Free Time Travel: A History by James Gleick

a time traveler,” he asked, “going back in time—whether ten years or ten million years—partake in the life of that time and mingle in with its people; or must he remain suspended in his own time-dimension, a spectator who merely looks on but is powerless to do more?” A paradox loomed; Gernsback could see it plainly, and he put it into words:

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.
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    *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
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    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,