The Water That Wasn't There

The trip was canceled because of the rain, so Maya and Soren ended up in the wrong room. It had a long glass tank in it, longer than a bathtub, water running through it fast, and a woman in rubber boots frowning at a laptop.

"You're not the plumbers," she said.

"No," said Soren.

"Good. The plumbers keep leaving." She waved a wrench at two folding chairs. "Sit if you want. Don't touch the tank. If you touch the tank I have to start over, and I have started over eleven times today."

Maya was already at the glass. The water rushed down a slope in the tank, faster and faster, until at one spot it went so fast that little ripples on the surface stopped moving forward.

"The ripples are stuck," she said.

"They're not stuck. They're trying," said the woman, whose name tag said DR. OKAFOR. "Watch upstream of the fast part. A ripple there can still swim back up. Watch downstream. It can't. The water's moving faster than the ripple can go."

Soren watched a ripple downstream. It leaned upstream like a person leaning into wind, and it lost. The fast water carried it away.

"So there's a line," he said. "Before the line, a ripple can escape. After the line, it can't. Ever."

"That line has a name," said Dr. Okafor.

Maya said it before Soren did, because the shape of it had already arrived in her head. "It's a horizon."

Dr. Okafor looked up from the laptop for the first time.

"A ripple that crosses it can never come back," Maya said. "Like light falling into a black hole."

"That is exactly why I built it," said Dr. Okafor. "I cannot fly to a black hole. I cannot make one. But sound in fast water behaves like light near a horizon. Same mathematics. So I study the black hole I can reach. This one. In a tank. With a plumbing problem."

Soren had his notebook out. He drew the slope, the fast part, the line. He drew a little arrow for a ripple that got away and a little arrow for one that didn't.

"But," he said, and stopped.

"But what," said Dr. Okafor, in the voice of someone who liked buts.

"If nothing comes back out," Soren said, "why are you excited? A place where things only fall in is just a drain."

Dr. Okafor smiled and turned the laptop toward them. On the screen was a graph, jittery, noisy, with a faint hump in it that she tapped with one finger.

"Because something does come out," she said. "Just barely. That is the whole reason I have started over eleven times. I am trying to hear it."

Maya leaned in. "Hear what?"

"Watch right at the line," said Dr. Okafor. "Not upstream. Not downstream. Right on it."

They watched. The water was never still. Even where there were no ripples, the surface trembled, tiny wrinkles appearing and vanishing, appearing and vanishing.

"The water's shaky," said Maya. "Even the empty part."

"The empty part is not empty," said Dr. Okafor. "It is never perfectly still. There is always the smallest jitter. Now. A wrinkle is born right on the line. Half of it lands on the fast side and gets swallowed. The other half is on the slow side."

Soren got there first this time. "The other half can escape."

"The other half escapes," said Dr. Okafor.

Maya "So the horizon leaks. Not stuff falling in. Stuff coming out. Made right at the edge. Made out of the shakiness."

"Say that again," said Soren, writing.

"A wrinkle gets born on the line," Maya said. "One half falls in forever. One half gets away. So from outside, it looks like the black hole is spitting."

Dr. Okafor sat down on an upturned bucket like her legs had decided for her.

"A man named Hawking worked that out about real black holes," she said. "Fifty years ago. He said the empty space right at a horizon can't be perfectly still, quantum things never are, and so a black hole must give off the faintest glow of particles. Which means a black hole is not a drain. It is slowly, slowly leaking itself away. Evaporating."

"The whole thing?" said Soren. "The entire black hole just... goes?"

"Over a length of time so long the number is silly," said Dr. Okafor. "Far longer than the universe has existed. But yes. Given enough time, the thing that swallows everything empties out."

Maya was staring at the tank like it had insulted her and she liked it.

"Nobody's ever seen it," she said. It was not a question. "On a real black hole. Have they."

"No," said Dr. Okafor. "It is too faint. Fainter than starlight from the far side of the sky. We are almost certain it is real, the mathematics is beautiful and it agrees with everything else, but no human eye or telescope has ever caught it happening out there." She tapped the noisy graph, the little hump. "So I make one I can reach. And I try to catch the leak in my tank. And the plumbers keep leaving."

Soren looked at the faint hump on the screen. "That bump. That's the leak."

"That might be the leak," said Dr. Okafor. "On a good day. Under the noise. I am not sure yet. That is the honest answer."

Maya pressed her nose almost to the glass, right at the line, where the wrinkles were being born and dying, born and dying, one half swimming free and one half swept away, over and over, faster than she could count.

"So somewhere," she said, "a real one. A giant one. Bigger than the sun. It's doing this. Right now. Just too quiet for anyone to hear."

"Right now," said Dr. Okafor. "Getting lighter by the tiniest crumb. All of them are. Every black hole in the sky, very slowly, coming undone."

The rain kept going against the high windows. Soren put his hand flat on the cool glass side of the tank, then remembered and pulled it back before she could yell.

Upstream, a single ripple caught the fast water, leaned, tried, and made it back.

Downstream, at the line, a new wrinkle rose out of the trembling water and split in two.