But, of course, we hadn't yet replied at all; another popular interpretation was that the Rosetta stone was being withheld until humanity did did reply. reply.

There's an old concept in SETI that said that signals would likely be sent at a group of frequencies called "the water hole"-between the emission frequency for hydrogen, at 1420 megahertz, and for hydroxyl, at 1667 megahertz. Hydrogen (H) and hydroxyl (OH) are the components of water (H20), and Earth's atmosphere is most transparent to radio waves at that range of frequencies, while interstellar s.p.a.ce is largely free of interference there. Since all life as we know it began in water, this area of the spectrum seemed a natural gathering place for those species looking to undertake interstellar communications.

But the Centauri signals weren't anywhere near the water hole-another example of what we expected to be a shared view of reality not turning out to be shared at all.

Could there, Heather wondered, be other water holes-other common grounds that would have to be shared by any being that existed in the same universe we did, regardless of its biology or the nature of its planet?

She was supposed to meet her friend Judy for lunch at the Faculty Club at 12:15. She'd stick around until today's message began to arrive, then head off.

Still ten minutes to go. Heather wasn't one to waste time. She had the latest issue of The Journal of Jungian Studies The Journal of Jungian Studies on her datapad; she started working her way through it. on her datapad; she started working her way through it.

After a while, the phone rang. Heather finished the paragraph in front of her, then absently reached for the handset. "h.e.l.lo?"

"Heather? Did you forget?"

Heather glanced at her watch. "Oh, G.o.d! Sorry, Judy!" She looked over at her computer. "I was waiting for today's message-I was going to leave as soon as the incoming-message signal sounded." She moved over to her computer and told it to go directly to the Alien Signal Center homepage. Nothing.

"Judy, I can't make it. The alien message is late today."

"Are you sure you've got the right time?"

"Positive. Look, I've got to go. Maybe lunch tomorrow?"

"Sure, I'll call you."

"Thanks." Heather replaced the handset. As soon as she did, the phone rang again. She picked it up. "h.e.l.lo?"

"Heather," said a different female voice, "it's Salme van Horne."

"Salme! Where are you? Here in Canada?"

"No, I'm still in Helsinki. Have you tried to download today's message?"

"Yes. There doesn't seem to be one coming through."

"This has never happened before, has it? The Centaurs have never missed a day, have they?"

"Never. They've never even been late."

"Do you suppose the problem is at our end?" asked Salme. "Whose turn is it to receive the message?"

"Arecibo is designated prime, isn't it? But there are backups, and-oh, wait. Something's going up on the Web page."

"I see it, too."

"d.a.m.n holograms-ah, here it is: 'No technical malfunction at receiving end. Apparently no message was sent.' "

"That can't be the end of the transmissions," said Salme. "There has to be a key."

"Maybe they got tired of waiting for us to reply," said Heather. "Maybe they won't send again until we do do reply." reply."

"Or maybe-"

"What?" asked Heather.

"Drake equation, final term."

Heather was quiet for a moment. "Oh," she said softly.

The Drake equation estimated the number of radio-broadcasting civilizations in the galaxy. It had seven terms: R* f fp n ne f fl f fi f fc L L The rate of star formation, times the fraction of stars with planets, times the number of those planets that are suitable for life, times the fraction of such planets on which life actually appears, times the fraction of life forms that are intelligent, times the fraction of such life forms that actually develop radio, times . . .

Times big L: the lifetime of such a civilization.

A civilization that had radio probably also had nuclear weapons, or other equally dangerous things.

Civilizations could be wiped out in a matter of moments-certainly in less than a single thirty-one-hour day.

"They can't be dead," said Salme.

"They're either dead, or they voluntarily stopped, or the message is complete."

There was a knock at the door. Heather covered the mouthpiece. "Come in!"

The departmental a.s.sistant stuck his head in. "Sorry to bother you, Professor Davis, but the CBC is on the phone. They want to talk to you about what happened to the aliens."

10.

Kyle's lab was crowded. The dean leaned against one wall, the department chair had his b.u.t.t perched on the shelf jutting out of the bottom of Cheetah's console, a lawyer from the university's patent unit sat in Kyle's usual chair, and the five grad students who worked on Kyle's quantum-computing team were milling around as well.

"Okay," said Kyle to the group. "As you know, there's been a technique available since nineteen ninety-six for producing simple quantum-logic gates; that technique was based on using nuclear magnetic resonance to measure atomic spins. But it was hampered by the fact that as you added bits, the output signal got exponentially weaker: a thirty-bit quantum computer based on that principle produces output only one-billionth as strong as that from a one-bit computer based on the same technique.

"Well, the method we're going to demonstrate today is, we believe, the long-sought-after breakthrough: a quantum computer that, in theory, can employ an unlimited number of bits with no reduction in output quality. For our demo today, we're going to try to factor a randomly generated three-hundred-digit number. To do that on the department's ECB-5000 would take approximately one hundred years of constant calculation. If we're right-if this works-we'll have an answer about thirty seconds after I commence the experiment."

He moved across the room.

"Our prototype quantum computer, which we call Democritus, has not just thirty registers, but one thousand, one thousand, each of which consists of a single atom. The results will be a series of interference patterns, which another computer-that one over there-will a.n.a.lyze and reduce to a numeric readout." He looked from face to face. "All set? Let's go." each of which consists of a single atom. The results will be a series of interference patterns, which another computer-that one over there-will a.n.a.lyze and reduce to a numeric readout." He looked from face to face. "All set? Let's go."

Kyle walked over to the simple black console containing the Democritus computer. For the sake of drama, they'd built a large knife switch, worthy of Frankenstein's lab, into the side of the cabinet. Kyle pulled it down, its blade touching the metal contacts. A bright red LED came on and- -and everyone held their breath. Kyle kept watching Democritus, which, of course, was operating absolutely silently. Part of him missed the old days of clicking relays. Others were watching the digital clock mounted next to the red EXIT sign on the curving wall.

Ten seconds went by.

Then ten more.

Then a final ten.

And then the LED went dark.

Kyle let out his breath.

"Done," he said, heart pounding.

He gestured for everyone to follow him across the room. There, another computer was a.n.a.lyzing the output from Democritus.

"It'll take about five minutes to decode the interference pattern," said Kyle. He allowed himself a smile. "If you're thinking that that's a lot longer than it took to produce the pattern, you're right-but we're now dealing with a conventional computer."

"How many computations would it take to factor a number that big?" asked the dean, her voice clearly intrigued.

"Approximately ten to the five hundredth," said Kyle.

"And there's no way to do it in fewer steps?" she asked. "This isn't a case of Democritus taking a shortcut?"

Kyle shook his head. "No, it really does take ten to the five hundredth steps to factor a number that big."

"But Democritus didn't do that many steps."

"This Democritus didn't-in fact, it performed only one calculation, using a thousand atoms as the stones in its abacus, so to speak, to do so. But if all went well, 10 Democritus didn't-in fact, it performed only one calculation, using a thousand atoms as the stones in its abacus, so to speak, to do so. But if all went well, 10500 other Democrituses in other universes will also each have done one calculation-involving, of course, a total of a thousand times 10 other Democrituses in other universes will also each have done one calculation-involving, of course, a total of a thousand times 10500 atoms, which is 10 atoms, which is 10503 atoms. And that, my friends, is a very significant number." atoms. And that, my friends, is a very significant number."

"How so?" asked the department chair.

"Well, the precise value isn't important. What is is important is how it relates to the number of atoms in our entire universe." Kyle smiled, waiting for the inevitable question. important is how it relates to the number of atoms in our entire universe." Kyle smiled, waiting for the inevitable question.

"And how many atoms are there in our universe?" asked the dean.

"I called up Holtz over in the McLennan Physical Labs and asked her," said Kyle. "The answer, plus or minus a couple of orders of magnitude, is that there are ten to the eightieth atoms in the universe."

A few jaws dropped.

"Do you see?" said Kyle. "In that thirty-second period, to factor our test number, Democritus must have accessed many trillions of times more atoms than there are in our entire universe. Other, earlier quantum-computing demonstrations have never involved enough bits to actually exceed the quant.i.ty of atoms available to them in our universe, leaving open some doubt as to whether they'd actually accessed parallel worlds, but if this experiment works, the only only answer will be that our Democritus worked in tandem with computers in other universes." answer will be that our Democritus worked in tandem with computers in other universes."

The conventional computer they were standing in front of beeped and one of its monitors came to life. Precisely two strings of numbers appeared on the screen, each dozens of digits long.

"Are those the first two factors?" asked the lawyer, clearly anxious to start notarizing things.

Kyle felt his heart sink. "Ah, no. No." He swallowed; his stomach was roiling. "I mean, yes, certainly, they are doubtless factors of our source number, but-but. . ."

One of Kyle's grad students looked at him and then said the words that, at that moment, Kyle himself couldn't get out. "The display shouldn't have appeared until all the factors are ready. Unless by some miracle, the source number has only two factors, then the experiment didn't work."

The department head loomed in at the screen and placed his index finger on the last digit of the second number; it was a four. "That's an even number, so there have got to be smaller factors that aren't displayed." He straightened up. "What went wrong?"

Kyle was shaking his head. "It worked-sort of. Our Democritus did do only one calculation. The other number must must have come from a parallel universe." have come from a parallel universe."

"You can't prove that," said the dean. "Only two calculations means that only two thousand atoms were involved."

"I know," said Kyle. He breathed out. "Sorry, everyone. We'll keep working on it."

The dean frowned, presumably thinking of all the money that had already been spent. She left the room. The department head laid a hand briefly on Kyle's slumped shoulder before he, too, left, followed by the lawyer.

Kyle looked at his grad students and shrugged. Nothing was going his way these days . . .

After the students went home, Kyle sat down in his chair in front of Cheetah's console.

"I'm sorry," said Cheetah.

"Yeah," said Kyle. He shook his head. "It should have worked."

"I'm confident you'll figure out what went wrong."

"I suppose." He looked up at the print of "Christus Hypercubus." "But maybe it'll never work; researchers have been trying to accomplish this for over twenty years without success. He dropped his gaze to the floor. "I just keep wasting my time on projects that never bear fruit."

"Like me," said Cheetah, without rancor.

Kyle said nothing.

"I have faith in you," said Cheetah.

Kyle made a sound in his throat, a laugh aborted.

"What?"

"I dunno. Maybe that's the whole problem. Maybe it's my lack of faith."

"You mean G.o.d is punishing you for being an atheist?"

Kyle did laugh, but it was humorless. "Not that kind of faith. I mean my faith in quantum physics." He paused. "When I was a grad student, nothing excited me like quantum mechanics-it was mind-expanding, endlessly fascinating. But I felt sure that someday it would all click, click, you know-all make sense. Someday I'd really see. But I never have. Oh, I understand the equations in an abstract way, but I don't get it, you know? Maybe I don't even really believe it." you know-all make sense. Someday I'd really see. But I never have. Oh, I understand the equations in an abstract way, but I don't get it, you know? Maybe I don't even really believe it."

"You've lost me," said Cheetah.

Kyle spread his arms, trying to find a way to explain it. "I was at a party once, and this fat guy comes in, and he's got a slice through a geode held to his forehead by a headband. I never asked about it-guy comes in with something like that, you don't ask. But his companion, a scrawny woman, must have noticed me looking at the geode, so she comes over and says, 'That's Cory-he's gifted with the third eye.' And I'm thinking, Good Christ, let me out of here. Later, Cory comes up to me and says, 'Hey, man, what time is it?' And I'm thinking what good is the third eye if you don't even know what f.u.c.king time time it is?" it is?"

Cheetah was quiet for a while. "And your point would be . . . ?"

"My point is that maybe you do do need some special insight to understand-really, deeply understand-quantum mechanics. Einstein never did, you know; he was never comfortable with it, calling it 'spooky action at a distance.' But some of these guys in quantum mechanics, they need some special insight to understand-really, deeply understand-quantum mechanics. Einstein never did, you know; he was never comfortable with it, calling it 'spooky action at a distance.' But some of these guys in quantum mechanics, they do do get it-either that or they fake it really well. Me, I always thought I'd be one of those who'd get it, too-that it get it-either that or they fake it really well. Me, I always thought I'd be one of those who'd get it, too-that it would would click at some point. But it hasn't. I never developed the third eye." click at some point. But it hasn't. I never developed the third eye."

"Maybe you should get a geode slice from the Earth Sciences Centre."

Kyle grunted. "Maybe. I guess down deep, at some basic level, I just don't buy quantum mechanics. I feel like a bit of a charlatan."

"Democritus did indeed communicate with at least one other alternative reality. That seems to confirm the many-worlds interpretation."

Kyle looked at Cheetah's lenses. "That's it," he said simply. "That's the problem. This type of quantum computing hinges on the many-worlds interpretation, but, come on, really, how plausible is that? Surely not every conceivable universe exists, but rather only the ones that have at least some likelihood of having occurred."

"For instance?" asked Cheetah.