Time Travel. A History

In the macroscopic world, the world we inhabit, time has a definite direction. When the technology of cinema was still new, filmmakers discovered they could create amusing effects by reversing their strips of celluloid. The Lumière brothers reversed their short Charcuterie mécanique to show a sausage unmade and a pig unbutchered. In a backward movie an omelet could organize into white and yolk and return to the egg, with shell fragments neatly reassembling themselves. A rock flies out of a turbulent pond, a reverse fountain of droplets closing in to seal the hole. Smoke pours down a fireplace into the flames as coals grow into logs. Not to mention life itself: the quintessential irreversible process. William Thomson, Lord Kelvin, saw the problem in 1874—and saw that consciousness and memory were part of the problem: “Living creatures would grow backward, with conscious knowledge of the future, but no memory of the past, and would become again unborn.”

Every so often it is good to remind ourselves that most natural processes are not reversible. They work only one way, forward in time. For starters here is a little list from Lord Kelvin: “friction of solids; imperfect fluidity of fluids; imperfect elasticity of solids [all these imperfects]; inequalities of temperature, and consequent conduction of heat produced by stresses in solids and fluids; imperfect magnetic retentiveness; residual electric polarization of dielectrics; generation of heat by electric currents inducted by motion; diffusion of fluids, solutions of solids in fluids, and other chemical changes; and absorption of radiant heat and light.” That last is where Johnny and Dick came in.

At some point we have to talk about entropy.

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THERE’S A CATCHPHRASE, the arrow of time, familiarly used by scientists and philosophers in many languages (la flèche du temps, Zeitpfeil, zamanın oku, ось времени) as shorthand for a complex fact that everyone knows: time has a direction. The phrase spread widely in the 1940s and 1950s. It came from the pen of Arthur Eddington, the British astrophysicist who first championed Einstein. In a series of lectures at the University of Edinburgh in the winter of 1927 Eddington was attempting to comprehend the great changes under way in the nature of scientific thought. The next year he published his lectures as a popular book, The Nature of the Physical World.

It struck him that all previous physics was now seen to be classical physics, another new expression. “I am not sure that the phrase ‘classical physics’ has ever been closely defined,” he told his listeners. No one called it classical until it broke down. (Now “classical physics” is a retronym, like acoustic guitar, dial telephone, and cloth diaper.)^* Millennia had gone by without scientists needing special shorthand like “time’s arrow” to state the obvious—the great thing about time is that it goes on. Now, however, it was no longer obvious. Physicists were writing laws of nature in a way that made time directionless, a mere change of sign separating +t from –t. But one law of nature is different: the second law of thermodynamics. That’s the one about entropy.

“Newton’s equations go forwards and backwards, they do not care which way,” explains Thomasina, the teenage prodigy invented by Tom Stoppard in Arcadia. “But the heat equation cares very much, it goes only one way.”

The universe tends inexorably toward disorder. Energy is indestructible, but it dissipates. This is not a microscopic law. Is it a “fundamental” law, like F = ma? Some argue that it is not. From one point of view, laws governing individual constituents of the world—single particles, or a very few—are primary, and laws about multitudes must be derived from them. But to Eddington this second law of thermodynamics was the fundamental law: the one that holds “the supreme position among the laws of Nature”; the one that gives us time.

In Minkowski’s world past and future lie revealed before us like east and west. There are no one-way signs. So Eddington added one: “I shall use the phrase ‘time’s arrow’ to express this one-way property of time which has no analogue in space.” He noted three points of philosophical import:

1. It is vividly recognized by consciousness.

2. It is equally insisted upon by our reasoning faculty.

3. It makes no appearance in physical science except…