Time Travel: A History (2016)
“Time is a mental concept,” said Pringle. “They looked for time everywhere else before they located it in the human mind. They thought it was a fourth dimension. You remember Einstein.”
—Clifford D. Simak (1951)
BEFORE WE HAVE clocks we experience time as fluid, mercurial, and inconstant. Pre-Newtonians did not assume that time was a universal, trustworthy, absolute affair. Time was well known to be relative—to use that word in its psychological sense, not to be confused with the newer sense that came into being circa 1905. Time travels in divers paces with divers persons.*1 Clocks reified time and then Newton made time…let’s say, official. He made it an essential part of science: time t, a factor to be plugged into equations. Newton regarded time as part of the “sensorium of God.” His view is handed down to us as if engraved on tablets of stone:
Absolute, true, and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly…
The cosmic clock ticks invisibly and inexorably, everywhere the same. Absolute time is God’s time. This was Newton’s credo. He had no evidence for it, and his clocks were rubbish compared to ours.
It may be, that there is no such thing as an equable motion, whereby time may be accurately measured. All motions may be accelerated and retarded, but the flowing of absolute time is not liable to any change.
Besides religious conviction, Newton was motivated by mathematical necessity: he needed absolute time, as he needed absolute space, in order to define his terms and express his laws. Motion is defined as the change in place over time; acceleration is the change in velocity over time. With a backdrop of absolute, true, and mathematical time, he could build an entire cosmology, a System of the World. This was an abstraction; a convenience; a framework for calculating. But for Newton it was also a statement about the world. You may believe it, or not.*2
Albert Einstein believed it. Up to a point.
He believed in an edifice of laws and computation that had grown from a bare stone church into a grand ornate cathedral, supported by colonnades and flying buttresses, layered with carving and tracery—work still in progress, with hidden crypts and ruined chapels. In this edifice time t played an indispensable part. No one could grasp the whole structure, but Einstein understood more than most and had encountered a problem. There was an internal contradiction. The great achievement of the last century’s physics was James Clerk Maxwell’s unification of electricity, magnetism, and light—the achievement that was so visibly wiring the whole world. Electric currents, magnetic fields, radio waves, and light waves were one and the same. Maxwell’s equations made it possible to calculate the speed of light, for the first time. But they were not meshing perfectly with the laws of mechanics. Those light waves, for example—so clearly waves, according to the mathematics, but waves in what? Sound needs air or water or other substance to carry the vibrations. Light waves likewise implied an unseen medium, the so-called ether—“luminiferous,” or light bearing. Naturally experimentalists were trying to detect this ether, with no success. Albert Michelson and Edward Morley came up with a clever experiment in 1887 to measure the difference between the speed of light in the direction of the earth’s motion and the speed of light at right angles to it. They couldn’t find any difference at all. Was the ether necessary? Or was it possible to think purely of an electrodynamics of moving bodies, through empty space?
We know now that the speed of light in empty space is constant, 299,792,458 meters per second. No rocket ship can overtake a flash of light or reduce that number in the slightest. Einstein struggled (“psychic tension”; “all sorts of nervous conflicts”) to make sense of that: to discard the luminiferous ether, to accept the speed of light as absolute. Something else had to give. On a fine bright day in Bern (as he told the story later), he talked it over with his friend Michele Besso. “Next day I came back to him again and said to him, without even saying hello, ‘Thank you. I’ve completely solved the problem.’ An analysis of the concept of time was my solution.” If light speed is absolute, then time itself cannot be. We must abandon our faith in perfect simultaneity: the assumption that two events can be said to happen at the same time. Multiple observers experience their own present moments. “Time cannot be absolutely defined,” said Einstein—it can be defined, but not absolutely—“and there is an inseparable relation between time and signal velocity.”
The signal carries information. Suppose six sprinters line up at the start line for the hundred-meter run, with their hands and one knee touching the ground and their feet in the starting blocks, awaiting the sound of the gun. The signal velocity in this case will be about a few hundred meters per second, the speed of sound through air. That’s slow nowadays, so Olympic events have scrapped starting pistols in favor of signals wired (at light speed) into loudspeakers. To think about simultaneity more carefully, it becomes necessary also to consider the signal velocity of light traveling to the eyes of the runners, the judges, and the spectators. In the end, there is no one instant, no “point in time,” that can be the same for everyone.
Suppose lightning strikes a railway embankment (trains are more usual than horses in these stories) at two different points, distant from each other. Can you—a physicist, with the most excellent modern equipment—establish whether the two flashes were simultaneous? You cannot. It turns out that a physicist riding the train will disagree with a physicist standing at the station. Every observer owns a reference frame, and each reference frame has its own clock. There is no one cosmic clock, no clock of God or Newton.
The revelation is that we can share no now—no universal present moment. But was that altogether a surprise? Before Einstein was born, John Henry Newman, poet and priest, wrote that “time is not a common property; / But what is long is short, and swift is slow/And near is distant, as received and grasped / By this mind and by that, / And every one is standard of his own chronology.” For him it was intuitive.
“Your now is not my now,” wrote Charles Lamb in England to his friend Barron Field in Australia, the far side of the earth, in 1817, “your then is not my then; but my now may be your then, and vice versa. Whose head is competent to these things?”
Nowadays we are all competent to these things. We have time zones. We can contemplate the International Date Line, where an imaginary boundary divides Tuesday from Wednesday.*3 Even when we suffer from jet lag—the quintessential disease of time travel—we are shrewd in our suffering and can nod wisely at William Gibson’s account of “soul delay”:
Her mortal soul is leagues behind her, being reeled in on some ghostly umbilical down the vanished wake of the plane that brought her here, hundreds of thousands of feet above the Atlantic. Souls can’t move that quickly, and are left behind, and must be awaited, upon arrival, like lost luggage.
We know that the light of the stars is ancient light, that distant galaxies reveal themselves to us only as they once were, not as they now are. As John Banville reminds us in his novel of that name, ancient light is all we have: “Even here, at this table, the light that is the image of my eyes takes time, a tiny time, infinitesimal, yet time, to reach your eyes, and so it is that everywhere we look, everywhere, we are looking into the past.”*4 (Can we peer into the future as well? That clever time traveler Joyce Carol Oates says via Twitter, “As minutes are required for the sun’s light to reach us, we are living always in a sunlit past. Just the reverse, reading bound galleys.”)
When everything reaching our senses comes from the past, when no observer lives in the now of any other observer, the distinction between past and future begins to decay. Events in our universe can be connected, such that one is the cause of the other, but, alternatively, they can be close enough in time and far enough apart that they cannot be connected and no one can even say which came first. (Outside the light cone, says the physicist.) We are more isolated, then, than we may have imagined, alone in our corners of spacetime. You know how fortune-tellers pretend to know the future? It turns out, said Richard Feynman, that no fortune-teller can even know the present.
Einstein’s powerful ideas spread in the public press as rapidly as in the physics journals and disrupted the placid course of philosophy. The philosophers were surprised and outgunned. Bergson and Einstein clashed publicly in Paris and privately by post and seemed to be speaking different languages: one scientific, measured, practical; the other psychological, flowing, untrustworthy. “ ‘The time of the universe’ discovered by Einstein and ‘the time of our lives’ associated with Bergson spiraled down dangerously conflicting paths, splitting the century into two cultures,” notes the science historian Jimena Canales. We are Einsteinian when we search for simplicity and truth, Bergsonian when we embrace uncertainty and flux. Bergson continued to place human consciousness at the center of time, while Einstein saw no place for spirit in a science that relied on clocks and light. “Time is for me that which is most real and necessary,” wrote Bergson; “it is the necessary condition of action. What am I saying? It is action itself.” Before an audience of intellectuals at the Société Française de Philosophie in April 1922, Einstein was adamant: “The time of the philosophers does not exist.” Einstein, it seems, prevailed.
What does his framework mean for our understanding of the true nature of things? His biographer Jürgen Neffe sums up the situation judiciously. “Einstein provided no explanations for these phenomena,” he says. “No one knows what light and time really are. We are not told what something is. The special theory of relativity merely provides a new rule for measuring the world—a perfectly logical construct that surmounts earlier contradictions.”
HERMANN MINKOWSKI READ Einstein’s 1905 paper on special relativity with special interest. He had been Einstein’s mathematics teacher in Zurich. He was forty-four years old and Einstein was twenty-nine. Minkowski saw that Einstein had knocked the concept of time “from its high seat,” had shown, indeed, that there is no time, but only times. But he thought that his former student had left the big job unfinished—had stopped short of stating the new truth about the nature of all reality. So Minkowski prepared a lecture. He delivered it at a scientific meeting in Cologne on September 21, 1908, and it is famous.
“Raum und Zeit” was his title, “Space and Time,” and his mission was to declare both concepts null and void. “The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength,” he began grandly. “They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.”
He reminded his listeners that space is denoted by three orthogonal coordinates, x, y, z, for length, breadth, and thickness. Let t denote time. With a piece of chalk, he said, he could draw four axes on the blackboard: “the somewhat greater abstraction associated with the number 4 does not hurt the mathematician.” And so on. He was excited. This was “a new conception of space and time,” he declared; “the first of all laws of nature.” He called this conception the “principle of the absolute world.”
Four numbers, x, y, z, t, define a “world point.” Together, all the world points that trace an object’s existence from birth to death form a “world line.” And what shall we call the whole shebang?
The multiplicity of all thinkable x, y, z, t systems of values we will christen the world.
Die Welt! Good name. But we just call it spacetime now. (The continuum.) If we resist (“Because I know that time is always time / And place is always and only place,” said T. S. Eliot), we do so in vain.
It was a bit of misdirection for Minkowski to begin by saying his lecture was grounded in experimental physics. His true subject was the power of abstract mathematics to reshape our understanding of the universe. He was a geometer above all. The physicist and historian Peter Galison puts it this way: “Where Einstein manipulated clocks, rods, light beams, and trains, Minkowski played with grids, surfaces, curves, and projections.” He thought in terms of the most profound visual abstraction.
“Mere shadows,” Minkowski said. That was not mere poetry. He meant it almost literally. Our perceived reality is a projection, like the shadows projected by the fire in Plato’s cave. If the world—the absolute world—is a four-dimensional continuum, then all that we perceive at any instant is a slice of the whole. Our sense of time: an illusion. Nothing passes; nothing changes. The universe—the real universe, hidden from our blinkered sight—comprises the totality of these timeless, eternal world lines. “I would fain anticipate myself,” said Minkowski in Cologne, “by saying that in my opinion physical laws might find their most perfect expression as reciprocal relations between these world lines.” Three months later he was dead of a ruptured appendix.
Thus the idea of time as a fourth dimension crept forward. It did not happen all at once. In 1908 Scientific American “simply explained” the fourth dimension as a hypothetical space analogous to the first three: “For passing into the fourth dimension, we should pass out of our present world.” The next year the magazine sponsored an essay contest on the topic “The Fourth Dimension,” and not one of the winners or runners-up considered it to be time—notwithstanding the German physicists and the English writer of fantastic fiction. The space-time continuum was radical indeed. Max Wien, an experimental physicist, described his initial reaction as “a slight brain-shiver—now space and time appear conglomerated together in a gray, miserable chaos.”*5 It offends common sense. “The texture of Space is not that of Time,” cries Vladimir Nabokov, “and the piebald four-dimensional sport bred by relativists is a quadruped with one leg replaced by the ghost of a leg.” If these critics sound Filbyish, even Einstein did not immediately embrace Minkowski’s vision: “überflüssige Gelehrsamkeit,” he called it—superfluous learnedness. But Einstein came around. When his friend Besso died in 1955, Einstein consoled his family with words that have been quoted many times:
Now he has departed from this strange world a little ahead of me. That means nothing. People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion.
Einstein died three weeks later.
FUNNY IRONY, though.
A century after Einstein discovered that perfect simultaneity is a chimera, the technology of our interconnected world relies on simultaneity as never before. When telephone-network switches get out of sync, they drop calls. While no physicist “believes in” absolute time, humanity has established a collective official timescale, preached by a choir of atomic clocks maintained at a temperature near absolute zero in vaults at the United States Naval Observatory in Washington, the Bureau International des Poids et Mesures near Paris, and elsewhere. They bounce their networked light-speed signals to one another, make the necessary relativistic corrections, and thus the world sets its myriad clocks. Confusion about past and future cannot be tolerated.
To Newton this would make perfect sense. International atomic time has the effect of codifying the absolute time that he created, and for the same reason: it lets the equations work out and the trains run on time. A century before Einstein, this technical achievement in simultaneity would have been almost impossible to conceive. The very notion of simultaneity scarcely existed. It was a rare philosopher who considered the question of what time it might be in a faraway place. One could hardly even hope to know, said the doctor and philosopher Thomas Browne in 1646,
It being no ordinary or Almanack business, but a probleme Mathematical, to finde out the difference of hours in different places; nor do the wisest exactly satisfy themselves in all. For the hours of several places anticipate each other, according to their Longitudes; which are not exactly discovered of every place.
All time was local. “Standard time” had no use before the railroad came and could not be established before the telegraph. England began synchronizing its clocks (new expression) to railway time in the mid-nineteenth century, when telegraph signals went out from the new electromagnetic clock at the Royal Observatory in Greenwich and the Electric Time Company in London. Also to the newly coordinated clock towers and electric street clocks of Bern.*6These were technologies on which the ideas of Einstein depended, and also the ideas of H. G. Wells.
So now, on a hilltop near the Potomac River, the United States maintains a Directorate of Time, a subdepartment of the navy and by law the country’s official timekeeper. Likewise in Paris is the BIPM, which also owns the international prototype of the kilogram. These are the keepers of temps universel coordonné, or coordinated universal time, or UTC—which I think we can admit is arrogantly named. Let’s just call it Earth time.
All the chronometric paraphernalia of modernity: scientific, and yet arbitrary. Railroads made time zones inevitable, and in hindsight we can see that time zones already entailed a sense of time travel. They were not organized all at once, by fiat. They had many beginnings. For example, on November 18, 1883, a Sunday, known afterward as “the Day of Two Noons,” James Hamblet, general superintendent of the Time Telegraph Company in New York City, reached out his hand and stopped the pendulum of the standard clock in the Western Union Telegraph Building. He waited for a signal and then restarted it. “His clock is adjusted to hundredth parts of a second,” reported the New York Times, “a space of time so infinitesimal as to be almost beyond human perception.” Around the city, tickers announced the new time and jewelers’ shops adjusted their clocks. The newspaper explained the new setup in science-fictional terms:
When the reader of The Times consults his paper at 8 o’clock this morning at his breakfast table it will be 9 o’clock in St. John, New-Brunswick, 7 o’clock in Chicago, or rather in St. Louis—for Chicago authorities have refused to adopt the standard time, perhaps because the Chicago meridian was not selected as the one on which all time must be based—6 o’clock in Denver, Col., and 5 o’clock in San Francisco. That is the whole story in a nut-shell.
Of course, that was nothing like the whole story. Arbitrary as they were, the railroads’ time zones did not please everyone, and a new oddity followed: Daylight Saving Time, as it was known in North America, or, as Europeans called it, Summer Time. Even now, after a century of experience, some people find this twice-yearly time jump disturbing, and even physically uncomfortable. (And philosophically unsettling. Where does the hour go?) Germany was the first to impose Sommerzeit, during World War I, hoping to save coal. Soon after, the United States adopted it, then repealed it, then reimposed it. In England, seeking evening light for hunting, King Edward VII had the clocks on the royal estate set to “Sandringham Time,” a half hour ahead of Greenwich. When the Nazis occupied France, they ordered all the clocks moved an hour forward, to Berlin time.
It was not just a matter of minutes and hours. The days and the years, too, confounded a world whose farthest parts were now in close communication. When, finally, would humanity agree on a uniform calendar? The new League of Nations took up the question after World War I. Its Committee on Intellectual Cooperation chose the philosopher Bergson as its president; another member, briefly, was Einstein. The League tried to impose the Gregorian calendar, itself the product of centuries of strife and revision, on nations less concerned with computing the proper dates of Easter feasts. The prospect of leaping forward or back in time created anxiety. Those nations did not fall in line. Bulgarians and Russians complained that their citizens could not be made suddenly to age by thirteen days, to surrender thirteen days of their lives in the name of globalization. Conversely, when France condescended to join Greenwich time, the Parisian astronomer Charles Nordmann said, “Some people may have consoled themselves with the reflection that to grow younger by 9 minutes and 21 seconds, on the authority of the law, was a pleasure worth having.”
Had time become a thing over which dictators and kings could exercise power? “The Problem of Summer Time” is the English title of a new sort of time-travel story, published in 1943 by a darkly satirical Parisian writer, Marcel Aymé. In French it is “Le décret”—The Decree—issued after scientists and philosophers discover how easy it is to advance the time an hour forward each summer and back again each winter. “Little by little,” says the narrator, “the realization spread that time was under man’s control.” Humans are time’s dynamic masters: they may hurry up or slow down to suit themselves. Anyway “the old, stately pace was over.”
There was much talk of relative time, physiological time, subjective time and even compressible time. It became obvious that the notion of time, as our ancestors had transmitted it down the millennia, was in fact absurd claptrap.
With time now seemingly at their command, the authorities see a way to escape the nightmare of a war that seemed endless. They decide to advance the years by seventeen: 1942 leaps forward to 1959. (In the same spirit, moviemakers in Hollywood began to tear pages from calendars and spin the hands of clocks so as to move time along for their viewers.) The decree makes the world and all its people seventeen years older. The war has come to an end. Some have died, others have been born, and everyone has some catching up to do. It’s all rather disorienting.
Aymé’s narrator travels from Paris by railroad into the countryside. A surprise awaits him there. Apparently the decree has not spread everywhere. A storm, some wine, a troubled sleep, and in a distant village he encounters active German soldiers, and, sure enough, the mirror now shows him the thirty-nine-year-old he was, not the fifty-six-year-old he had become. On the other hand, he still has his newly acquired memories of those seventeen years. This is disturbing—indeed, impossible. “To be from an era, I thought, is to behold the world and oneself in a certain way that belongs to that era.” Is he fated to relive the same life, burdened with memories of times to come?
He feels the existence of two parallel worlds, seventeen years apart yet existing simultaneously. Worse, after these “mysterious leaps and turns through time,” why should there be only two?
Now I accepted the nightmare of an infinity of universes, in which the official time represented only the relative displacement of my consciousness from one to another, and then on to another.
Now— and now— and then another now.
Three o’clock—I am aware of the world in which I feature holding a pen. Three o’clock and one second—I am aware of the next universe in which I feature putting down my pen, etc.
It’s too much for the human mind to comprehend; mercifully, his memories begin to fade, as all memories do. What he has written of the past—the future, and then the past—begins to seem like a dream. “Only once in a while, more and more infrequently, do I have the very ordinary sensation of déjà vu.”
What is memory, for a time traveler? A conundrum. We say that memory “takes us back.” Virginia Woolf called memory a seamstress “and a capricious one at that.” (“Memory runs her needle in and out, up and down, hither and thither. We know not what comes next, or what follows after.”)
“I can’t remember things before they happen,” says Alice, and the Queen retorts, “It’s a poor sort of memory that only works backwards.” Memory both is and is not our past. It is not recorded, as we sometimes imagine; it is made, and continually remade. If the time traveler meets herself, who remembers what, and when?
In the twenty-first century the paradoxes of memory grow more familiar. Steven Wright remarks: “Right now I’m having amnesia and déjà vu at the same time. I think I’ve forgotten this before.”
*1 Rosalind adds: “I’ll tell you who time ambles withal, who time trots withal, who time gallops withal, and who he stands still withal.”
*2 The philosopher and physicist Ernst Mach, a forebear of relativity, objected to absolute time in 1883: “It is utterly beyond our power to measure the changes of things by time….Time is an abstraction at which we arrive by means of the changes of things.” Einstein quoted that approvingly when he wrote Mach’s obituary in 1916, but he himself could not go so far in expunging the convenient abstraction. Time remained an essential property of his universe.
*3 Time travel by circumnavigation? Poe seems to have been the first to make literary use of the possibility, in 1841 (“A Succession of Sundays,” Saturday Evening Post), before Jules Verne made it the surprise ending of Around the World in Eighty Days.
*4 The same thought came as a revelation to Israel Zangwill when he reviewed The Time Machine in 1895: “The star whose light reaches us to-night may have perished and become extinct a thousand years ago, the rays of light from it having so many millions of miles to travel that they have only just impinged upon our planet. Could we perceive clearly the incidents on its surface, we should be beholding the Past in the Present, and we could travel to any given year by travelling actually through space to the point at which the rays of that year would first strike upon our consciousness. In like manner the whole Past of the earth is still playing itself out—to an eye conceived as stationed to-day in space, and moving now forwards to catch the Middle Ages, now backwards to watch Nero fiddling over the burning of Rome.”
*5 Wien was the inventor of the Löschfunkensender, an early radio transmitter used, for example, on the Titanic.
*6 Peter Galison, an authority on this matter, suggests that Einstein and Besso, conversing on that fateful day in May 1905, must have been standing on a hill in northeast Bern, where they could simultaneously see both Bern’s old clock tower and another to the north, in the town of Muri.