The Time Traveller's Almanac

by Ann VanderMeer

  Another writer who famously did his homework was Carl Sagan. For his novel Contact, he wanted a physically plausible way for his heroine to travel to the star Vega and back quickly. He came up with a method that an incredibly advanced alien culture might develop, and mailed a description to Kip Thorne to ask his opinion. Dr. Thorne had a better idea. He shared it with Dr. Sagan, who incorporated it into the book.

  Dr. Thorne’s suggestion is known to the initiated as an Einstein-Rosen bridge and to producers and consumers of science fiction as a wormhole. (Check out the Wikipedia article on wormholes for details and pictures, including a formally ray-traced image of a wormhole connecting two places on Earth.) Simply connect the throat of a black hole near Vega with that of another near the Earth, and bingo! We’ve built a shortcut to the stars, and the universe is ours.

  But wait, it sounds like we’re talking about faster-than-light travel. What does this have to do with time travel?

  Everything. Remember, the central tenet of Einstein’s relativity is that space and time are different aspects of the same fundamental thing. Bend one, and you twist the other.

  Dr. Thorne and other theorists have suggested that it might be possible to turn a wormhole into a time machine. Leave one mouth of the tunnel at home, and take the other on a Twin Paradox sortie out into space. The travelling mouth experiences less time than the homebound one. When it returns, you can enter the latter and come out the former in the past. It’s a bonafide time machine. (Physicists use the term “closed timelike curves” when discussing them in print, in an attempt to head off media headlines screaming about scientists inventing time machines.)

  The wormhole time machine is limited. It’s hard to adjust the time interval between the two mouths. You can do it only by taking one mouth or the other on a high-speed jaunt. And you can never go back to a time before you built the wormhole, a disappointment to people interested in altering the outcomes of still-earlier elections, sporting events, or armed conflicts. But you could still use it to make a fortune on Wall Street, or assassinate an ancestor and finally put to rest all philosophical posturing about the Grandfather Paradox.

  General relativity may allow for the possibility of wormholes, but that doesn’t mean they’re a done deal. There are some construction challenges we don’t yet know how to overcome. First, every normal black hole contains an evil singularity in the center. Anything that crosses the hole’s horizon must fall into that singularity, be disrupted by it, and become one with it. Ouch. Next, there is not an obvious way to coax two black holes to connect with one another. Finally, theorists predict that if two holes are somehow spliced together, the resulting tunnel will pinch itself off before anything could pass through. Considerable intellectual energy has been invested in these topics, though, and there could be a way to solve them.

  It might be possible to make two connected and singularity-free black holes out of something besides ordinary mass, which could then counteract the natural tendency of the tunnel to collapse. Theory suggests that this requirement would be met by a substance with negative mass and negative pressure. That’s right: to build a traversable wormhole, we’ll need to use something that weighs less than nothing and is emptier than a perfect vacuum. (Did you not believe me when I said it was going to get weird?) Engineers joke about “unobtainium” for applications that demand materials with unrealistic physical properties, but this stuff takes the cake.

  Yet off in the fringes of physics there do seem to be things that exert negative pressure. The mysterious “dark energy” that is accelerating the expansion of our universe against the pull of its own gravity might be one. Another is the Casimir effect. It may be possible to build a traversable wormhole using the Casimir effect, so it’s worth covering here.

  Physicists believe that at the tiniest-size scales and the briefest flickers of time, our universe is a seething froth of instability, constantly creating pairs of sub-atomic particles that recombine and vanish before they can be detected. These are called “virtual particles.” Among the virtual particles are photons, the wiggles of electricity and magnetism that make up light, radio, X-rays, and so on. Photons both real and virtual cannot travel very well through electrical conductors such as metals. So if we take two very smooth flat metal surfaces, and place them very close together, they’ll suppress the creation of virtual photons with a wavelength longer than the separation between them. But outside the plates are virtual photons of all wavelengths, which exert a tiny bit more radiation pressure on the back sides of the plates than does the restricted range of wavelengths available between them. If all of this weirdness is really true, then there should be a very tiny force – effectively a negative pressure – pushing the plates toward one another.

  This force exists and has been measured in experiments.

  There are some difficulties with building wormholes using the Casimir effect. It operates only over very short distances. Also it’s rather weak. It’s not as heavy a hammer for knocking holes in spacetime as, say, the collapsing core of a massive star. But if our kids are terraforming planets and we don’t want to be outdone, we should go for it. We start by building a spherical metal shell with the diameter of the orbit of Pluto, a supersized Dyson sphere. We then build another one surrounding the first, carefully maintaining the gap between them at one Ångstrom unit, roughly the size of an atom. If we accomplish these things, says Dr. Thorne, the Casimir effect will warp space so that we will no longer be able to tell which sphere is the inner one and which is the outer. We will have built a wormhole that allows us to travel the massive distance of one ten-billionth of a meter. Not a practical transportation device, unfortunately. But it’s a real wormhole, and by sending one end on a high-speed trip we might possibly be able to turn it into a real time machine.

  Back to the Present

  Unfortunately, our own less-than-incredibly-advanced culture won’t be building Matryoshka-doll Dyson spheres and accelerating them to relativistic speeds any time soon. But that doesn’t diminish the appeal of time travel. It remains a fruitful topic for both science fiction and theoretical physics. As in the case of Contact, sometimes the interplay between the two helps make both stronger. And as our train moves inexorably forward at 3600 seconds per hour, the day when we can engineer time machines must be moving just as inexorably closer. Maybe, somewhere up the track, they’re sending people even further along, to still more distant futures where they can send people back.

  REACTIONARIES AND REVOLUTIONARIES

  A SOUND OF THUNDER

  Ray Bradbury

  Ray Bradbury was one of the most celebrated twentieth-century American writers. He wrote science fiction, horror, and mystery fiction. Many of Bradbury’s works have been adapted into comic books, television shows, and films. This story was first published in Collier’s Magazine in 1952. The term “the Butterfly Effect” was coined because of this famous story – which may be the most reprinted science fiction tale in history.

  The sign on the wall seemed to quaver under a film of sliding warm water. Eckels felt his eyelids blink over his stare, and the sign burned in this momentary darkness:

  TIME SAFARI, INC.

  SAFARIS TO ANY YEAR IN THE PAST.

  YOU NAME THE ANIMAL.

  WE TAKE YOU THERE.

  YOU SHOOT IT.

  Warm phlegm gathered in Eckels’ throat; he swallowed and pushed it down. The muscles around his mouth formed a smile as he put his hand slowly out upon the air, and in that hand waved a check for ten thousand dollars to the man behind the desk.

  “Does this safari guarantee I come back alive?”

  “We guarantee nothing,” said the official, “except the dinosaurs.” He turned. “This is Mr. Travis, your Safari Guide in the Past. He’ll tell you what and where to shoot. If he says no shooting, no shooting. If you disobey instructions, there’s a stiff penalty of another ten thousand dollars, plus possible government action, on your return.”

  Eckels glanced across the vast office at a mass
and tangle, a snaking and humming of wires and steel boxes, at an aurora that flickered now orange, now silver, now blue. There was a sound like a gigantic bonfire burning all of Time, all the years and all the parchment calendars, all the hours piled high and set aflame.

  A touch of the hand and this burning would, on the instant, beautifully reverse itself. Eckels remembered the wording in the advertisements to the letter. Out of chars and ashes, out of dust and coals, like golden salamanders, the old years, the green years, might leap; roses sweeten the air, white hair turn Irish-black, wrinkles vanish; all, everything fly back to seed, flee death, rush down to their beginnings, suns rise in western skies and set in glorious easts, moons eat themselves opposite to the custom, all and everything cupping one in another like Chinese boxes, rabbits into hats, all and everything returning to the fresh death, the seed death, the green death, to the time before the beginning. A touch of a hand might do it, the merest touch of a hand.

  “Unbelievable,” Eckels breathed, the light of the Machine on his thin face. “A real Time Machine.” He shook his head. “Makes you think. If the election had gone badly yesterday, I might be here now running away from the results. Thank God Keith won. He’ll make a fine President of the United States.”

  “Yes,” said the man behind the desk. “We’re lucky. If Deutscher had gotten in, we’d have the worst kind of dictatorship. There’s an anti-everything man for you, a militarist, anti-Christ, anti-human, anti-intellectual. People called us up, you know, joking but not joking. Said if Deutscher became President they wanted to go live in 1492. Of course it’s not our business to conduct Escapes, but to form Safaris. Anyway, Keith’s President now. All you got to worry about is—”

  “Shooting my dinosaur,” Eckels finished it for him.

  “A Tyrannosaurus Rex. The Tyrant Lizard, the most incredible monster in history. Sign this release. Anything happens to you, we’re not responsible. Those dinosaurs are hungry.”

  Eckels flushed angrily. “Trying to scare me!”

  “Frankly, yes. We don’t want anyone going who’ll panic at the first shot. Six Safari leaders were killed last year, and a dozen hunters. We’re here to give you the severest thrill a real hunter ever asked for. Traveling you back sixty million years to bag the biggest game in all of Time. Your personal check’s still there. Tear it up.”

  Mr. Eckels looked at the check. His fingers twitched.

  “Good luck,” said the man behind the desk. “Mr. Travis, he’s all yours.”

  They moved silently across the room, taking their guns with them, toward the Machine, toward the silver metal and the roaring light.

  First a day and then a night and then a day and then a night, then it was day-night-day-night. A week, a month, a year, a decade! A.D. 2055. A.D. 2019. 1999! 1957! Gone! The Machine roared.

  They put on their oxygen helmets and tested the intercoms.

  Eckels swayed on the padded seat, his face pale, his jaw stiff. He felt the trembling in his arms and he looked down and found his hands tight on the new rifle. There were four other men in the Machine. Travis, the Safari Leader, his assistant, Lesperance, and two other hunters, Billings and Kramer. They sat looking at each other, and the years blazed around them.

  “Can these guns get a dinosaur cold?” Eckels felt his mouth saying.

  “If you hit them right,” said Travis on the helmet radio. “Some dinosaurs have two brains, one in the head, another far down the spinal column. We stay away from those. That’s stretching luck. Put your first two shots into the eyes, if you can, blind them, and go back into the brain.”

  The Machine howled. Time was a film run backward. Suns fled and ten million moons fled after them. “Think,” said Eckels. “Every hunter that ever lived would envy us today. This makes Africa seem like Illinois.”

  The Machine slowed; its scream fell to a murmur. The Machine stopped.

  The sun stopped in the sky.

  The fog that had enveloped the Machine blew away and they were in an old time, a very old time indeed, three hunters and two Safari Heads with their blue metal guns across their knees.

  “Christ isn’t born yet,” said Travis. “Moses has not gone to the mountains to talk with God. The Pyramids are still in the earth, waiting to be cut out and put up. Remember that. Alexander, Caesar, Napoleon, Hitler – none of them exists.”

  The man nodded.

  “That” – Mr. Travis pointed – “is the jungle of sixty million two thousand and fifty-five years before President Keith.”

  He indicated a metal path that struck off into green wilderness, over streaming swamp, among giant ferns and palms.

  “And that,” he said, “is the Path, laid by Time Safari for your use. It floats six inches above the earth. Doesn’t touch so much as one grass blade, flower, or tree. It’s an anti-gravity metal. Its purpose is to keep you from touching this world of the past in any way. Stay on the Path. Don’t go off it. I repeat. Don’t go off. For any reason! If you fall off, there’s a penalty. And don’t shoot any animal we don’t okay.”

  “Why?” asked Eckels.

  They sat in the ancient wilderness. Far birds’ cries blew on a wind, and the smell of tar and an old salt sea, moist grasses, and flowers the color of blood.

  “We don’t want to change the Future. We don’t belong here in the Past. The government doesn’t like us here. We have to pay big graft to keep our franchise. A Time Machine is finicky business. Not knowing it, we might kill an important animal, a small bird, a roach, a flower even, thus destroying an important link in a growing species.”

  “That’s not clear,” said Eckels.

  “All right,” Travis continued, “say we accidentally kill one mouse here. That means all the future families of this one particular mouse are destroyed, right?”

  “Right.”

  “And all the families of the families of the families of that one mouse! With a stamp of your foot, you annihilate first one, then a dozen, then a thousand, a million, a billion possible mice!”

  “So they’re dead,” said Eckels. “So what?”

  “So what?” Travis snorted quietly. “Well, what about the foxes that’ll need those mice to survive? For want of ten mice, a fox dies. For want of ten foxes a lion starves. For want of a lion, all manner of insects, vultures, infinite billions of life forms are thrown into chaos and destruction. Eventually it all boils down to this: fifty-nine million years later, a caveman, one of a dozen on the entire world, goes hunting wild boar or saber-toothed tiger for food. But you, friend, have stepped on all the tigers in that region. By stepping on one single mouse. So the caveman starves. And the caveman, please note, is not just any expendable man, no! He is an entire future nation. From his loins would have sprung ten sons. From their loins one hundred sons, and thus onward to a civilization. Destroy this one man, and you destroy a race, a people, an entire history of life. It is comparable to slaying some of Adam’s grandchildren. The stomp of your foot, on one mouse, could start an earthquake, the effects of which could shake our earth and destinies down through Time, to their very foundations. With the death of that one caveman, a billion others yet unborn are throttled in the womb. Perhaps Rome never rises on its seven hills. Perhaps Europe is forever a dark forest, and only Asia waxes healthy and teeming. Step on a mouse and you crush the Pyramids. Step on a mouse and you leave your print, like a Grand Canyon, across Eternity. Queen Elizabeth might never be born, Washington might not cross the Delaware, there might never be a United States at all. So be careful. Stay on the Path. Never step off!”

  “I see,” said Eckels. “Then it wouldn’t pay for us even to touch the grass?”

  “Correct. Crushing certain plants could add up infinitesimally. A little error here would multiply in sixty million years, all out of proportion. Of course maybe our theory is wrong. Maybe Time can’t be changed by us. Or maybe it can be changed only in little subtle ways. A dead mouse here makes an insect imbalance there, a population disproportion later, a bad harvest further on, a depres
sion, mass starvation, and finally, a change in social temperament in far-flung countries. Something much more subtle, like that. Perhaps only a soft breath, a whisper, a hair, pollen on the air, such a slight, slight change that unless you looked close you wouldn’t see it. Who knows? Who really can say he knows? We don’t know. We’re guessing. But until we do know for certain whether our messing around in Time can make a big roar or a little rustle in history, we’re being careful. This Machine, this Path, your clothing and bodies, were sterilized, as you know, before the journey. We wear these oxygen helmets so we can’t introduce our bacteria into an ancient atmosphere.”

  “How do we know which animals to shoot?”

  “They’re marked with red paint,” said Travis. “Today, before our journey, we sent Lesperance here back with the Machine. He came to this particular era and followed certain animals.”

  “Studying them?”

  “Right,” said Lesperance. “I track them through their entire existence, noting which of them lives longest. Very few. How many times they mate. Not often. Life’s short. When I find one that’s going to die when a tree falls on him, or one that drowns in a tar pit, I note the exact hour, minute, and second. I shoot a paint bomb. It leaves a red patch on his side. We can’t miss it. Then I correlate our arrival in the Past so that we meet the Monster not more than two minutes before he would have died anyway. This way, we kill only animals with no future, that are never going to mate again. You see how careful we are?”