Time Travel. A History

We go back and forth between being time’s master and its victim. Time is ours to use, and then we are at its mercy. I wasted time, and now doth time waste me, says Richard II; For now hath time made me his numbering clock. If you say that an activity wastes time, implying a substance in finite supply, and then you say that it fills time, implying a sort of container, have you contradicted yourself? Are you confused? Are you committing a failure of logic? None of those. On the contrary, you are a clever creature, when it comes to time, and you can keep more than one idea in your head. Language is imperfect; poetry, perfectly imperfect. We can occupy the time and pass the time in the same breath. We can devour time or languish in its slow-chapp’d power.

Newton, who invented the idea of mass, knew that time didn’t have any, that it’s not a substance, yet he said that time “flows.” He wrote this in Latin: tempus fluit. The Romans said tempus fugit, time flees, or at any rate that motto began appearing on English sundials in the Middle Ages. Newton would have seen that. True, the hours speed by and are gone, once we learn to measure them, but how can time flee? It’s another figure of speech. And how can time flow, if it has no substance?

Newton took pains to distinguish two kinds of time. We might call them physical time and psychological time, but he lacked those words, so he had to struggle a bit. The first kind he called, with a flurry of adjectives, “time absolute true and mathematical” (tempus absolutum verum & Mathematicum). The other was time as conceived by the common people—the vulgus—and this he called “relative” and “apparent.” True time—mathematical time—he inferred from a technological feature of his world, the consistency of clocks. He and the clockmakers both leaned on Galileo here—it was Galileo who established that a swinging pendulum of a given length divides time into regular pieces. He measured time by using his pulse. Shortly thereafter, doctors began using clocks to time pulses. The ancients looked to the heavens for measuring time: the sun, the stars, the moon—those were reliable. They gave us our days, months, and years. (When Joshua needed more time to smite the Amorites, he asked God to halt the sun and moon in their tracks—“Sun, stand thou still upon Gibeon; and thou, Moon, in the valley of Ajalon.” Who among us has not wanted to stop time?) Now machinery takes over the reckoning.

Another circularity creeps in—a chicken-and-egg problem. Time is how we measure motion. Motion is how we measure time. Newton tried to escape that by fiat. He made Absolute Time axiomatic. He needed a reliable backbone for his laws of motion. The first law: an object moves at a constant velocity, unless acted upon by some external force. But what is velocity? Distance per unit time. When Newton declared that time flows equably, aequabiliter fluit, he meant that we can count on unit time. Hours, days, months, years: they are the same everywhere and always. In effect, he imagined the universe as its own clock, the cosmic clock, perfect and mathematical. He wanted to say that when two of our earthly clocks differ, it’s because of some fault in the clocks, not because the universe speeds up and slows down hither and yon.

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NOW IT IS fashionable among physicists and philosophers to ask whether time is even “real”—whether it “exists.” The question is debated at conferences and symposia and analyzed in books. I have put quotation marks around those words because they are so problematic in themselves. The nature of reality hasn’t been settled either. We know what it means to say that unicorns are not real. Likewise Santa Claus. But when scholars say time is not real, they mean something different. They haven’t lost faith in their wristwatches or their calendars. They use “real” as code for something else: absolute, special, or fundamental.