Suppose I can travel back into time, let me say 200 years; and I visit the homestead of my great great great grandfather….I am thus enabled to shoot him, while he is still a young man and as yet unmarried. From this it will be noted that I could have prevented my own birth; because the line of propagation would have ceased right there.
Henceforth this would be known as the grandfather paradox. It turns out that one person’s objection is another’s story idea. Gernsback invited comments from readers by mail and received quite a few, over a period of years. A boy in San Francisco suggested yet another paradox, “the last knock on time traveling”: What if a man were to travel into the past and marry his mother? Could he be his own father?
Page Einstein indeed.
*1 Sir Boyle is also remembered for this: “Why should we put ourselves out of our way to do anything for posterity, for what has posterity ever done for us?”—a joke that reads differently now that we have time travel. Posterity does plenty for us: sends us assassins and bounty hunters on covert missions to change the course of history, for example.
*2 When the American astronaut Scott Kelly returned to Earth in March 2016 after nearly a year of high-speed orbit, he was reckoned to be 8.6 milliseconds younger, relative to his groundling twin brother, Mark. (Then again, Mark had lived through only 340 days while Scott experienced 10,944 sunrises and sunsets.)
*3 J. B. Priestley, who loved Wells and credited him with inspiring his Time Plays, said, “Although he was never rude about it he deplored the way in which I was bothering my head about Time in the thirties. He was like a man who, having wrongly given up playing an instrument for which he had a flair, then refused to listen to anybody else playing it.” Another disappointed admirer, W. M. S. Russell, echoed Priestley’s complaint at a centennial symposium in 1995: “More than a century after his wonderful achievement, let us be remembering, not the disillusioned elder, but the young creator of The Time Machine.”
*4 Spoken aloud: “One to foresee…”
*5 Kingsley Amis also took the time to read this book. “Ralph 124C 41+ concerns the technological marvels invented or demonstrated by the ridiculously resourceful eponymous hero….After some trouble with a pair of rival suitors, one human, the other Martian, Ralph restores a dead girl to life by a complicated deep-freeze and blood-transfusion technique. Other wonders include the hypnobioscope…and three-dimensional color television, a term which Gernsback is credited, if that is the word, with having invented.”
*6 He also proposed a few “don’ts,” including, “Don’t make your professor, if you have one, talk like a military policeman or an Eighth Avenue ‘cop.’ Don’t put cheap jokes in his mouth. Read semi-technical magazines and reports of speeches to get the flavor of academic phraseology.”
*7 An editor’s note explained: “Stories of traveling in time are always exceedingly interesting reading, mainly for the reason that the feat has not yet been accomplished; though no one can say that it cannot be done in the future, when we have reached a much higher plane of scientific achievement. Traveling in time, either forward or backward, may well become a possibility.”
FOUR
Ancient Light
“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.”