Time Travel. A History

And what of our persistent impressions to the contrary? We experience time in our bones. We remember the past, we await the future. But the physicist notes that we are fallible organisms, easily fooled and not to be trusted. Our prescientific ancestors experienced the flat earth and traveling sun. Could our experience of time be equally naïve? Perhaps—but scientists have to come back to the evidence of our senses in the end. They must test their models against experience.

“People like us, who believe in physics,” Einstein said, “know that the distinction between past, present, and future is only a stubbornly persistent illusion.” Who believe in physics—I detect something wistful in that. “In physics,” repeats Freeman Dyson, “the division of space-time into past, present, and future is an illusion.” These formulations retain a bit of humility that is sometimes lost in the quoting. Einstein was consoling a bereaved sister and son, and perhaps thinking as well of his own pending mortality. Dyson was expressing hopeful bonds of kinship with people of the past and people of the future: “They are our neighbors in the universe.” These are beautiful thoughts, but they were not intended as final statements about the nature of reality. As Einstein himself said on an earlier occasion, “Time and space are modes by which we think, and not conditions in which we live.”

There is something perverse about a scientist’s believing that the future is already complete—locked down tight, no different from the past. The first motivation for the scientific enterprise, the prime directive, is to gain some control over our headlong tumble into an unknown future. For ancient astronomers to forecast the movements of heavenly bodies was vindication and triumph; to predict an eclipse was to rob it of its terror; medical science has labored for centuries to eradicate diseases and extend the lifetimes that fatalists call fixed; in the first powerful application of Newton’s laws to earthly mechanics, students of gunnery computed the parabolic trajectories of cannonballs, the better to send them to their targets; twentieth-century physicists not only managed to change the course of warfare but then dreamt of using their new computing machines to forecast and even control the earth’s weather. Because, why not? We are pattern-recognition machines, and the project of science is to formalize our intuitions, do the math, in hopes not just of understanding—a passive, academic pleasure—but of bending nature, to the limited extent possible, to our will.

Remember Laplace’s perfect intelligence, vast enough to comprehend all the forces and the positions and to submit them to analysis. “To it nothing would be uncertain, and the future as the past would be present to its eyes.” This is how the future becomes indistinguishable from the past. Tom Stoppard joins the parade of philosophers wittily paraphrasing him: “If you could stop every atom in its position and direction, and if your mind could comprehend all the actions thus suspended, then if you were really, really good at algebra you could write the formula for all the future; and although nobody can be so clever as to do it, the formula must exist just as if one could.” It bears asking—because so many modern physicists still believe something like this—why? If no intelligence can be so comprehensive, no computer can do so much computing, why must we treat the future as though it were predictable?

The implicit answer, sometimes explicit, is that the universe is its own computer. It computes its own destiny, step by step, bit by bit (or qubit by qubit). The computers we know, in the early twenty-first century, not counting the tantalizing quantum variety, operate deterministically. A given input always leads to the same output. Our input, again, is the totality of initial conditions and our program is the laws of nature. These are the whole kit and caboodle: the entire future is already there. No information needs to be added, nothing remains to be discovered. There shall be no novelty, no surprise. Only the clanking of the logical gears remains—a mere formality.