Marek frowned. "More slits mean fewer bars? Why?"

"The usual explanation is what I've drawn  -  the light passing through the slits acts like two waves that overlap. In some places they add to each other, and in other places they cancel each other out. And that makes a pattern of alternating light and dark on the wall. We say the waves interfere with each other, and that this is an interference pattern."

Chris Hughes said, "So? What's wrong with all that?"

"What's wrong," Gordon said, "is that I just gave you a nineteenth-century explanation. It was perfectly acceptable when everybody believed that light was a wave. But since Einstein, we know that light consists of particles called photons. How do you explain a bunch of photons making this pattern?"

There was silence. They were shaking their heads.

David Stern spoke for the first time. "Particles aren't as simple as the way you have described them. Particles have some wavelike properties, depending on the situation. Particles can interfere with one another. In this case, the photons in the beam of light are interfering with one another to produce the same pattern."

"That does seem logical," Gordon said. "After all, a beam of light is zillions and zillions of little photons. It's not hard to imagine that they would interact with one another in some fashion, and produce the interference pattern."

They were all nodding. Yes, not hard to imagine.

"But is it really true?" Gordon said. "Is that what's going on? One way to find out is to eliminate any interaction among the photons. Let's just deal with one photon at a time. This has been done experimentally. You make a beam of light so weak that only one photon comes out at a time. And you can put very sensitive detectors behind the slits  -  so sensitive, they can register a single photon hitting them. Okay?"

They nodded, more slowly this time.

"Now, there can't be any interference from other photons, because we are dealing with a single photon only. So: the photons come through, one at a time. The detectors record where the photons land. And after a few hours, we get a result, something like this."

"What we see," Gordon said, "is that the individual photons land only in certain places, and never others. They behave exactly the same as they do in a regular beam of light. But they are coming in one at a time. There are no other photons to interfere with them. Yet something is interfering with them, because they are making the usual interference pattern. So: What is interfering with a single photon?"

Silence.

"Mr. Stern?"

Stern shook his head. "If you calculate the probabilities - "

"Let's not escape into mathematics. Let's stay with reality. After all, this experiment has been performed  -  with real photons, striking real detectors. And something real interferes with them. The question is, What is it?"

"It has to be other photons," Stern said.

"Yes," Gordon said, "but where are they? We have detectors, and we don't detect any other photons. So where are the interfering photons?"

Stern sighed. "Okay," he said. He threw up his hands.

Chris said, "What do you mean, Okay? Okay what?"

Gordon nodded to Stern. "Tell them."

"What he is saying is that single-photon interference proves that reality is much greater than just what we see in our universe. The interference is happening, but we can't see any cause for it in our universe. Therefore, the interfering photons must be in other universes. And that proves that the other universes exist."

"Correct," Gordon said. "And they sometimes interact with our own universe."

"I'm sorry," Marek said. "Would you do that again? Why is some other universe interfering with our universe?"

"It's the nature of the multiverse," Gordon said. "Remember, within the multiverse, the universes are constantly splitting, which means that many other universes are very similar to ours. And it is the similar ones that interact. Each time we make a beam of light in our universe, beams of light are simultaneously made in many similar universes, and the photons from those other universes interfere with the photons in our universe and produce the pattern that we see."

"And you are telling us this is true?"

"Absolutely true. The experiment has been done many times."

Marek frowned. Kate stared at the table. Chris scratched his head.

Finally David Stern said, "Not all the universes are similar to ours?"

"No."

"Are they all simultaneous to ours?"

"Not all, no."

"Therefore some universes exist at an earlier time?"

"Yes. Actually, since they are infinite in number, the universes exist at all earlier times."

Stern thought for a moment. "And you are telling us that ITC has the technology to travel to these other universes."

"Yes," Gordon said. "That's what I'm telling you."

"How?"

"We make wormhole connections in quantum foam."

"You mean Wheeler foam? Subatomic fluctuations of space-time?"

"Yes."

"But that's impossible."

Gordon smiled. "You'll see for yourself, soon enough."

"We will? What do you mean?" Marek said.

"I thought you understood," Gordon said. "Professor Johnston is in the fourteenth century. We want you to go back there, to get him out."

No one spoke. The flight attendant pushed a button and all the windows in the cabin slid closed at the same time, blocking out the sunshine. She went around the cabin, putting sheets and blankets on the couches, making them up as beds. Beside each she placed large padded headphones.

"We're going back?" Chris Hughes said. "How?"

"It will be easier just to show you," Gordon said. He handed them each a small cellophane packet of pills. "Right now, I want you to take these."

"What are they?" Chris said.

"Three kinds of sedative," he said. "Then I want you all to lie down and listen on the headphones. Sleep if you like. The flight's only ten hours, so you won't absorb very much, anyway. But at least you'll get used to the language and pronunciation."

"What language?" Chris said, taking his pills.

"Old English, and Middle French."

Marek said, "I already know those languages."

"I doubt you know correct pronunciation. Wear the headphones."

"But nobody knows the correct pronunciation," Marek said. As soon as he said it, he caught himself.

"I think you will find," Gordon said, "that we know."

Chris lay down on one bed. He pulled up the blanket and slipped the headphones over his ears. At least they blotted out the sound of the jet.

These pills must be strong, he thought, because he suddenly felt very relaxed. He couldn't keep his eyes open. He listened as a tape began to play. A voice said, "Take a deep breath. Imagine you are in a beautiful warm garden. Everything is familiar and comforting to you. Directly ahead, you see a door going down to the basement. You open the door. You know the basement well, because it is your basement. You begin to walk down the stone steps, into the warm and comforting basement. With each step, you hear voices. You find them pleasant to listen to, easy to listen to."

Then male and female voices began to alternate.

"Give my hat. Yiff may mean haht."

"Here is your hat. Hair baye thynhatt."

"Thank you. Grah mersy."

"You are welcome. Ayepray thee."

The sentences became longer. Soon Chris found it difficult to follow them.

"I am cold. I would rather have a coat. Ayeam chillingcold, ee wolld leifer half a coot."

Chris was drifting gently, imperceptibly, to sleep, with the sensation that he was still walking down a flight of stairs, deeper and deeper into a cavernous, echoing, comforting place. He was peaceful, though the last two sentences he remembered gave a tinge of concern:

"Prepare to fight. Dicht theeselv to ficht."

"Where is my sword? Whar beest mee swearde?"

But then he exhaled, and slept.

BLACK ROCK

"Risk everything, or gain

nothing."

GEOFFREY DE CHARNY, 1358

The night was cold and the sky filled with stars as they stepped off the airplane onto the wet runway. To the east, Marek saw the dark outlines of mesas beneath low-hanging clouds. A Land Cruiser was waiting off to one side.

Soon they were driving down a highway, dense forest on both sides of the road. "Where exactly are we?" Marek said.

"About an hour north of Albuquerque," Gordon said. "The nearest town is Black Rock. That's where our research facility is."

"Looks like the middle of nowhere," Marek said.

"Only at night. Actually, there are fifteen high-tech research companies in Black Rock. And of course, Sandia is just down the road. Los Alamos is about an hour away. Farther away, White Sands, all that."

They continued down the road for several more miles. They came to a prominent green-and-white highway sign that read ITC BLACK ROCK LABORATORY. The Land Cruiser turned right, heading up a twisting road into the forested hills.

From the back seat, Stern said, "You told us before that you can connect to other universes."

"Yes."

"Through quantum foam."

"That's right."

"But that doesn't make any sense," Stern said.

"Why? What is quantum foam?" Kate said, stifling a yawn.

"It's a remnant of the birth of the universe," Stern said. He explained that the universe had begun as a single, very dense pinpoint of matter. Then, eighteen billion years ago, it exploded outward from that pinpoint  -  in what was known as the big bang.

"After the explosion, the universe expanded as a sphere. Except it wasn't an absolutely perfect sphere. Inside the sphere, the universe wasn't absolutely homogeneous  -  which is why we now have galaxies clumped and clustered irregularly in the universe, instead of being uniformly distributed. Anyway, the point is, the expanding sphere had tiny, tiny imperfections in it. And the imperfections never got ironed out. They're still a part of the universe."

"They are? Where?"

"At subatomic dimensions. Quantum foam is just a way of saying that at very small dimensions, space-time has ripples and bubbles. But the foam is smaller than an individual atomic particle. There may or may not be wormholes in that foam."

"There are," Gordon said.

"But how could you use them for travel? You can't put a person through a hole that small. You can't put anything through it."

"Correct," Gordon said. "You also can't put a piece of paper through a telephone line. But you can send a fax."

Stern frowned. "That's entirely different."

"Why?" Gordon said. "You can transmit anything, as long as you have a way to compress and encode it. Isn't that so?"

"In theory, yes," Stern said. "But you're talking about compressing and encoding the information for an entire human being."

"That's right."

"That can't be done."

Gordon was smiling, amused now. "Why not?"

"Because the complete description of a human being  -  all the billions of cells, how they are interconnected, all the chemicals and molecules they contain, their biochemical state  -  consists of far too much information for any computer to handle."

"It's just information," Gordon said, shrugging.

"Yes. Too much information."

"We compress it by using a lossless fractal algorithm."

"Even so, it's still an enormous - "

"Excuse me," Chris said. "Are you saying you compress a person?"

"No. We compress the information equivalent of a person."

"And how is that done?" Chris said.

"With compression algorithms  -  methods to pack data on a computer, so they take up less space. Like JPEG and MPEG for visual material. Are you familiar with those?"

"I've got software that uses it, but that's it."

"Okay," Gordon said. "All compression programs work the same way. They look for similarities in data. Suppose you have a picture of a rose, made up of a million pixels. Each pixel has a location and a color. That's three million pieces of information  -  a lot of data. But most of those pixels are going to be red, surrounded by other red pixels. So the program scans the picture line by line, and sees whether adjacent pixels are the same color. If they are, it writes an instruction to the computer that says make this pixel red, and also the next fifty pixels in the line. Then switch to gray, and make the next ten pixels gray. And so on. It doesn't store information for each individual point. It stores instructions for how to re-create the picture. And the data is cut to a tenth of what it was."

"Even so," Stern said, "you're not talking about a two-dimensional picture, you're talking about a three-dimensional living object, and its description requires so much data - "

"That you'd need massive parallel processing," Gordon said, nodding. "That's true."

Chris frowned. "Parallel processing is what?"

"You hook several computers together and divide the job up among them, so it gets done faster. A big parallel-processing computer would have sixteen thousand processors hooked together. For a really big one, thirty-two thousand processors. We have thirty-two billion processors hooked together."

"Billion?" Chris said.

Stern leaned forward. "That's impossible. Even if you tried to make one . . ." He stared at the roof of the car, calculating. "Say, allow one inch between motherboards . . . that makes a stack . . . uh . . . two thousand six hundred . . . that makes a stack half a mile high. Even reconfigured into a cube, it'd be a huge building. You'd never build it. You'd never cool it. And it'd never work anyway, because the processors would end up too far apart."

Gordon sat and smiled. He was looking at Stern, waiting.

"The only possible way to do that much processing," Stern said, "would be to use the quantum characteristics of individual electrons. But then you'd be talking about a quantum computer. And no one's ever made one."

Gordon just smiled.

"Have they?" Stern said.

"Let me explain what David is talking about," Gordon said to the others. "Ordinary computers make calculations using two electron states, which are designated one and zero. That's how all computers work, by pushing around ones and zeros. But twenty years ago, Richard Feynman suggested it might be possible to make an extremely powerful computer using all thirty-two quantum states of an electron. Many laboratories are now trying to build these quantum computers. Their advantage is unimaginably great power  -  so great that you can indeed describe and compress a three-dimensional living object into an electron stream. Exactly like a fax. You can then transmit the electron stream through a quantum foam wormhole and reconstruct it in another universe. And that's what we do. It's not quantum teleportation. It's not particle entanglement. It's direct transmission to another universe."

The group was silent, staring at him. The Land Cruiser came into a clearing. They saw a number of two-story buildings, brick and glass. They looked surprisingly ordinary. This could be any one of those small industrial parks found on the outskirts of many American cities. Marek said, "This is ITC?"

"We like to keep a low profile," Gordon said. "Actually, we chose this spot because there is an old mine here. Good mines are getting hard to find now. So many physics projects require them."

Off to one side, working in the glare of floodlights, several men were getting ready to launch a weather balloon. The balloon was six feet in diameter, pale white. As they watched, it moved swiftly up into the sky, a small instrument bundle hanging beneath. Marek said, "What's that about?"

"We monitor the cloud cover every hour, especially when it's stormy. It's an ongoing research project, to see if the weather is the cause of any interference."

"Interference with what?" Marek asked.

The car pulled up in front of the largest building. A security guard opened the door. "Welcome to ITC," he said with a big smile. "Mr. Doniger is waiting for you."

Doniger walked quickly down the hallway with Gordon. Kramer followed behind. As he walked, Doniger scanned a sheet of paper that listed everybody's names and backgrounds. "How do they look, John?"

"Better than I expected. They're in good physical shape. They know the area. They know the time period."

"And how much persuading will they need?"

"I think they're ready. You just have to be careful talking about the risks."

"Are you suggesting I should be less than entirely honest?" Doniger said.

"Just be careful how you put it," Gordon said. "They're very bright."

"Are they? Well, let's have a look."

And he threw the door open.

Kate and the others had been left alone in a plain, bare conference room  -  scratched Formica table, folding chairs all around. On one side was a large markerboard with formulas scrawled on it. The formulas were so long that they ran the entire width of the board. It was completely mysterious to her. She was about to ask Stern what the formulas were for, when Robert Doniger swept into the room.

Kate was surprised by how young he was. He didn't look much older than they were, especially dressed in sneakers, jeans and a Quicksilver T-shirt. Even late at night, he seemed full of energy, going around the table quickly, shaking hands with each of them, addressing them by name. "Kate," he said, smiling at her. "Good to meet you. I've read your preliminary study on the chapel. It's very impressive."

Surprised, she managed to say, "Thank you," but Doniger had already moved on.

"And Chris. It's nice to see you again. I like the computer-simulation approach to that mill bridge; I think it will pay off."

Chris had time only to nod before Doniger was saying, "And David Stern. We haven't met. But I gather you're also a physicist, as I am."

"That's right. . . ."

"Welcome aboard. And Andre. Not getting any shorter! Your paper on the tournaments of Edward I certainly set Monsieur Contamine straight. Good work. So: please, all of you, please sit down."

They sat, and Doniger moved to the head of the table.

"I will get right to the point," Doniger said. "I need your help. And I will tell you why. For the last ten years, my company has been developing a revolutionary new technology. It is not a technology of war. Nor is it a commercial technology, to be sold for profit. On the contrary, it is an entirely benign and peaceful technology that will provide a great benefit to mankind. A great benefit. But I need your help."

"Consider for a moment," Doniger continued, "how unevenly technology has impacted the various fields of knowledge in the twentieth century. Physics employs the most advanced technology  -  including accelerator rings many miles in diameter. The same with chemistry and biology. A hundred years ago, Faraday and Maxwell had tiny private labs. Darwin worked with a notebook and a microscope. But today, no important scientific discovery could be made with such simple tools. The sciences are utterly dependent on advanced technology. But what about the humanities? During this same time, what has happened to them?"

Doniger paused, rhetorically. "The answer is, nothing. There has been no significant technology. The scholar of literature or history works exactly as his predecessors did a hundred years before. Oh, there have been some minor changes in authentication of documents, and the use of CD-ROMs, and so forth. But the basic, day-to-day work of the scholar is exactly the same."

He looked at each of them in turn. "So we have an inequity. The fields of human knowledge are unbalanced. Medieval scholars are proud that in the twentieth century their views have undergone a revolution. But physics has undergone three revolutions in the same century. A hundred years ago, physicists argued about the age of the universe and the source of the sun's energy. No one on earth knew the answers. Today, every schoolchild knows. Today, we have seen the length and breadth of the universe, we understand it from the level of galaxies to the level of subatomic particles. We have learned so much that we can speak in detail about what happened during the first few minutes of the birth of the exploding universe. Can medieval scholars match this advance within their own field? In a word, no. Why not? Because no new technology assists them. No one has ever developed a new technology for the benefit of historians  -  until now."

A masterful performance, Gordon thought. One of Doniger's best  -  charming, energetic, even excessive at moments. Yet the fact was, Doniger had just given them an exciting explanation for the project  -  without ever revealing its true purpose. Without ever telling them what was really going on.

"But I told you I needed your help. And I do."

Doniger's mood changed. He spoke slowly now, somber, concerned. "You know that Professor Johnston came here to see us because he thought we were withholding information. And in a way, we were. We did have certain information that we hadn't shared, because we couldn't explain how we got it."

And, Gordon thought, because Kramer screwed up.

"Professor Johnston pushed us," Doniger was saying. "I'm sure you know his way. He even threatened to go to the press. Finally we showed him the technology we are about to show you. And he was excited  -  just as you will be. But he insisted on going back, to see for himself."

Doniger paused. "We didn't want him to go. Again, he threatened. In the end, we had no choice but to let him go. That was three days ago. He is still back there. He asked you for help, in a message he knew you would find. You know that site and time better than anyone else in the world. You have to go back and get him. You are his only chance."

"What exactly happened to him after he went back?" Marek said.

"We don't know," Doniger said. "But he broke the rules."

"Rules?"

"You have to understand that this technology is still very new. We've been cautious about how we use it. We have been sending observers back for about two years now  -  using ex-marines, trained military people. But of course they are not historians, and we have kept them on a tight leash."

"Meaning what?"

"We haven't ever let our observers enter the world back there. We haven't allowed anyone to stay longer than an hour. And we haven't allowed anyone to go more than fifty yards from the machine. Nobody has ever just left the machine behind and gone off into the world."

"But the Professor did?" Marek said.

"He must have, yes."

"And we'll have to, too, if we're going to find him. We'll have to enter the world."

"Yes," Doniger said.

"And you're saying we're the first people ever to do this? The first people ever to step into the world?"

"Yes. You, and the Professor before you."

Silence.

Suddenly, Marek broke into a broad grin. "Terrific," he said. "I can't wait!"

But the others said nothing. They looked uneasy, edgy.

Stern said, "About this guy they found in the desert. . . ."

"Joe Traub," Doniger said. "He was one of our best scientists."

"What was he doing in the desert?"

"Apparently, he drove there. They found his car. But we don't know why he went."

Stern said, "Supposedly, he was all messed up, there was something about his fingers. . . ."

"That wasn't in the autopsy report," Doniger said. "He died of a heart attack."

"Then his death had nothing to do with your technology?"

"Nothing at all," Doniger said.

There was another silence. Chris shifted in his chair. "In layman's terms  -  how safe is this technology?"

"Safer than driving your car," Doniger said without hesitation. "You will be thoroughly briefed, and we'll send you back with our experienced observers. The trip will last a maximum of two hours. You'll just go back and get him."

Chris Hughes drummed his fingers on the table. Kate bit her lip. Nobody spoke.

"Look, this is all voluntary," Doniger said. "It's entirely up to you whether you go or not. But the Professor has asked for your help. And I don't think you would let him down."

"Why don't you just send the observers?" Stern said.

"Because they don't know enough, David. As you're aware, it's an entirely different world back there. You have the advantage of your knowledge. You know the site, and you know the time, in detail. You know languages and customs."

"But our knowledge is academic," Chris said.

"Not anymore," Doniger said.

The group filed out of the room, heading off with Gordon to see the machines. Doniger watched them go, then turned as Kramer entered the room. She had been watching everything on the closed-circuit television.

"What do you think, Diane?" Doniger said. "Will they go?"

"Yeah. They'll go."

"Can they pull it off?"

Kramer paused. "I'd say it's fifty-fifty."

They walked down a broad concrete ramp, large enough for a truck to drive down. At the bottom was a pair of heavy steel doors. Marek saw a half-dozen security cameras mounted in different locations around the ramp. The cameras turned, following them as they walked down to the doors. At the bottom of the ramp, Gordon looked up at the security cameras, and waited.

The doors opened.

Gordon led them through into a small room beyond. The steel doors clanged shut behind them. Gordon went forward to an inner set of doors, again waited.

Marek said, "You can't open them yourself?"

"No."

"Why? They don't trust you?"

"They don't trust anybody," Gordon said. "Believe me, nobody gets in here unless we intend for them to get in."

The doors opened.

They walked into an industrial-looking metal cage. The air was cold, faintly musty. The doors closed behind them. With a whir, the cage began to descend.

Marek saw that they were standing in an elevator.

"We're going down a thousand feet," Gordon said. "Be patient."

The elevator stopped and the doors opened. They walked down a long concrete tunnel, their footsteps echoing. Gordon said, "This is the control and maintenance level. The actual machines are another five hundred feet below us."

They came to a pair of heavy doors that were dark blue and transparent. At first, Marek thought they were made of extremely thick glass. But as the doors slid open on a motorized track, he saw slight movement beneath the surface. "Water," Gordon said. "We use a lot of water shielding here. Quantum technology is very sensitive to random outside influences  -  cosmic rays, spurious electronic fields, all of that. That's why we're down here in the first place."

Up ahead, they saw what appeared to be the doors to an ordinary laboratory hallway. Passing through another set of glass doors, they entered a hallway painted antiseptic white, with doors opening off on either side. The first door on the left said PREPACK. The second, FIELDPREP. And further down the hallway, they saw a sign marked simply TRANSIT.

Gordon rubbed his hands together. He said, "Let's get right to the packing."

The room was small and reminded Marek of a hospital laboratory; it made him uneasy. In the center of the room stood a vertical tube, about seven feet high and five feet in diameter. It was hinged open. Inside were dull strips. Marek said, "A suntanning machine?"

"Actually, it's an advanced resonance imager. Basically it's a high-powered MRI. But you'll find it's good practice for the machine itself. Perhaps you should go first, Dr. Marek."

"Go in there?" Marek pointed to the tube. Seen up close, it looked more like a white coffin.

"Just remove your clothes and step inside. It's exactly like an MRI  -  you won't feel anything at all. The entire process takes about a minute. We'll be next door."

They went through a side door with a small window, into another room. Marek couldn't see what was in there. The door clanged shut.

He saw a chair in the corner. He went over and took his clothes off, then walked into the scanner. There was the click of an intercom and he heard Gordon say, "Dr. Marek, if you will look at your feet."

Marek looked down at his feet.

"You see the circle on the floor? Please make sure your feet are entirely within that circle." Marek shifted his position. "Thank you, that's fine. The door will close now."

With a mechanical hum, the hinged door swung shut. Marek heard a hiss as it sealed. He said, "Airtight?"

"Yes, it has to be. You may feel some cold air coming in now. We'll give you added oxygen while we calibrate. You're not claustrophobic, are you?"

"I wasn't, until now." Marek was looking around at the interior. The dull strips, he now saw, were plastic-covered openings. Behind the plastic he saw lights, small whirring machines. The air became noticeably cooler.

"We're calibrating now," Gordon said. "Try not to move."

Suddenly, the individual strips around him began to rotate, the machines clicking. The strips spun faster and faster, then suddenly jerked to a stop.

"That's good. Feel all right?"

"It's like being inside a pepper mill," Marek said.

Gordon laughed. "Calibration is completed. The rest is dependent on exact timing, so the sequence is automatic. Just follow the instructions as you hear them. Okay?"

"Okay."

A click. Marek was alone.

A recorded voice said, "The scan sequence has begun. We are turning on lasers. Look straight ahead. And do not look up."

Instantly, the interior of the tube was a bright, glowing blue. The air itself seemed to be glowing.

"Lasers are polarizing the xenon gas, which is now being pumped into the compartment. Five seconds."

Marek thought, Xenon gas?

The bright blue color all around him increased in intensity. He looked down at his hand and could hardly see it for the shimmering air.

"We have reached xenon concentration. Now we will ask you to take a deep breath."

Marek thought, Take a deep breath? Of xenon?

"Hold your position without moving for thirty seconds. Ready? Stand still  -  eyes open  -  deep breath  -  hold it. . . . Now!"

The strips suddenly began to spin wildly, then one by one, each strip started to jerk back and forth, almost as if it were looking, and sometimes had to go back for a second look. Each strip seemed to be moving individually. Marek had the uncanny sense of being examined by hundreds of eyes.

The recorded voice said, "Very still, please. Twenty seconds remaining."

All around him, the strips hummed and whirred. And then suddenly, they all stopped. Several seconds of silence. The machinery clicked. Now the strips began to move forward and back, as well as laterally.

"Very still, please. Ten seconds."

The strips began to spin in circles now, slowly synchronizing, until finally they were all rotating together as a unit. Then they stopped.

"The scan is completed. Thank you for your cooperation."

The blue light clicked off, and the hinged door hissed open. Marek stepped out.

In the adjacent room, Gordon sat in front of a computer console. The others had pulled up chairs around him.

"Most people," Gordon said, "don't realize that the ordinary hospital MRI works by changing the quantum state of atoms in your body  -  generally, the angular momentum of nuclear particles. Experience with MRIs tells us that changing your quantum state has no ill effect. In fact, you don't even notice it happening.

"But the ordinary MRI does this with a very powerful magnetic field  -  say, 1.5 tesla, about twenty-five thousand times as strong as the earth's magnetic field. We don't need that. We use superconducting quantum interference devices, or SQUIDs, that are so sensitive they can measure resonance just from the earth's magnetic field. We don't have any magnets in there."

Marek came into the room. "How do I look?" he said.

The image on the screen showed a translucent picture of Marek's limbs, in speckled red. "You're looking at the marrow, inside the long bones, the spine, and the skull," Gordon said. "Now it builds outward, by organ systems. Here's the bones"  -  they saw a complete skeleton - " and now we're adding muscles. . . ."

Watching the organ systems appear, Stern said, "Your computer's incredibly fast."

"Oh, we've slowed this way down," Gordon said. "Otherwise you wouldn't be able to see it happening. The actual processing time is essentially zero."

Stern stared. "Zero?"

"Different world," Gordon said, nodding. "Old assumptions don't apply." He turned to the others. "Who's next?"

They walked down to the end of the corridor, to the room marked TRANSIT. Kate said, "Why did we just do all that?"

"We call it prepacking," Gordon said. "It enables us to transmit faster, because most of the information about you is already loaded into the machine. We just do a final scan for differences, and then we transmit."

They entered another elevator, and passed through another set of water-filled doors. "Okay," Gordon said. "Here we are."

They came out into an enormous, brightly lit, cavernous space. Sounds echoed. The air was cold. They were walking on a metal passageway, suspended a hundred feet above the floor. Looking down, Chris saw three semicircular water-filled walls, arranged to form a circle, with gaps between large enough for a person to walk through. Inside this outer wall were three smaller semicircles, forming a second wall. And inside the second wall was a third. Each successive semicircle was rotated so that the gaps never lined up, giving the whole thing a mazelike appearance.

In the center of the concentric circles was a space about twenty feet across. Here, half a dozen cagelike devices stood, each about the size of a phone booth. They were arranged in no particular pattern. They had dull-colored metal tops. White mist drifted across the enclosure. Tanks lay on the floor, and heavy black power cables snaked everywhere. It looked like a workroom. And in fact, some men were working on one of the cages.

"This is our transmission area," Gordon said. "Heavily shielded, as you can see. We're building a second area over there but it won't be ready for several months." He pointed across the cavernous space, where a second series of concentric walls were going up. These walls were clear; they hadn't been filled with water yet.

From the gangway, a cable elevator went down to the space in the center of the glass walls.

Marek said, "Can we go down there?"

"Not yet, no."

A technician looked up and waved. Gordon said, "How long until the burn check, Norm?"

"Couple of minutes. Gomez is on her way now."

"Okay." Gordon turned to the others. "Let's go up to the control booth to watch."

Bathed in deep blue light, the machines stood on a raised platform. They were dull gray in color and hummed softly. White vapor seeped along the floor, obscuring their bases. Two workmen in blue parkas were down on their hands and knees, working inside the opened base of one of them.

The machines were essentially open cylinders, with metal at the top and bottom. Each machine stood on a thick metal base. Three rods around the perimeter supported the metal roof.

Technicians were dragging a tangle of black cables down from an overhead grid and then attaching the cables to the roof of one machine, like gas station attendants filling a car.

The space between the base and the roof was completely empty. In fact, the whole machine seemed disappointingly plain. The rods were odd, triangular-shaped, and studded along their length. Pale blue smoke seemed to be coming from under the roof of the machine.

The machines didn't look like anything Kate had ever seen. She stared at the huge screens inside the narrow control room. Behind her, two technicians in shirtsleeves sat at two console desks. The screens in front of her gave the impression you were looking out a window, though in fact the control room was windowless.

"You are looking at the latest version of our CTC technology," Gordon said. "That stands for Closed Timelike Curve  -  the topology of space-time that we employ to go back. We've had to develop entirely new technologies to build these machines. What you see here is actually the sixth version, since the first working prototype was built three years ago."

Chris stared at the machines and said nothing. Kate Erickson was looking around the control room. Stern was anxious, rubbing his upper lip. Marek kept his eye on Stern.

"All the significant technology," Gordon continued, "is located in the base, including the indium-gallium-arsenide quantum memory, the computer lasers and the battery cells. The vaporizing lasers, of course, are in the metal strips. The dull-colored metal is niobium; pressure tanks are aluminum; storage elements are polymer."

A young woman with short dark red hair and a tough manner walked into the room. She wore a khaki shirt, shorts and boots; she looked as if she were dressed for a safari. "Gomez will be one of your aides when you go take your trip. She's going back right now to do what we call a 'burn check.' She's already burned her navigation marker, fixing the target date, and now she's going to make sure it's accurate." He pushed the intercom. "Sue? Show us your nav marker, would you?"

The woman held up a white rectangular wafer, hardly larger than a postage stamp. She cupped it easily in her palm.

"She'll use that to go back. And to call the machine for the return  -  show us the button, would you, Sue?"

"It's a little hard to see," she said, turning the wafer on edge. "There's a tiny button here, you push it with your thumbnail. That calls the machine when you're ready to return."

"Thank you, Sue."

One of the technicians said, "Field buck."

They turned and looked. On his console, one screen showed an undulating three-dimensional surface with a jagged upswinging in the middle, like a mountain peak. "Nice one," Gordon said. "Classic." He explained to the others. "Because our field-sensing equipment is SQUID-based, we're able to detect extremely subtle discontinuities in the local magnetic field  -  we call them 'field bucks.' We'll register them starting as early as two hours before an event. And in fact, these started about two hours ago. It means a machine is returning here."

"What machine?" Kate said.

"Sue's machine."

"But she hasn't left yet."

"I know," he said. "It doesn't seem to make sense. Quantum events are all counterintuitive."

"You're saying you get an indicator that she is returning before she has left?"

"Yes."

"Why?" Kate said.

Gordon sighed. "It's complicated. Actually, what we are seeing in the field is a probability function  -  the likelihood that a machine is going to return. We don't usually think about it that way. We just say it's coming back. But to be accurate, a field buck is really telling us that it is highly probable a machine is coming back."

Kate was shaking her head. "I don't get it."

Gordon said, "Let's just say that in the ordinary world, we have beliefs about cause and effect. Causes occur first, effects second. But that order of events does not always occur in the quantum world. Effects can be simultaneous with causes, and effects can precede causes. This is one minor example of that."

The woman, Gomez, stepped into one of the machines. She pushed the white wafer into a slot in the base in front of her. "She's just installed her nav marker, which guides the machine out and back."

"And how do you know you'll get back?" Stern said.

"A multiverse transfer," Gordon said, "creates a sort of potential energy, like a stretched spring that wants to snap back. So the machines can come home relatively easily. Outbound is the tricky part. That's what's encoded in the ceramic."

He leaned forward to press an intercom button. "Sue? How long are you gone?"

"I'll be a minute, maybe two."

"Okay. Synch elapsed."

Now the technicians began to talk, flipping switches at a console, looking at video readouts in front of them.

"Helium check."

"Read as full," a technician said, looking at her console.

"EMR check."

"Check."

"Stand by for laser alignment."

One of the technicians flipped a switch, and from the metal strips, a dense array of green lasers fired into the center of the machine, putting dozens of green spots on Gomez's face and body as she stood still, her eyes closed.

The bars began to revolve slowly. The woman in the center remained still. The lasers made green horizontal streaks over her body. Then the bars stopped.

"Lasers aligned."

Gordon said, "See you in a minute, Sue." He turned to the others. "Okay. Here we go."

The curved water shields around the cage began to glow a faint blue. Once again the machine began to rotate slowly. The woman in the center stood motionless; the machine moved around her.

The humming grew louder. The rotation increased in speed. The woman stood, calm and relaxed.

"For this trip," Gordon said, "she'll use up only a minute or two. But she actually has thirty-seven hours in her battery cells. That's the limit these machines can remain in a location without returning."

The bars were spinning swiftly. They now heard a rapid chattering sound, like a machine gun.

"That's the clearance check: infrared sensors verify the space around the machine. They won't proceed without two meters on all sides. They check both ways. It's a safety measure. We wouldn't want the machine emerging in the middle of a stone wall. All right. They're releasing xenon. Here she goes."

The humming was now very loud. The enclosure spun so rapidly, the metal strips were blurred. They could see the woman inside quite clearly.

They heard a recorded voice say, "Stand still  -  eyes open  -  deep breath  -  hold it. . . . Now!"

From the top of the machine, a single ring descended, scanning quickly to her feet.

"Now watch closely. It's fast," Gordon said.

Kate saw deep violet lasers fire inward from all the bars toward the center. The woman inside seemed to glow white-hot for an instant, and then a burst of blinding white light flashed inside the machine. Kate closed her eyes, turned away. When she looked back again, there were spots in front of her eyes, and for a moment she couldn't see what had happened. Then she realized that the machine was smaller. It had pulled away from the cables at the top, which now dangled free.

Another laser flash.

The machine was smaller. The woman inside was smaller. She was now only about three feet high, and shrinking before their eyes in a series of bright laser flashes.

"Jesus," Stern said, watching. "What does that feel like?"

"Nothing," Gordon said. "You don't feel a thing. Nerve conduction time from skin to brain is on the order of a hundred milliseconds. Laser vaporization is five nanoseconds. You're long gone."

"But she's still there."

"No, she's not. She was gone in the first laser burst. The computer's just processing the data now. What you see is an artifact of compression stepping. The compression's about three to the minus two. . . ."

There was another bright flash. The cage now shrank rapidly. It was three feet high, then two. Now it was close to the floor  -  less than a foot tall. The woman inside looked like a little doll in khakis.

"Minus four," Gordon said. There was another bright burst, near the floor. Now Kate couldn't see the cage at all.

"What happened to it?"

"It's there. Barely."

Another burst, this time just a pinpoint flash on the floor.

"Minus five."

The flashes came more quickly now, winking like a firefly, diminishing in strength. Gordon counted them out.

"And minus fourteen. . . . Gone."

There were no more flashes.

Nothing.

The cage had vanished. The floor was dark rubber, empty.

Kate said, "We're supposed to do that?"

"It's not an unpleasant experience," Gordon said. "You're entirely conscious all the way down, which is something we can't explain. By the final data compressions, you are in very small domains  -  subatomic regions  -  and consciousness should not be possible. Yet it occurs. We think it may be an artifact, a hallucination that bridges the transition. If so, it's analogous to the phantom limb that amputees feel, even though the limb isn't there. This may be a kind of phantom brain. Of course, we are talking about very brief time periods, nanoseconds. But nobody understands consciousness anyway."

Kate was frowning. For some time now, she had been looking at what she saw as architecture, a kind of "form follows function" approach: wasn't it remarkable how these huge underground structures had concentric symmetry  -  slightly reminiscent of medieval castles  -  even though these modern structures had been built without any aesthetic plan at all. They had simply been built to solve a scientific problem. She found the resulting appearance fascinating.

But now that she was confronted by what these machines were actually used for, she struggled to make sense of what her eyes had just seen. And her architectural training was absolutely no help to her. "But this, uh, method of shrinking a person, it requires you to break her down - "

"No. We destroy her," Gordon said bluntly. "You have to destroy the original, so that it can be reconstructed at the other end. You can't have one without the other."

"So she actually died?"

"I wouldn't say that, no. You see - "

"But if you destroy the person at one end," Kate said, "don't they die?"

Gordon sighed. "It's difficult to think of this in traditional terms," he said. "Since you're instantaneously reconstructed at the very moment you are destroyed, how can you be said to have died? You haven't died. You've just moved somewhere else."

Stern felt certain  -  it was a visceral sense  -  that Gordon wasn't being entirely honest about this technology. Just looking at the curved water shields, at all the different machines standing on the floor, gave him the sense that there was quite a bit more that was being left unexplained. He tried to find it.

"So she is in the other universe now?" he asked.

"That's right."

"You transmitted her, and she arrived in the other universe? Just like a fax?"

"Exactly."

"But to rebuild her, you need a fax machine at the other end."

Gordon shook his head. "No, you don't," he said.

"Why not?"

"Because she's already there."

Stern frowned. "She's already there? How could that be?"

"At the moment of transmission, the person is already in the other universe. And therefore the person doesn't need to be rebuilt by us."

"Why?" Stern said.

"For now, just call it a characteristic of the multiverse. We can discuss it later if you like. I'm not sure everybody needs to be bothered with these details," he said, nodding to the others.

Stern thought, There is something more. Something he doesn't want to say to us. Stern looked back at the transmission area. Trying to find the odd detail, the thing that was out of place. Because he was sure that something here was out of place.

"Didn't you tell us that you've only sent a few people back?"

"That's right, yes."

"More than one at a time?"

"Almost never. Very rarely two."

"Then why do you have so many machines?" Stern said. "I count eight in there. Wouldn't two be enough?"

"You're just seeing the results of our research program," Gordon said. "We are constantly working to refine our design."

Gordon had answered smoothly enough, but Stern was certain he had seen something  -  some buried glint of uneasiness  -  in Gordon's eyes.

There is definitely something more.

"I would have thought," Stern said, "that you'd make refinements to the same machines."

Gordon shrugged again, but did not answer.

Definitely.

"What are those repairmen doing in there?" Stern said, still probing. He pointed to the men on their hands and knees, working on the base of one machine. "I mean by the machine in the corner. What exactly are they repairing?"

"David," Gordon began. "I really think - "

"Is this technology really safe?" Stern said.

Gordon sighed. "See for yourself."

On the big screen, a sequence of rapid flashes appeared on the floor of the transit room.

"Here she comes," Gordon said.

The flashes grew brighter. They heard the chattering sound again, first faintly, then louder. And then the cage was full-size; the humming died away; the ground mist swirled, and the woman climbed out, waving to the spectators.

Stern squinted at her. She appeared absolutely fine. Her appearance was identical to what it had been before.

Gordon looked at him. "Believe me," he said. "It's perfectly safe." He turned to the screen. "How'd it look back there, Sue?"

"Excellent," she said. "Transit site is on the north side of the river. Secluded spot, in the woods. And the weather's pretty good, for April." She glanced at her watch. "Get your team together, Dr. Gordon. I'm going to go burn the spare nav marker. Then let's go back there and pull that old guy out before somebody hurts him."

"Lie on your left side, please." Kate rolled over on the table and watched uneasily as an elderly man in a white lab coat raised what looked like a glue gun and placed it over her ear. "This will feel warm."

Warm? She felt a burning rush of heat in her ear. "What is that?"

"It's an organic polymer," the man said. "Nontoxic and nonallergenic. Give it eight seconds. All right, now please make chewing motions. We want a looser fit. Very good, keep chewing."

She heard him going down the line. Chris was on the table behind her, then Stern, then Marek. She heard the old man say, "Lie on your left side, please. This will feel warm. . . ."

Not long after, he was back. He had her turn over, and injected the hot polymer into her other ear. Gordon was watching from the corner of the room. He said, "This is still a bit experimental but so far it works quite well. It's made of a polymer that begins to biodegrade after a week."

Later, the man had them stand up. He expertly popped the plastic implants out of their ears, moving down the line.

Kate said to Gordon, "My hearing is fine, I don't need a hearing aid."

"It's not a hearing aid," Gordon said.

Across the room, the man was drilling out the center of the plastic earpieces and inserting electronics. He worked surprisingly quickly. When the electronics were in place, he capped the hole with more plastic.

"It's a machine language translator and a radio mike. In case you need to understand what people are saying to you."

"But even if I understand what they're saying," she said, "how can I answer back?"

Marek nudged her. "Don't worry. I speak Occitan. And Middle French."

"Oh, that's good," she said sarcastically. "You going to teach it to me in the next fifteen minutes?" She was tense, she was about to be destroyed or vaporized or whatever the hell they did in that machine, and the words just popped out of her mouth.

Marek looked surprised. "No," he said seriously. "But if you stay with me, I will take care of you."

Something about his earnestness reassured her. He was such a straight arrow. She thought, He probably will take care of me. She felt herself relaxing.

Soon after, they were all fitted with flesh-colored plastic earpieces. "They're turned off now," Gordon said. "To turn them on, just tap your ear with your finger. Now, if you'll come over here . . ."

Gordon handed them each a small leather pouch. "We've been working on a first-aid kit; these are the prototypes. You're the first to enter the world, so you may have a use for them. You can keep them out of sight, under your clothing."

He opened one pouch and brought out a small aluminum canister about four inches high and an inch in diameter. It looked like a little shaving cream can. "This is the only defense we can provide you. It contains twelve doses of ethylene dihydride with a protein substrate. We can demonstrate for you with the cat, H.G. Where are you, H.G?"

A black cat jumped onto the table. Gordon stroked it, and then shot a burst of gas at its nose. The cat blinked, made a snuffling sound, and fell over on its side.

"Unconsciousness within six seconds," Gordon said, "and it leaves a retroactive amnesia. But bear in mind that it's short acting. And you must fire right in the person's face to ensure any effect."

The cat was already starting to twitch and revive as Gordon turned back to the pouch and held up three red paper cubes, roughly the size of sugar cubes, each covered in a layer of pale wax. They looked like fireworks.

"If you need to start a fire," he said, "these will do it. Pull the little string, and they catch fire. They're marked fifteen, thirty, sixty  -  the number of seconds before the fire starts. Wax, so they're waterproof. A word of warning: sometimes they don't work."