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Wrong Way Street
by Larry Niven
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Copyright (c)1965 by Larry Niven
First published in Galaxy, April 1965

Fictionwise Contemporary
Science Fiction
Nebula Award(R) Nominee

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       Mike Capoferri turned out to be at one time in his life the loneliest man on the Moon. But it was not his first venture into aloneness. He had felt it before, almost twenty years before, when he was twelve and his eight-year-old brother died.
       Young Tony had been riding a Flexy, a kind of bobsled on wheels, down the hill road above Venice Boulevard. At the bottom of the hill he had turned hard right onto Venice. The Flexy had flipped over on its back, and its blunt rubber handle had poked hard into Tony's stomach.
       One of the first things the doctor did was to take Tony's blood pressure. It was low, which meant shock. It started to fall almost as soon as the blood pressure cuff was removed, but the doctor didn't know that until it was too late. Tony's spleen was ruptured.
       Mike had loved his younger brother. He sat in his room most of the time, unable to get used to his loss, and not really trying. After four weeks of it his father neglected his own grief long enough to take Mike to a child psychologist.
       Mike was a recent but ardent science fiction fan. "I want to change it, Doctor Stuart," he said earnestly. "I want to go back to four weeks ago and take away Tony's Flexy." He meant it, of course.
       Doctor Stuart had worked hard to get Mike to say those words. If he was thinking in terms of sibling rivalries and guilt feelings, it didn't show. "You can't do that, Mike. Time is a one-way street with no parking spaces. You just have to keep going."
       "Until you have an accident," Mike said bitterly.
       Doctor Stuart nodded. "Or run out of gas," he added, because he himself was old enough for the analogy to apply. They talked for almost three hours, with Mike doing most of the talking. Afterward Mike gradually stopped mourning.
         * * * *
When Mike Capoferri graduated from high school he had become intensely interested in space travel. His first year at Cal Tech was the year Walnikov landed on Mars. Mike was determined to follow.
       In a way he traveled further than Walnikov. He never got to Mars, but he did make it to the Moon. And unlike Mars, the Moon once had intelligent visitors.
       Mike was one of many. Thirty men and women had come to the alien base, determined to probe all its secrets. By this time Mike was thirty-one years old and held doctorates in physics, mathematics and philosophy. He was tall, dark, not too homely, a little too earnest. He got over that at the base, where the only defense against strangeness was a sense of humor.
       Besides the base, the aliens had thoughtfully provided one spaceship. It rested on its side near the base, a fat cylinder with conical ends and asymmetrical bulges in unexpected places. Mike began going through the ship before he ever entered the base, and he kept at it through the years. This wasn't unusual. The ship was thought to be the real treasure, for its star charts showed (in hard-to-read notations in ultraviolet ink) that it had cruised between widely separated stars. It may have had a faster-than-light drive.
       The base personnel lived in the base itself, with their own air regenerating system and their own airlock built into the open alien airlock. There was plenty of room for them. The aliens had averaged ten feet tall, and there were things which must have been bunks for forty-eight of them.
       Still, the base took getting used to. The alien engineers had put steps and ledges in the floor wherever the ground beneath wasn't exactly level. Newcomers learned not to sit on the "bunks," which looked like pieces of free-form sculpture, felt like foam rubber with a metal core, and changed shape without warning. They were told not to touch mosaic designs which had been marked with paint, for the design might hide a control of some kind.
         * * * *
It was four years since Mike had landed on the Moon. In that time the human tenants had made a great deal of progress.
       An emergency repair kit from the ship had yielded a method of creating artificial crystals of almost any shape from almost any solid, by building them up atom by atom. Already ships had lifted on rocket motors built from large diamonds.
       A box which held perfectly preserved sections from some non-terrestrial animals, possibly used as food, had given them a field which would interrupt any chemical process. The applications were numerous and varied. A short-range death ray. A beam to fight forest fires. A new method of inducing suspended animation, very useful in surgery.
       A sculpting implement, used by the aliens as a means of recreation (the base was infested with the statues they had left behind), had become a disintegrator. Turning it on had been heartbreakingly difficult. Mike had solved that problem in his second year, but had never been able to turn it off. The alien rec room had to be kept in vacuo, with a separate airlock, because air disappeared into the little ball of nothing at the end of the sculpting tool.
       Enough progress had been made on the alien number system that it was possible to do calculus with it. The money system, however, remained a complete mystery.
       Aside from the crystal maker and the airlock controls, the ship was as great and as fascinating an enigma as ever. The rows of "bunks" near the back -- suppose a bunk changed shape and dumped its occupant during a 5G maneuver? The controls, in plain sight on a common-sense control board in the bunk section -- what did it take to make them work? And what was the purpose of the dull red tetrahedron, seven feet on a side, which was set in the rear wall of the passenger section?
       Mike was taking a coffee break with Terry Holmes, a pretty, cheerful, blonde little Doctor of Languages, the day he first said, "I think I know what the central pyramid is for."
       Many people had said that, of course, but Mike was not addicted to wild guesses. "What is it?" Terry asked eagerly.
       "It's a time machine," he said.
       Terry got mad and left the table. The Halloween before Mike had dressed to imitate an alien statue and had frighteningly "come to life" before her horrified eyes. Since then she had been sensitive about his jokes.
       "No, really," he told her during the afternoon coffee break. "The idea makes a great deal of sense. We can be sure that the aliens had suspended animation, can't we?"
       "Sure." The reaction damping field was perfect for that purpose.
       "Right. So if they had time travel to go with it, it adds up to an FTL drive. They can sleep through a hundred-year journey and then move back a hundred years."
       "You're only guessing," Terry told him. "If the pyramid is an interstellar drive they didn't need time travel. If they had suspended animation they could have spent generations on one trip. We'll have to do that ourselves, probably."
       "Sure, but the idea of a time-travel device in the center of a spaceship is at least logical. I've been working on the thing for quite a while, and I think that's what it is. I've made it produce a weak gravitational field, so I know it can distort four-space."
       "Then it's for artificial gravity." She laughed as his face fell. "Mike, I dub thee world's champion rationalizer. And now I've got to get back to work."
       For a month nothing important happened. Carlos found a way to turn on the alien television set and got three-dimensional, technicolor static. Terry made some progress with the alien money; she had a tentative ordering of coins into either ascending or descending value, if in fact they were coins.
       Then one day the ship disappeared.
         * * * *
Mike was trying something new. He had set up a magnetic field around the control board and pushed one of the pyramid knobs. There were two of these, the same shape and color as the massive machine behind him. Now he put a block of glass between the poles of his generator and cut the current. The knob lit with an almost invisible blue glow. Suddenly everything was in free fall.
       "Eureka," Mike said absently, meaning: at last I've gotten some action out of the beast. When he turned his head he saw that the big red tetrahedron was base forward. He'd heard no sound of motion.
       A faint purple line grew across the top of the board.
       There were too many unknowns crawling into his experiment. Mike looked back so that he could see the big pyramid turn around, and switched his generator back on. Results came instantly.
       Mike sat up trying to rub the pain out of his eyes. It was several seconds before he could open them.
       The pyramid was apex-forward again. Mike got up and pulled out the pyramid knob, waited a moment for luck, then turned off the field generator. At last he sat down perspiring on an alien "bunk." What a sight that had been! He couldn't even remember it without his eyes hurting.
       Mike's bunk inconsiderately dropped him on the floor. He promptly got up and made for the airlock, feeling a crying need for coffee, Terry Holmes, conversation and familiar things. The strangeness had suddenly become too much to take.
       His momentary fear of the ship was gone by the time he left the airlock. What had started it, anyway? Merely the fact that he'd gotten things to working at last. Now they could make some real progress.
       He moved toward the base in easy four-foot leaping strides which splashed waves of dust when he landed. He was looking straight at the base airlock, but he was preoccupied by the thought of coffee and the familiar, instantly suppressed wish for a cigarette. He was half way there before he noticed...
       The base airlock was closed. The _alien_ airlock!
       Mike stopped short, staring. At first he was only bewildered, not horrified. How could the doors have closed? The bulk of the UN airlock would have stopped them. Or was the alien metal strong enough to -- 
       Mike made a strangling noise. The human airlock must have been crushed flat!
       He ran.
         * * * *
It had taken the base team months to open those doors. Although Mike had arrived a year later, he knew how they had done it. But why had they closed it? Had some fool been meddling with the controls?
       With alien designing, practically anything could be a control. The aliens had cleverly hidden their knobs, buttons, and pressure sensitive surfaces in esthetically pleasing design. The doors could have been closed by somebody accidentally leaning against a wall. Nobody had ever bothered to find out how to open them from the inside.
       Mike began picking pressure points out of the mosaic on the outer door. He stopped to wonder if the base held air, then decided that it didn't matter. Anyone still alive would be wearing a spacesuit under emergency regulations.
       He was taking a breather when he noticed that the UN ship was gone.
       Had they started to evacuate the base? No, the ship only held four people and cargo. They must have gone for help.
       The lock had been designed for use by two ten-footers with fourteen-foot branched tentacles. Mike needed forty minutes and a great deal of ingenuity, but finally the lock swung open.
       There was no wreckage in the lock.
       "Dust," Mike told himself. There was almost no moondust on the worn path between the ship and the base. Yet dust had spurted beneath his boots ... and there was no Earth-built ship, and the station was locked.
       "Eureka," he said softly. "They haven't found the base yet. I've traveled in time."
       Yet there were other possibilities. Mike began seeking them out even as he was going to work on the inner door. Maybe he had gone forward in time, to when the base had been restored as a museum. Or, worse yet, to some time after the return of the legitimate owners. (That had once been a favorite joke around the base: "Hey, look, they're coming back!") He might even be in another time track, one in which the base had never been abandoned. After all, he really didn't know much about the machine he'd been running.
       One look through a telescope would have told all. He could see the Earth from where he was standing, huge and full, of course, he could not make out the shapes of the continents.
       He kept working.
       He was rigidly tensed as the doors folded back. Had the station been abandoned yet? Was his the honor, God help him, of meeting the first inhuman intelligence? But nobody came to meet him.
       His air pressure dial read 22.4 pounds/square inch. This must be alien air.
         * * * *
He walked through the base, slowly and cautiously; after four and a half years he was used to watching where he stepped. The base was like a haunted house. There was an air of strangeness here that he had never known before, not even when he had first come. Not Commander Link Day of UN Flight Four, but Mike Capoferri, was the first man to set foot in this place.
       Could he get back? Sure he could. The other button must be the one that controlled flight into the future.
       But even then, he might not be able to tell anyone.
       _Hey,_ he told himself proudly, _I'm a time traveler! Wait here,_ he answered solicitously, _I'll call the medic. No,_ he protested, _I can prove it. Get in the ship and I'll show you._ But that could go wrong in a dozen ways. He'd want to know more about what he was doing before he tried this again.
       _Kilroy was here_, he thought.
       If he left marks of his visit, they would still be here when he returned to his own time. He could scratch his initials -- hmm.
       He turned right. When he reached the rec room he went to one of the sculpting machines and began to take it apart.
       The tool itself looked like a big, fat mechanical pencil. It was set in a brace which could be moved to any part of the work. The brace allowed the tool to move freely under pressure and held it steady otherwise. The pointed business end of the tool generated a sphere of emptiness into which matter vanished without trace.
       Removing the tool from the brace was almost easy. Turning it on took just under an hour. Once Mike almost gave up the idea, but he kept at it, for he had nothing else which would mar any of the indestructible materials used to build the base.
       He held the device like a pencil, but more carefully. His first thought had been to put a portrait of Commander Link Day on the statue of an alien female in the bunk room. He'd changed his mind. It would be dangerous and stupid to change his own past. He had to do something which would not be discovered before he arrived at the base, in 1985.
       The inner side of the outer door would be a good place to hide a carving, because nobody had ever seen it. It folded against the airlock wall when the lock was open.
       A wind blew toward his hand as he walked. There must be a way to shut air out of the disintegrator, but he hadn't the time to find it. He couldn't remember whether the team had found air in the base. If they had, he was changing the past right now.
       What should he write? "The world is my ash tray," he decided, and slammed his toe into a ledge. He threw both hands out to break his fall, and changed his mind too late. Horrified, he watched the sculpting pencil vanish into the floor. It left a neat cylindrical hole.
       Well, Mike thought furiously, that takes care of that. I've made my mark.
         * * * *
He plugged the hole with cement from the meteor repair kit on his suit belt. There was now a machine missing from the base, one that had been there in his own time, but he couldn't do anything about that. He did manage to close the airlock doors as he left.
       The breathtaking beauty of the full Earth stopped him outside the ship. He gazed at the magnificent bluish-white disk, trying to decide what made it seem different. Was there more cloud area? Whatever the reason, the sight was more impressive than ever.
       Then it came to him. The Earth was bigger! It was probably twice as large as he had ever seen it. Of course, there was nothing nearby to compare it to, which was why he hadn't noticed before.
       Mike was smiling as he entered the lock. The Moon has been moving outward from the Earth since creation, picking up energy from the slowing of the Earth's rotation. He must be a long way into the past. About three billion years...
       He pushed through the inner door and stood a moment, looking down the three broken rows -- one along the floor of the ship, the others down the sides -- of amethyst portholes. It would have been nice to be able to see out, but the glassy material was transparent only to a wide range of ultraviolet light.
       He went through the motions at the control panel. Right pyramid knob in -- and it had better be the right move. Generator on. Glass block between the poles. Generator off.
       He floated.
       Suddenly, remembering the sight of the central pyramid "turning," Mike was glad that he could not see the ship traveling through time. Obviously the aliens could stand the sight -- but they could also look at the central pyramid, for they had done nothing to protect themselves from it.
       A green line crept across the board, covering and wiping out the faint purple line. Mike let it grow until the purple line was gone, then slipped on his generator.
       Wrong, Wrong! He was still in free fall!
         * * * *
In hideous indecision he watched the board, waiting for it to tell him -- it didn't matter what, for the board was quiet and dark. In the end he left the knob in and the generator on and pushed himself aft. He had to get a look outside.
       He braced himself in the airlock, suspiciously examining the brilliant sky for any sign that he was still traveling in time. There was nothing. Mike turned on his shoes and gingerly stepped out onto the hull. When he looked down, the Moon wasn't there.
       A misty white planet floated nearby. It was a heavy atmosphere type, as uniform and featureless as a piece of bedsheet. It was Venus, if he was still in the solar system. Otherwise -- heavy atmospheres are the norm in space.
       It seemed obvious now that he'd guessed wrong. The knob on the left must control time travel; the one on the right, space travel. It was a chance he'd had to take.
       Mike watched the white disk slowly setting toward one horizon of the ship. As the last thing he might see in life, it left a lot to be desired. Still, blank as it was, he could tell quite a bit about it. It couldn't be very large, for instance. If it were a giant, its atmosphere would be banded. It must be bigger than Mars to have an opaque atmosphere, but, unless an oversized moon had stripped away most of the air, it couldn't be much larger than Earth.
       When he saw its star he could try guessing its surface temperature.
       He sat down on the hull. There were two days' worth of oxygen in the ship, and little chance that it would get him home. He was lost in both space and time. He didn't know how to go into the future; if there was a way, he could expect to spend months looking for it. It was time to face death.
       Besides, he'd been running for hours, torn by conflicting emotions, through a world whose laws were more black magic than physics. It was high time for a coffee break.
       Mike licked dry lips. That last, lost cup of coffee would have tasted wonderful. A cigarette would have torn his throat out after four and a half years, but it would have felt natural and smelled good smoldering between his fingers.
       He'd left precious little legacy for the others at the base, a spare spacesuit that couldn't fit anyone else, three sets of lounging overalls, and a few interesting discoveries. He'd taken the spaceship; they'd cuss him out good for that...
       Or had he ever lived at all? He had died before he was born. Perhaps there would be no Mike Capoferri, ever.
       But UN Flight Four would find his anonymous traces when they opened the base. Footprints in moondust. A sculpting tool missing from the rec room. A hole in the floor; his cement was sure to disintegrate in three billion years. Would they ever guess how deep it was? The damn thing must have fallen all the way to the center of the Moon.
       Searing light stabbed his eyes. Mike groped blindly for his filter switch, and found it. The light became bearable.
         * * * *
A sun was rising over the hull. It looked very much like the Sun as seen from the Moon; but that only meant that it wasn't. Seen from a Venus orbit, the Sun would have been much larger. He was in another solar system.
       Could the ship have come home by itself? Was that the home world of the base race? No, of course not. The aliens had had a water metabolism, and there would be no water down there. That world, in an Earth-like orbit around a type G yellow dwarf, must have a surface temperature of around five hundred degrees Fahrenheit.
       Mr. Parkman in Physics 1B had told the class one day that "The Earth's atmosphere goes way past the Moon." He seemed surprised by their laughter. It was his highly successful way of holding their attention. "No, it's true. Of course, it gets pretty thin. The idea is that the Earth's atmosphere ends where its density drops to the density of surrounding space. In the same way the Sun's atmosphere goes out beyond Mars.
       "Well, the air is thin enough to behave like separate particles at that distance. So the Moon is constantly whipping through this cloud of gas molecules" -- he made frantic motions with his hands -- "and it pulls some of them up to escape velocity every time it goes by. Naturally they're never heard from again. The air keeps replacing itself, more or less, by volcanic action.
       "Now, most planets don't have giant moons, so they grow tremendous air envelopes. Like Venus. Here's where the greenhouse effect comes in..."
       Mike snapped back to the present because of something small and dark and spinning. With the light filter over his eyes he couldn't see more. He looked away. Something was worrying at the bottom of his mind.
       Again his mind's eye watched the sculpting tool fall into a tunnel of its own making. He saw it lying at the center of the Moon, perhaps carving out a little pit for itself...
       Wrong. There would be millions of tons of pressure to flatten any cavity into oblivion...
       Any cavity but one. Now the picture was right.
       The Sun had dropped below the hull, though part of the corona still showed. Mike raised his filter and searched for the spinning blob. He knew what it was, now.
       At first glance it looked like a walnut shell; but not quite, for the shape was wrong and the convolutions were too deep. What it really resembled was a deflated beach ball which somebody has crushed between his hands.
       The Moon had had a long time to push itself through a sphere an inch and a half in diameter. Probably it had not taken more than a few millennia. Afterward there had been nothing but this crumpled ball of waste, too light and rigid for gravity to compress it further. For three billion years the Earth had been moonless.
       "...Six to eight hundred degrees!" Mr. Parkman waited a moment while the scribblers caught up. "They knew about the greenhouse effect, but they hadn't dreamed that it would apply to little Venus. You could melt lead in such a greenhouse!
       "The point is, the astronomers were using Earth as a norm. It isn't. The Earth-Moon system is an astronomical freak. A normal planet in Earth's orbit would have an opaque, very thick atmosphere, so thick that wind and light and temperature changes would never reach the surface. An eternal searing black calm."
         * * * *
Mike turned and crawled into the airlock, moving as fast as he dared in free fall. He could have gone mad waiting for the inner door to open, but he didn't dare. The knowledge of certain death had been better than this aching sense of responsibility.
       The door opened and he jumped toward the control board. Already he was planning. He had to go back some time before his first arrival. Then -- remove the sculpting tools from the rec room, or somehow scramble the controls of the base airlock, or leave a message for "himself" on the outer door. Anything to restore the past.
       The glass block had not floated out of place. All he had to do was cut the magnetic field. He watched the purple line until he was sure that it was longer than it had been before. When he flipped the generator back on his feet thumped satisfyingly against the floor. Half the battle.
       Ghosts from his childhood whispered to him while he waited for the outer door to open. Parkman was there but Mike refused to listen to him. He remembered Tony; which was unfair, because he'd only robbed Tony of eight years.
       The door opened on the Moon. Mike bounded toward the base ... Or had he? He really should have known better than to loan Tony his Flexy. His Flexy, because Tony's had a broken wheel. Had he told Doctor Stuart that? No.
       "Time is a one-way street," said Doctor Stuart, sympathetically but firmly. He was wrong, dead wrong.
       Mike stood before the base airlock wriggling his fingers like a clarinet player. How far back had he come this time? He turned left to see the size of the Earth.
       It wasn't there.
       But it was always there! Bewildered, Mike peered around him. The Moon must still be rotating...
       To his right, the Earth was a vast, incredible crescent -- and the plain was full of ships. They were of many different sizes, but they all had the same lumpy cylindrical shape. Tiny figures moved among them.
       Stuart was right, he thought idiotically. You go the wrong way on a one-way street, you've got to have accidents. He turned and ran.
       Behind him the lock swung open. Two ten-foot tripeds turned to each other and gestured rapidly, like nests of striking snakes. Then one of them hopped after him and picked him up.

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