Time Travel: A History (27 page)

Not that time travel settles the matter. “A variety of incoherencies might be alleged here,” the encyclopedia cautions, “including the incoherency of changing what is already fixed (causing the past), of being both able and unable to kill one’s own ancestors, or of generating a causal loop.” Brave writers are willing to risk an incoherency or two. Philip K. Dick ran the clocks backward (as it were) in
Counter-Clock World,
and so did Martin Amis in
Time’s Arrow.

We do seem to be traveling in circles.

—

“THE RECENT RENAISSANCE
of wormhole physics has led to a very disturbing observation,” wrote Matt Visser, a mathematician and cosmologist in New Zealand in 1994 in
Nuclear Physics B
(the forking path of
Nuclear Physics
devoted to “theoretical, phenomenological, and experimental high energy physics, quantum field theory and statistical systems”). Evidently the “renaissance” of wormhole physics was well established, though these supposed tunnels through spacetime remained (and remain) entirely hypothetical. The disturbing observation was this: “If traversable wormholes exist then it appears to be rather easy to transform such wormholes into time machines.” It was not just disturbing. It was
extremely
disturbing: “This extremely disturbing state of affairs has led Hawking to promulgate his chronology protection conjecture.”

Hawking is, of course, Stephen Hawking, the Cambridge physicist who by then had become the world’s most famous living scientist, in part because of his dramatic decades-long struggle with an inexorably paralyzing motor neuron disease and in part because of his flair for popularizing the knottiest problems of cosmology. No wonder he was attracted to time travel.

“Chronology Protection Conjecture” was the title of a paper he wrote in 1991 for
Physical Review D.
He explained the motivation as follows: “It has been suggested that an advanced civilization might have the technology to warp spacetime so that closed timelike curves would appear, allowing travel into the past.” Suggested by whom? An army of science-fiction writers, of course, but Hawking cited the physicist Kip Thorne (yet another Wheeler protégé) of the California Institute of Technology, who had been working with his graduate students on “wormholes and time machines.”

At some point the term “sufficiently advanced civilization” became a trope. As in: Even if we humans can’t do it, could a sufficiently advanced civilization? This is useful not just for SF writers but for physicists, too. So Thorne and Mike Morris and Ulvi Yurtsever wrote in
Physical Review Letters
in 1988, “We begin by asking whether the laws of physics permit an arbitrarily advanced civilization to construct and maintain wormholes for interstellar travel.” Not coincidentally, twenty-six years later, Thorne served as executive producer and science advisor for the 2014 big-budget movie
Interstellar.
“One can imagine an advanced civilization pulling a wormhole out of the quantum foam,” they wrote in the 1988 paper, and they included an illustration captioned “Spacetime diagram for conversion of a wormhole into a time machine.” They were contemplating wormholes with mouths in motion: a spaceship might enter one mouth and exit another mouth
in the past.
Fittingly, they concluded by posing a paradox, only this time it isn’t the grandfather who dies:

Can an advanced being measure Schrödinger’s cat to be alive at an event
P
(thereby “collapsing its wave function” onto a “live” state), then go backward in time via the wormhole and kill the cat (collapse its wave function onto a “dead” state) before it reaches
P
?

They left that question unanswered.

Hawking stepped in. He analyzed the wormhole physics as well as the paradoxes (“all sorts of logical problems, if you were able to change history”). He considered the possibility of evading the paradoxes “by some modification of the concept of free will,” but free will is seldom a happy topic for a physicist, and Hawking saw a better approach: what he proposed to call the chronology protection conjecture. A great deal of calculation was required, and when the calculating was done, Hawking was convinced: the
laws of physics
would protect history from the supposed time travelers. Notwithstanding Kurt Gödel, they must forbid the appearance of closed timelike curves. “It seems there is a chronology protection agency,” he wrote sci-fi-ishly, “which prevents the appearance of closed timelike curves and so makes the universe safe for historians.” And he concluded with a flourish—the kind of thing Hawking could get away with in the
Physical Review.
He had more than a theory. He had “evidence”:

There is also strong experimental evidence in favor of the conjecture from the fact that we have not been invaded by hordes of tourists from the future.

Hawking is one of those physicists who knows that time travel is impossible but also knows it’s fun to talk about. He points out that we are all traveling through time, one second at a time. He describes black holes as time machines, reminding us that gravitation slows the passage of time locally. And he often tells the story of the party he threw for time travelers—invitations sent only after the fact: “I sat there a long time, but no one came.”

In fact, the chronology protection conjecture had been floating about long before Stephen Hawking gave it a name. Ray Bradbury, for example, stated it in his 1952 story about time-traveling dinosaur hunters: “Time doesn’t permit that sort of mess—a man meeting himself. When such occasions threaten, Time steps aside. Like an airplane hitting an air pocket.” Notice that time has agency here: time
doesn’t permit,
and time
steps aside.
Douglas Adams offered his own version: “Paradoxes are just the scar tissue. Time and space heal themselves up around them and people simply remember a version of events which makes as much sense as they require it to make.”

Perhaps that seems a bit magical. Scientists prefer to credit
the laws of physics.
Gödel thought a robust, paradox-free universe was simply a matter of logic. “Time travel is possible, but no person will ever manage to kill his past self,” he told a young visitor in 1972.
^*8
“The
a priori
is greatly neglected. Logic is very powerful.” At some point chronological protection became part of the ground rules. It even became a cliché. In her 2008 story “The Region of Unlikeness,” Rivka Galchen can take all that old stage business for granted:

Science fiction writers have arrived at analogous solutions to the grandfather paradox: murderous grandchildren are inevitably stopped by something—faulty pistols, slippery banana peels, their own consciences—before the impossible deed can be carried out.

Region of unlikeness
comes from Augustine: “I perceived myself to be far off from Thee, in the region of unlikeness”—
in regione dissimilitudinis.
He is not fully realized. Nor are any of us, bound as we are in time and space. “I beheld the other things below Thee, and I perceived that they neither altogether are, nor altogether are not.” God is eternity, remember, and we are not, much to our sorrow.

Galchen’s narrator falls into a friendship with two older men, philosophers maybe, scientists, it’s all a bit vague. The relationships are not well defined. The narrator feels that she is a bit undefined herself. The men speak in riddles. “Oh, time will tell,” one of them says. And: “Time is our tragedy, the substance we have to wade through as we try to move closer to God.” They vanish from her life for a while. She watches the obituary pages. An envelope appears mysteriously in her mailbox—diagram, billiard balls, equations. She thinks of an old joke: “Time flies like an arrow and fruit flies like a banana.” One thing becomes clear: everyone in this story knows a lot about time travel. A fateful loop—the same paradox as ever—begins to emerge from the shadows. Some rules are explained: that “contrary to popular movies, travel into the past didn’t alter the future, or, rather, that the future was already altered, or, rather, that it was all far more complicated than that.” Fate seems to be tugging at her, in a gentle way. Can anyone evade destiny? Look what happened to Laius. All she can say is, “Surely our world obeys rules still alien to our imagination.”

—

WE BEGIN AGAIN.
A woman stands at the end of a “pier”—the open-air observation platform at Orly Airport (
la grande jetée d’Orly
), overlooking a sea of concrete on which the great metal jetliners rest, pointed like arrows toward the future. The sun is pale in a charcoal sky. We hear shrill jet blasts, a ghostly choir, murmuring voices. The woman almost smiles as the wind ruffles her hair. A child holds on to the railing, watching the planes on a warm Sunday. He sees the woman raise her hands to her face in horror and sees also, out of the corner of his eye, a blur, a falling shape.
Later, he knew he had seen a man die,
the narrator intones. Not long afterward, World War III begins. A nuclear holocaust destroys Paris, and the rest of the world, too.

This is
La jetée,
a 1962 film by Chris Marker—the pen name of Christian François Bouche-Villeneuve, born in 1921, a philosophy student, a resistance fighter with the Maquis, and then a roving journalist and photographer.
^*9
He was rarely photographed without a mask and lived to be ninety-one. In the fifties, after he worked with Alain Resnais on his Holocaust documentary
Night and Fog,
Resnais said: “A theory is making the rounds, and not without some grounds, that Marker could be an extra-terrestrial. He looks like a human, but perhaps he could be from the future or another planet.” Marker called
La jetée
a “photo-novel”: it is composed of still photographs, fading and dissolving, shifting points of view, to create, as one critic observed, the “illusion of a time-space continuum.” We are told that it is the story of a man marked by a memory from his childhood.
The sudden roar, the woman’s gesture, the crumpling body, and the cries of the crowd on the jetty blurred by fear.
The memory—and the marking—make him a candidate for time travel.

Now the world is dead and radioactive. Ruined churches, cratered streets. Survivors inhabit the tunnels and catacombs under Chaillot; a few men rule over prisoners in a camp. They despair. Their one hope lies in finding an emissary to send back to the past.
Space was off limits. The only link with the means of survival passed through Time. A loophole in Time and then maybe it would be possible to reach food, medicine, sources of energy.
Camp scientists experiment cruelly on one prisoner after another, driving them to madness or death, until finally they come to the nameless man “whose story we are telling.” What makes this man different from the others is his obsession with the past—with a particular
image
from the past.
If they could imagine or dream of another time, perhaps they would be able to reenter it. The camp police spied even on dreams.
The message here is that time travel is for the imaginative: an idea that recurs in the literature, for example in Jack Finney’s
Time and Again.
Time travel begins in the mind’s eye. Here, in
La jetée,
it’s a matter not just of transportation but of survival.
The human mind balked. To wake up in another time meant to be born a second time, as an adult. The shock would be too much.

Credit 11.1

He lies in a hammock. A mask, with electrodes, covers his eyes. A large hypodermic needle injects drugs into his veins, while background voices whisper in German.
He suffers. They continue. On the tenth day, images begin to ooze, like confessions. A peacetime morning. A peacetime bedroom—a real bedroom. Real children. Real birds. Real cats. Real graves. On the sixteenth day he is on the pier at Orly. Empty.

Sometimes he sees a woman, who might be the one he seeks. She is standing on the pier, or driving a car, smiling. A headless body is carved in ruined stone. These are images from a timeless world. He recovers from his trance, but the experimenters send him back again.

This time, he is close to her, speaks to her. She welcomes him without surprise. They are without plans, without memories. Time builds itself simply around them, their only landmarks the flavor of the moment and the markings on the walls.
They explore a natural history museum, filled with animals from other times. For her, he is a man of mystery—vanishing periodically, wearing a curious necklace, dog tags from the war to come.
She calls him her Ghost.
It occurs to him that in his world, his time, she is already dead.

Many people, seeing
La jetée
with no foreknowledge, are not aware that they are seeing a series of still images. Then, twenty minutes into the film, the woman, asleep, her hair askew on her pillow, opens her eyes, looks directly at the viewer, breathes, and blinks. Time shudders, becomes momentarily real again. The frozen images have been timeless—memories, crystalized. Perhaps memory is the time traveler’s subject. Marker once said, “I will have spent my life trying to understand the function of remembering, which is not the opposite of forgetting, but its other side.” And he liked to quote George Steiner: “It is not the past that rules us—it is the image of the past.”
Jetée
is a pun, too:
j’étais,
I was.