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  Logically all such changes in the past should occur at once. From the moment the final switch was thrown, therewould always have beenErics and Seans scattered all up and down the time-stream across the whole span of the experiment’s 190 million years.

  But there wasn’t anything logical about time-travel in the first place, Sean knew. It gloriously defied all the laws of cause and effect. And so evidently each swing of the pendulum was going to produce a completely new version of the past. Reality would be fluid from now on, and no one within that shifting reality would ever be aware of the changes. They could never remember the past as it used to be. Themoment the change was made, the past would always have been the way it was now.

  Only he and Eric, the daring young men on the flying trapeze, moving as outside observers, would be able to comprehend the havoc they were wreaking as they flashed back and forth across the fabric of time, reweaving it as they went. And even they would start to lose track of the changes as the paradoxes mounted.

  He walked over to Eric and Sean1. God, they looked pale and sweaty! That was embarrassing. They were really nervous. He didn’t remember having felt that nervous himself when it had been his turn to be Sean1, fifty minutes ago. He thought of himself as having waited calmly, coolly, confidently for his launching into the time-shunt.

  But he realized that he was probably kidding himself. The way Sean1 looked now was the way he himself must have looked fifty minutes ago, because he had been Sean1 then. There was no hiding from the truth of that. He had been scared stiff. Fifty minutes ago he had been sitting there waiting to be converted into a cluster of tachyons—particles that move faster than light and travel backward in time in an anti-time universe. What the singularity coupling did was turn him into a tachyonic replica of himself, throwing off showers of anti-time energy that would be exactly balanced by the time-force liberated in the opposite direction. At least that was what it was supposed to do. And he had been sitting there wondering if it would.

  Well, it had. And here he was.

  They were staring goggle-eyed at him. As though he had no business being there. As though he were some evil being who had come to haunt the place.

  Sean smiled.

  “Relax,” he said. “It’s all going to be fine. Just let yourself-go, and don’t try to fight it. You won’t like it at first, butall the worst stuff comes right at the beginning, and then it’s okay.”

  A little comfort, a little friendly cheer. It was the least he could do for them, he thought. For Ricky. And also for Sean, who was sitting there looking so pale and miserable. His brother and his other self. If there’s anyone in the world who’s closer and dearer to you than your twin brother, Sean thought, it’s your other self.

  5.

  Eric

  -5×102minutes

  The big room was oddly peaceful, here on the night before the experiment. It was about two in the morning. The overhead lights were turned off, and the only illumination came from a couple of green security lamps off to the side.Nobody seemed to be there when Eric stepped off the shunt platform after a moment of arrival vertigo blessedly more brief than it had been the last time. He looked around. Nobody here at all? That was peculiar. They knew what time he’d be due to arrive on this swing. Even if most of them would be asleep at this hour, resting up for the big day ahead of them tomorrow,somebodyshould have been here to debrief him when he showed up.

  Then he noticed one of the younger scientists—a quasiconductor man named John Terzunian—dozing in the darkness.

  Eric went over to him. Touched him gently on the shoulder.

  “Johnny? Johnny, wake up. It’s me, Eric, making the minus-five-hundred-minute shunt.”

  “What? Who?” A look of sudden panic. “Oh, God, I must have slumped off.”

  “Happens to anyone,” Eric said. The other man looked hardly older than he was, maybe twenty-five, twenty-six, barely past his doctorate. His hair was thinning already. His eyes were jet black and very bloodshot. “Don’t worry. I won’t tell. Everyone else is asleep, huh?”

  Terzunian nodded. “The last one left an hour ago. We drew lots for who would stay till you came in.”

  “And you lost.”

  A sheepish smile. “Nobody’s had much sleep for three or four nights in a row, now. I wouldn’t mind being sacked out right this minute. But somebody had to be here to meet you.”

  “Sure,” Eric said. “I understand.”

  He thought of Sean1 and Eric1, snoring away in the dorm section a couple of hundred yards from the lab. For them, he knew, edginess had fought a battle with exhaustion and exhaustion had won.

  Well, it was a good idea for them to be sleeping. This would be the last chance they’d have to get a proper night’s sleep in the year 2016 for a long time to come. Little more than eight hours from now they were going to set out on a journey that would carry them some 95 million years in each direction before they saw their own home year again. Adrift in the time-stream, swinging back and forth, swooping through the eons.

  It was strange, thinking of Sean1 and Eric1 astheminstead ofus. But he had to. Those two guys sleeping down there in the dorm weren’t Sean and Eric Gabrielson at all, not really. Not to him. They were two entirely other people: Sean1 and Eric1. Yesterday’s selves. They hadn’t yet begunto oscillate in time. They still had no real idea of what any of this was going to be like.

  To them, if they thought of him at all, he would be Eric2, an Eric of the future, tomorrow’s Eric, an unreal Eric. That was all right. He didn’t feel unreal. He wasn’t living in tomorrow. He was living innow. It was a now that kept sliding around between past and future, but all the same it was the only now he had. He was real enough to himself: the true and authentic Eric. And the true and authentic Sean, for him, was the one who was nearly seventeen hours away just now, up there at the plus-500-minute level, at the opposite end of the time-travel seesaw that they were riding.

  Everything in balance. Everything symmetrical.

  It all had the intense bright clarity of a very powerful dream. Except it was actually happening to them, and it would go on happening for something like ninety-five million years.

  Terzunian said, “Can I get you anything? A drink of water? Something to eat?”

  “No, thanks,” Eric said. “So far as subjective time goes, this is still just the beginning of the experiment for me. I’ve only experienced a few minutes of elapsed time since the whole thing started.”

  “All right,” Terzunian said. “We’d better get down to work, then. I’m supposed to ask you questions about your psychological and physical state upon arrival. Here—the camera’s on. Testing. Testing.” He seemed twitchy, ill at ease, afraid of messing anything up. Well, Eric thought, he’s been involved with this project for years, and now here it is, actually happening.

  Actually happening. Yes.

  There were times when he had trouble believing that he and Sean had really agreed to do it. Of course they hadknown about the experiment for years—Project Pendulum had gotten underway when they were still in high school, as soon as the development of artificially produced mini-singularities had provided the technological basis for traveling in time.

  Sean had brought home a pile of theoretical papers about it. Explaining how the phase-linkage coupling of a minute black hole, identical to those that are found all over interstellar space, and its mathematical opposite, a “white hole,” created an incredibly powerful force that ripped right through the fabric of space-time—and how that force could be contained and controlled, like a bomb in a basket, so that it could be used as a transit tube for making two-way movements in time.

  Eric’s first reaction on hearing that was to imagine himself-running backward along the earth’s geological history as if seeing a film from back to front—soaring through the epochs, past the Pleistocene and the Pliocene into the days of the dinosaurs, the early amphibians, the trilobites, back even to the primordial days when there was nothing on the surface of the world except a bare
granite shield rising above a steaming sea. Tremendous! To see it all. Not to have to reconstruct it from compressed strata and scattered fossils, but to look at everything with your own eyes while it still existed.

  His second reaction was to think that the whole notion was completely crazy, a fantastic pipe dream.

  No, Sean had said. It really can work. Here, let me show you the equations—

  And Sean had scribbled equations for him until he begged for mercy. Math on Sean’s level was a mysterious language to him, as remote and inaccessible as the language the ancient Egyptians spoke in their dreams. The more Sean explained of it, the less Eric understood—or cared. But Seanwas convinced that the theory of time-shunting was correct, and Sean was usually right about anything he investigated with such passion. At least in the world of physics.

  That’s extraordinary, Eric had said, figuring that fifty or sixty years of heavy-duty work would be necessary, at the very least, before time travel was anything more than a set of fascinating equations. And then he put it all totally out of his mind. He had other things to think about that seemed more urgent, like going to college, and his graduate work in paleontology after that.

  But then came news that the first displacement machine had actually been built and tested. Eric paid some mild attention to that. Robots equipped with data-recording gear and cameras went off, so it was claimed, on safaris in time. The robots made their journey and returned to the same instant from which they had been sent off. To the watching scientists the elapsed time of the experiment was zero. So there was no way of telling that anything had happened, except for the power drain that the instrument measured— and except for the paradoxes.

  The paradoxes! Even though the robots hadn’t seemed to go anywhere, they turned up in the laboratory hours and days and weeks before they had been sent out. That gave everyone headaches, thinking about it. The past kept flowing and shifting around, and nobody’s memory was a safe place: things were always getting different from what you thought you remembered.

  And the robots also turned up an equal number of hours and days and weeksafterthe experiment, flashing suddenly into existence in the laboratory and staying around for a few minutes, maybe an hour or two, before vanishing again.

  The robots seemed to have suffered no ill effects from their mysterious journeys. They appeared still to be in fine working order. But the cameras they carried yielded nothingbut fogged film. Sean explained that film emulsion was evidently unable to withstand the tachyon storms to which it was exposed during the time shunts. The data-recording gear had produced scrambled digital readouts, just static, probably for the same reason.

  Oh, Eric had said. Tachyon storms, is that it?

  He didn’t bother asking for more elaborate explanations. Not then.

  They sent living creatures through the machine, too— turtles, frogs, rabbits. The usual nature organizations complained about that, but the animals all came back safely. Back from where? Who could say? No question that they had gonesomewhere. The usual time-displacement paradoxes had been observed: rabbits popping out of nowhere in the laboratory three days before the start of the experiment, and doing the same thing three days after the experiment, too.

  That was interesting, a remarkable achievement. If the rabbits could be sent three days backward and forward in time, they might well have gone a million years, or fifty million. Still, what could a turtle or a rabbit tell anybody about the way the Mesozoic really looks, or the world ofA.D.One Million? You could send a turtle to the end of time and back, and it wouldn’t give you one syllable of useful information about its trip.

  So of course they called for volunteers.

  He didn’t bother asking for more elaborate explanations. Not then.

  They sent living creatures through the machine, too— turtles, frogs, rabbits. The usual nature organizations complained about that, but the animals all came back safely. Back from where? Who could say? No question that they had gonesomewhere. The usual time-displacement paradoxes had been observed: rabbits popping out of nowhere in the laboratory three days before the start of the experiment, and doing the same thing three days after the experiment, too.

  That was interesting, a remarkable achievement. If the rabbits could be sent three days backward and forward in time, they might well have gone a million years, or fifty million. Still, what could a turtle or a rabbit tell anybody about the way the Mesozoic really looks, or the world ofA.D.One Million? You could send a turtle to the end of time and back, and it wouldn’t give you one syllable of useful information about its trip.

  Human time-travelers would have to go through the machine in order to get any significant results. Only a lunatic, Eric figured, would volunteer for a deal like that.

  The word went out that they wanted to use a pair of identical twins, because there had to be an exact balance of momentum down to the last milligram. Twins, because they had the same bone structure and pretty much the samedistribution of body fat, would make it that much easier to attain that balance.

  That’s nice, Eric thought. And went back to his doctoral thesis on Arctic amphibian life in the Mesozoic period.

  They’re looking for twins with scientific background, someone told him.

  Eric simply shrugged.

  Ideally they want one twin who’s a physicist and one who’s a paleontologist, someone else told him. In order to maximize the value of their observations.

  Right. Eric was a paleontologist. Sean was a physicist.

  That’s very interesting, Eric said, still showing no interest-at all. I suppose we’re not the only twins who meet that requirement. They’ll find someone sooner or later who’ll be willing to risk the trip.

  Then one day Sean turned up and said, “Don’t you think it could help your research a little if you got a look at somelivingMesozoic critters, Ricky?”

  And now here he was five hundred minutes in his own past, locked into an unstoppable series of ever-widening swings in time, back and forth, back and forth, minutes and hours and months and years and centuries and eons. Like a dream, a very strange and intense dream, a dream brighter and sharper than any reality he had ever experienced.

  “Go ahead,” Terzunian said. “This is the minus-fivehundred minute level, John Terzunian speaking. Eric Gabrielson has just arrived right on schedule: the third backswing.” He pointed at Eric to give him his cue. “Okay. Make your report.”

  “There’s not a lot to tell. Easy arrival, none of the queasiness I felt when I made the minus-five-minute shunt. Just a fast flicker of discomfort, then everything normal. Some minor spatial displacement: I came in a couple of feetto the left of my departure point. No fatigue so far. Maybe some mild uneasiness—no, uneasiness is too strong a word, a little edginess, maybe—”

  Terzunian was staring at him. There was a peculiar expression on his face, what seemed to be a mixture of fascination and envy and what might have been something like pity.

  “Well, look,” Eric said, “there really isn’t anything to report yet. Give me another few shunts and I’ll have plenty to say.Plenty.”

  But who will I say it to, he wondered? When I’m nine and a half years in the past? Or 950,000 years in the future?

  6.

  Sean

  + 5×102minutes

  This time it felt as if some giant had scooped him up, popped him into a slingshot and whirled him around, and tossed him with all his might. When he landed, the sides of the laboratory were circling around him like the rim of a big centrifuge and the floor was rocking wildly from side to side. The place might just as well have been a carnival funhouse. Sean flung himself down flat, hanging on for all he was worth.But the effect lasted only a moment or two. The wild funhouse gyrations slowed down and then they stopped altogether. He patted the floor to make certain it had finished moving. Apparently it had. He got carefully to his feet, steadying himself with his outstretched arms. He took two or three cautious steps. Everything was holding still, now. Fine. Fine.

  “
It takes a little getting used to,” he said to nobody in particular.

  He looked around. There were new changes in the laboratory. He was five hundred minutes in the future: eighthours and twenty minutes since the start of the experiment. Night had fallen. The fluorescent lights seemed harsher and brighter. The big room was weirdly quiet, almost ominous.

  “Tell us what you experienced,” Dr. Ludwig said.

  Dr. Ludwig and Dr. White were the only people in the room. The technicians must have been sent home. The shunt platform was strangely forlorn and abandoned with no one around it. The two metal seats that flanked the displacement cone might have been nothing more than a couple of classroom chairs. The cone itself seemed trivial, a mere chunk of inactive machinery.

  Staring at it, Sean had trouble believing that under that glossy lump of shielding lurked a symmetrical pair of laboratory-generated collapsed stars: a miniature black hole and its mirror image, a so-called white hole. Together they made up a pair of perfectly balanced singularities—zones of strangeness where nothing behaved according to the rules of the ordinary universe—held in an unbreakable coupling. Infinite energy forever circled in a loop between the interlocked event horizons of those singularities. Energy that had opened the time gate through which Sean and Eric had been shunted to begin their immense voyage through time and antitime.

  Dr. Ludwig’s eyes looked bleary and his plump cheeks were dark with stubble. It was the look of a man who has been in the office too long. When Sean had made his last trip through here at plus five minutes—hardly any time at all ago for Sean, eight hours and twenty minutes for them— Dr. Ludwig had been pink and freshly shaven.

  “The first time,” Sean said, “I thought I was losing my mind. The first forward swing, the plus-five-minutes one. Let me tell you, it was a truly hideous experience.”