Free Time Travel: A History by James Gleick Page B

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