Time Travel. A History

Richard Feynman said, “Time is what happens when nothing else happens,” which he knew was a wisecrack. “Maybe it is just as well if we face the fact that time is one of the things we probably cannot define (in the dictionary sense), and just say that it is what we already know it to be: it is how long we wait.”

When Augustine contemplated time, one thing he knew was that it was not space—“and yet, Lord, we perceive intervals of times, and compare them, and say some are shorter, and others longer.” We measure time, he said, though he had no clocks. “We measure times as they are passing, by perceiving them; but past, which now are not, or the future, which are not yet, who can measure?” You cannot measure what does not yet exist, Augustine felt, nor what has passed away.

In many cultures—but not all—people speak of the past as being behind them, while the future lies ahead. They visualize it that way, too. “Forgetting what is behind and straining toward what is ahead, I press on,” says Paul. To imagine the future or the past as a “place” is already to engage in analogy. Are there “places” in time, as there are in space? To say so is to assert that time is like space. The past is a foreign country: they do things differently there. The future, too. If time is a fourth dimension, that is because it is like the first three: visualizable as a line; measurable in extent. Still, in other ways time is unlike space. The fourth dimension differs from the other three. They do things differently there.

It seems natural to sense time as a spacelike thing. Accidents of language encourage that. We have only so many words; before and after have to do double duty as prepositions of space as well as time. “Time is a phantasm of motion,” said Thomas Hobbes in 1655. To count time, to compute time, “we make use of some motion or other, as of the sun, of a clock, of the sand in an hourglass.” Newton considered time to be absolutely different from space—after all, space remains always immovable, whereas time flows equably without regard to anything external, and by another name is called duration—but his mathematics created an inevitable analogy between time and space. You could plot them as axes on a graph. By the nineteenth century German philosophers in particular were groping toward some amalgam of time and space. Arthur Schopenhauer wrote in 1813, “In mere Time, all things follow one another, and in mere Space all things are side by side; it is accordingly only by the combination of Time and Space that the representation of coexistence arises.” Time as a dimension begins to emerge from the mists. Mathematicians could see it. Technology helped in another way. Time became vivid, concrete, and spatial to anyone who saw the railroad smashing across distances on a coordinated schedule—coordinated by the electric telegraph, which was pinning time to the mat. “It may seem strange to ‘fuse’ time and space,” explained the Dublin Review, but look—here is a “quite ordinary” space-time diagram:

Credit 1.1

So Wells’s Time Traveller can speak with conviction: “Scientific people know very well that Time is only a kind of Space. Here is a popular scientific diagram, a weather record. This line I trace with my finger shows the movement of the barometer….Surely the mercury did not trace this line in any of the dimensions of Space…but certainly it traced such a line, and that line, therefore, we must conclude was along the Time-Dimension.”

In the new century everything felt new; physicists and philosophers gazed upon Time, so often capitalized, with new eyes. Twenty-five years after The Time Machine the “new realist” philosopher Samuel Alexander put it this way:

If I were asked to name the most characteristic feature of the thought of the last twenty-five years I should answer: the discovery of Time. I do not mean that we have waited till to-day to become familiar with Time. I mean that we have only just begun in our speculation to take Time seriously and to realize that in some way or other Time is an essential ingredient in the constitution of things.

What is time? Time machines may help us understand.

—