The Folding Problem

The crane would not fold right.

Soren had the instructions flat on the kitchen table, and a square of paper that had already been creased and uncreased so many times it had gone soft, like cloth. Every time he followed the steps, he got a different bird. Same paper. Same folds. Different bird.

"That's not possible," Maya said. She was lying upside down on the couch, feet up the wall, because Soren's grandmother had said she could sit however she wanted as long as she didn't knock the plant over.

"It is possible. Look." Soren held up two cranes. One had a fat body. One had a long neck and no body to speak of. "Same instructions."

"Then you're doing the folds in a different order."

"I'm not."

Maya swung down off the couch. She picked up the soft square and started folding, slowly, watching the paper instead of the page. Halfway through she stopped.

"There," she said. "That fold. You can do it left first or right first. The instructions don't say. So you pick. And once you pick, everything after it is different."

Soren took it back and tried it her way. The neck came out where the neck was supposed to be.

"One sheet," he said slowly. "One set of folds. But the order you fold them changes the whole shape."

"Now imagine," said Maya, who had wandered to the laptop his grandmother lent them for the afternoon, "a sheet that folds itself."

"Paper doesn't fold itself."

"Proteins do." She said it like she was reading it off the inside of her head. "My mom's a biochemist, remember. A protein is just a long chain. A string of beads. And the string folds up into a shape all by itself, in like a millionth of a second, and the shape is the whole point. The shape is what it does."

Soren put the crane down. "What do you mean the shape is what it does."

"A protein shaped like a little claw grabs things. A protein shaped like a tube lets things through. If you know the string of beads, you still don't know the shape. That's the problem. That was the problem."

"Was?"

Maya was already typing. "My mom said scientists spent fifty years trying to figure out the shape from the string. Fifty years. People did whole careers on one protein. They'd freeze it and shoot it with X-rays just to see one shape."

Soren came over. On the screen was a ribbon, twisted and curled, glowing in soft blues and reds, turning slowly in black space.

"That's one protein?"

"That's one protein. And here's the thing." Maya turned the laptop toward him. "A computer predicted that shape. Just from the string of beads. It got it right. And then it didn't do one. It did almost all of them. Nearly every protein anybody had ever found. Two hundred million."

Soren did the math without meaning to. "That's not fifty years of work."

"No."

"That's more than fifty years of work. That's more shapes than every scientist who ever lived could have done by hand."

"In about a year," Maya said.

They were both quiet. Rain ran down the window in long bent lines.

Soren picked up his soft paper square again and turned it over.

"But how does the computer know," he said. "How does it know which fold comes first. We didn't know, with the crane. We had to try it."

Maya stopped. That was a real question, and she didn't have the answer in her head.

"It learned from all the shapes people already figured out," she said. "The hard ones. The fifty years of frozen X-ray ones. It looked at all of them and figured out the rules nobody could write down."

"So it learned the order." Soren turned the crane over in his hands. "The order we couldn't see."

"Yeah."

He looked at the ribbon on the screen, turning. Then he looked at the page of crane instructions, with its little numbered diagrams, somebody's careful drawings.

"Somebody had to fold the first ones," he said. "By hand. So the computer could learn from them."

"Lots of somebodies. For fifty years."

"They didn't know they were teaching it."

Maya looked at him. "No. They thought they were just doing one protein. Their one protein. The thing nobody else cared about."

Soren thought about a person in a cold room, year after year, working out the shape of one twisted string while everyone told them it was too small a thing to spend a life on.

"And it turned out," he said, "every single one of those mattered. Because the computer couldn't have learned without all of them."

"Every weird little one," Maya said. "The ones nobody thought were important."

She reached over and spun the protein on the screen with two fingers, the way you'd spin a globe.

"There are more," she said.

"More what?"

"Proteins it hasn't seen. New ones. Ones in animals we haven't found yet, in the bottom of the ocean, in dirt. Every time someone finds a new living thing, there's a whole new set of strings nobody's folded." She was talking faster now. "And shapes you could design on purpose. A protein shaped to grab a virus. A protein shaped to eat plastic. You could draw the shape you want and work backward to the string."

"People are doing that?"

"People are starting to do that."

Soren looked at his two wrong cranes, the fat one and the long-necked one, sitting side by side on the table. Two shapes from one string. He had thought he was failing. He had just been finding the other folds.

He set a fresh square of paper down and smoothed it flat with his palm.

"Make the left fold first," Maya said, leaning in beside him.

He made the left fold first. Then the right. The paper turned in his hands, and the neck rose up off the table on its own, exactly where it was supposed to be.