"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."

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."