“The universe?” asked Laz.

  “Sounds mystical,” said Ivy.

  “No it doesn’t,” said the Prof. “It’s scientific.”

  “Why? How?” asked Laz.

  “It’s scientific because I said it.”

  The two of them looked at the Prof with skeptical surprise.

  “Do you think I wouldn’t have tried to figure out the science? I’m just saying that whatever the consequence of my meeting myself would be, the universe refuses to let that happen. It’s like two south poles of the same magnetic strength, the way they push each other away, and the stronger the magnets, the wider the distance they force each other to stay apart.”

  “And that’s science,” said Laz.

  “It’s analogy,” said Ivy.

  “My explanation to two kids who haven’t even been to college was an analogy. Did you understand it?”

  “Yes,” said Laz.

  “Do you understand the science?” asked Ivy. “Or do you know it only as an analogy yourself?”

  “Nobody understands the science of anything,” said the Prof. “The cause of anything. All they understand is the best guess that hasn’t been contradicted by reality yet.”

  “And your story hasn’t been contradicted,” said Ivy.

  “So far,” said the Prof. “I trust it now—I don’t even have to think about avoiding myself in the timestreams, because I know it isn’t going to happen.”

  “But you don’t know it,” said Ivy.

  “I might only have another minute here, and you’re going to waste it arguing about epistemology?”

  Laz grinned at Ivy.

  “If you didn’t leave us those messages,” Ivy asked the Prof, “then who did? As a time traveler, is that something you could have done?”

  “Sure,” said the Prof. “If I scratched out those messages fast enough, sure.”

  “Silently,” said Ivy. “Invisibly.”

  “In a time I don’t belong in,” said the Prof, “I’m sliding. I can slow myself down long enough to pick up a stick and write a few letters, but I won’t actually be in any instant long enough for photons to bounce off me and hit your retinas. Or for sound waves to propagate enough to be audible to you. So yes, I not only could be silent and invisible, I wouldn’t have any choice.”

  “So that’s why you wouldn’t talk to us,” said Ivy.

  “Ivy,” said Laz, “it wasn’t him.”

  “Laz, why do you believe him?”

  “Because I wouldn’t lie about something like that,” said Laz.

  “I’m not talking about you, I’m talking about—” She fell silent as if somebody had covered her mouth.

  “I am talking about me,” said Laz. “And I’m talking about him. I’ve told plenty of lies in my life, but I’ve never withheld the truth just to torment somebody, and that’s what he’d be doing, telling us he didn’t leave those messages if he really did.”

  “Astonishing level of self-knowledge,” said the Prof.

  “Well, I’ve experienced a lot of things you never did,” said Laz.

  “Including lusting after my young assistant,” said the Prof.

  Ivy leaned close and rested her head on Laz’s shoulder. “We’re engaged,” said Ivy.

  “If you didn’t leave the messages,” said Laz, “how did you know where to find us? And why now instead of ten years ago?”

  “I felt the reverberation of your movement in time,” said the Prof. “I knew I wasn’t alone, sliding around in time. So I came to find you and warn you to stop.”

  “You didn’t know it was us?”

  “I’m used to feeling the thud every time you side step,” said the Prof. “I always know what timestream you’re in, because crossing that barrier makes a thunderclap in time-space.”

  “Didn’t know we were being so noisy,” said Ivy.

  “You especially,” said the Prof. “I never knew Ivy could learn how to side step.”

  “It still takes me way more effort than it takes Laz. You.”

  “I’m not surprised. The surprise is that you can do it at all,” said the Prof.

  “So I’m going to point out the elephant in the room,” said Laz.

  “We’re all pretty skinny,” said Ivy, “so what do you mean?”

  “We know of exactly three people in the universe who can time travel,” said Laz. “If the Prof here didn’t leave those messages, then they must have, or at least might have, been made by the other two.”

  That sat in the air unanswered.

  Then the Professor began to laugh. “You’ve been worried about messages you sent yourselves?”

  “You might have been thoughtful enough,” Ivy said to Laz, “to sign them.”

  “Me?” asked Laz. “I couldn’t have written those messages I saw.”

  “Why not?” asked Ivy.

  The Prof shook his head. “Weren’t you listening? The universe wouldn’t let Laz materialize that close to himself. Polar repulsion.”

  “It had to be you, Ivy,” said Laz. “You’re the only one who could have done it, if the Prof here didn’t.”

  “But why would I do it?” asked Ivy. “Did I detect some danger to you?”

  The Prof chuckled again. “There might be a dozen reasons or more why you’d want to warn him not to make that Portal,” he said. “But I know one really big reason.”

  Ivy and Laz waited expectantly, but the Prof didn’t speak.

  Then Laz realized why. “Because we know that one of us left that message.”

  Ivy scoffed. “That’s completely circular. We leave the messages now because we know that we already did, but our reason for leaving them then was because we knew that we’d leave them now.”

  “The universe is strange,” said the Prof. “You can’t get close to yourself because two copies of the molecules can’t co-exist, but there’s no problem with causality getting itself in a knot. You might not have been preventing anything dire from happening, except that by leaving those messages, you started yourselves—well, and the early Ivy and Laz—on the path that led to now.”

  Laz thought that through. “Without that earliest message, Ivy-Z and Z-Laz would have had no reason to stop working with Ron. Only because they wouldn’t make or maintain Portals anymore did Ron need to requisition two copies. Us.”

  “We exist because of that first message,” said Ivy.

  “Without it, you and I would not have been needed. We would not have been grown and decanted and put to work.”

  “So we caused ourselves to exist,” said Ivy. “A couple of virgin births.”

  “Not really,” said Laz. “If we left those messages, we were screwing with our fellow clones.”

  “Cute,” said the Prof. “But here’s the problem. I don’t know how you can make journeys like that without getting yourselves into my condition, riding a Ferris wheel that never stops long enough to get off.”

  “And we don’t actually want that,” said Laz.

  “Nobody would be able to come to our wedding,” said Ivy. “Including us.”

  “Would we even be able to stay together?” Laz asked the Prof.

  “I don’t know. Until now, I’ve been the only time traveler.”

  “Maybe the two of us together would be able to control it,” said Ivy.

  “Maybe we’d get hopelessly lost and never see each other again,” said Laz. “The way the Prof lost Mother Ivy.”

  The Professor winced. “I wasn’t in love with her. I was two decades older, she was my assistant, it would have been… exploitative. And as far as I could tell, she didn’t like me at all.”

  “What we know,” said Ivy, “is that she was definitely in love with you.”

  “Such a missed opportunity,” said Laz.

  The Prof thought about that. “I really think I shouldn’t see her at all, even if I stay here long enough.”

  “Why not?” asked Ivy.

  “Because I never felt that way about her. I never guessed she felt that way because it never occurred to me that anything like that could happen.”

  “That’s why Ivy and I were wakened at about the same age, at about the same time,” said Laz. “Precisely so that we could fall in love.”

  “Which you dutifully did.”

  “We weren’t given that as an assignment,” said Laz.

  “We spent about a year in a town where we were the only living humans,” said Ivy.

  “And a pack of four dogs,” said Laz. “But we never felt romantic toward them.”

  “Such a missed opportunity,” said the Prof. He grinned. Ivy laughed aloud. Laz was not terribly amused but he had to admit it was funny. “Let me guess,” said the Prof. “You never felt that way about those dogs.”

  “Never crossed my mind,” said Laz, “because I’m a human-only kind of guy.”

  “Thanks,” said Ivy. “So glad I met your minimum expectations.”

  “Me too,” said Laz.

  “So… go back and leave your messages,” said the Prof.

  “But…” said Ivy.

  “We don’t know what we’re doing,” said Laz. “We need to learn more from you.”

  “No,” said the Prof. “You don’t. You already time traveled.”

  “But how do I control the distance?” asked Laz.

  “You don’t. There aren’t any mile markers in time. You just stop and see where you are, and then go again in whichever direction seems right.”

  “Another thing,” said Ivy. “Do we have to start out in the geographical location where we want to end up?”

  “No,” said the Prof.

  “Then how do we change our destination?”

  “I’ve never tried with two people. I just think about where on the globe I want to be, and then start moving toward it. While zipping along through time. It just happens.”

  “Because you think of the place on the globe,” said Ivy.

  “Try it,” said the Prof. “I can’t think why it wouldn’t work for you as well as it does for me.”

  “How do we stay together? Come and go together? Wind up together?”

  The Professor smiled and shrugged. “I wish you well,” he said. “I know you’ll figure it out.”

  “Or we’ll get stuck on an endless treadmill through time, like you,” said Ivy.

  “Not really a treadmill,” said the Professor. “You don’t get tired, you don’t work up a sweat, you don’t even have to exert an effort, except to keep yourself from going too far in either direction.”

  “There!” said Laz. “I knew we had important questions left. How far is too far, and why?”

  “Well, I could just say, ‘You’ll see,’ but why make you go to all that bother?”

  “Why indeed?” said Ivy.

  “If you go far enough back, you hit an Ice Age, and even passing through, it gets cold and dry. I didn’t have anybody I wanted to visit back then, so I came back to warmer times.”

  “And forward, into the future?” asked Laz.

  “That’s just a hard stop, period.”

  “Why? What happens?” asked Ivy.

  “It doesn’t look like the world had ended yet. I come to a stop and for all I know I could stay, living one second per second. Regular minimum speed. Get off the treadmill.”

  “So why didn’t you?” asked Ivy.

  “Several reasons,” said the Prof. “I didn’t know anybody there, for one thing.”

  “Right, people only go where they already know people,” said Ivy.

  “Not the main reason,” said the Prof.

  “You going to tell us what the main reason is?” asked Laz.

  The Professor looked at the blank television screen. He was obviously reluctant to say.

  “Please,” whispered Ivy.

  “I was afraid,” the Prof said.

  “Of what?” asked Laz.

  “Because I knew that Earth was going to plunge into the Sun. There was nobody visible. It was downtown Washington, DC, on Massachusetts Avenue near Union Station. Always cars moving, always pedestrians.”

  “And nobody,” said Laz.

  “Not a soul. Nothing moving except flags in the breeze. Nothing smelled funny, no smell of death or chemicals or anything, just… nothing moving but the wind.”

  Ivy began, sounding a tiny bit scornful. “In Greensboro, we lived in a town without—”

  “Ivy,” said Laz. “He’s not finished.”

  “It was just a feeling,” he said. “And now it gets a little metaphysical. I began to feel like somebody was watching me. Somebody was coming for me. And then I realized that for the first time in my life, I didn’t see any timestreams but the one I was in. I couldn’t side step at all. I mean, I was still capable of it, it’s just that there was nowhere to step to.”

  Laz shuddered. “I don’t even know what that would feel like.”

  “You don’t want to know,” said the Prof. “I’ve been back to that spot several times. It’s like how an acrophobe is fascinated with the thousand-meter drop, has to go right up to the edge and look over, even though he’s trembling, terrified. I sometimes go back to that place before the end of Old Earth and study it, try to figure out why I feel like I’m being watched. And then that feeling of being watched gets too intense and I get back on the treadmill into the past.”

  “What if somebody did not leave for the New Place?” said Ivy. “And they watched you because you were the only other person alive in that place?”

  “I don’t know,” said the Professor. “I don’t want to know. Right now I can talk about it, but at the time, I had, I always have, this uncontrollable fear, and it keeps getting worse and worse…”

  “So we’ll be careful not to go back to the Ice Age, or to the end of Old Earth,” said Laz.

  “And we’ll hold hands,” said Ivy.

  “And we’ll refuse to get stuck on that treadmill,” said Laz.

  “All we need to do, first,” said Ivy, “is go back and leave that message for Zero-Laz, beside the road.”

  “While he’s standing right there,” said Laz.

  “And being close to him won’t repulse you because you never had a single molecule in common,” said the Professor. “Um…”

  They looked at him, since it was obvious he was about to say something.

  Then he was gone.

  “Back on the treadmill,” said Ivy.

  “Maybe if we figure stuff out, we can help him get off.”

  “Maybe,” said Ivy, “we can work on making sure we don’t get stuck before we worry about unsticking him.”

  “When should we do it?” asked Laz.

  “I have this strange reluctance to do it right now,” said Ivy.

  “But now or later, we’ll still be traveling in time,” said Laz. “So now is as good a time as any.”

  “I’m scared,” said Ivy. “It’s going to be a long jump. And what if we can’t find the right place?”

  “We’ll find Zero-Laz and follow him, and when he stops to look off in the distance, we’ll pause there and scratch the message.”

  “Do you even remember what the message is?” asked Ivy.

  “Ivy—the message will be whatever I write.”

  “You don’t have to match it with what the message said when he found it?”

  “It will match,” said Laz.

  “How do you know?” said Ivy.

  “Because it did. Whatever it said was whatever I wrote. I don’t have to remember it, I just have to do it.”

  “ ‘Winter coats on sale at Neiman Marcus’?” asked Ivy. “ ‘Free oil change while you wait’?”

  “I’m not going to write anything silly,” said Laz.

  “Because you never do silly things, right, Dim?”

  “Because I didn’t write anything silly,” said Laz. “Let’s get on with it.”

  “No,” said Ivy. “You just go on ahead without me.”

  “Because you don’t want to risk getting on the treadmill of time,” said Laz.

  “Maybe. I don’t know.”

  “You’d rather I go alone than risk giving up Central Time and all its wonders,” said Laz.

  “Laz, marry me and get me pregnant before you go,” said Ivy.

  “Ivy, that’s a powerful idea, a very appealing idea. But I’m going to go leave a message for the Zees. Come with me or not, separate from me or not, I can’t control that. But I’m going. Now.”

  Why was he being so hard with her? Why couldn’t he wait? It was time travel. No matter when he left, he would arrive at the same time.

  But he had to go now. And she had to stay. Both feelings that had no logical basis.

  Except that their unconscious minds had processed their superior array of data and reached different conclusions. Don’t go, in Ivy’s case. Go now, in Laz’s.

  Laz pulled her up from the couch and held her and kissed her as if it were the last kiss he’d ever get in his life. Which was quite possible. Maybe even probable.

  “Have a happy life without me,” he murmured.

  “I’d rather have a happy one with you,” she said softly.

  “Me too,” said Laz.

  Then he stepped onto the treadmill and found himself skimming into the past. Not just a half hour, but days. He could see and feel the passage of days and nights, of warm and cold. And Ivy wasn’t there anymore.

  30

  TWO THINGS BECAME clear immediately. First, targeting a particular location on the globe and going there was far more complicated than the Prof had said it would be. For the first couple of days, Laz would go back a few months in time, aiming for some particular location, and he would end up nowhere near it. On the other side of the world, far north, far south. It was as if Time had no sense of geography.

  But time was just a measurement of duration, so it had no sense of anything. How was it supposed to read Laz’s mind and lead him to the right place? If Laz himself didn’t know the way, how could Time find it?

  No good to think of Time as the name of some intelligent entity.

  But how else was he supposed to think of it? He needed the cooperation of Time to travel in time. Somehow that was making sense to him.

  He decided that it wasn’t because Time didn’t know where he wanted to go, it’s that Laz himself didn’t know where he was going. He could think of the name of a place, even its general location on a globe, but…