, and memory, even though complex murder plots are vanishingly rare in the real world of the police blotter.
Time travel is an exercise of the imagination. I just don’t expect to visit the past or the future anytime soon.