The Two Spinning Tops

The plan was simple. Drop a marble. Watch where it lands. Drop the same marble the same way and watch it land in the same place.

It would not.

"Again," said Maya.

Soren lifted the marble back to the top of the track. They had built the run out of foam tubing and three little wooden flippers that swung on nails, so a marble would hit a flipper, knock it sideways, roll down, hit the next one. He let go.

The marble rattled through, bounced off the second flipper, and dropped into the left cup.

"Left," said Soren.

"Last time it was right."

"Last time it was right twice and left once."

Maya crossed her arms. "We let it go from the same spot."

"We let it go from the same spot," Soren agreed.

They did it eleven more times. Soren wrote each landing in a column on the back of an envelope. Left, left, right, left, right, right, right, left. No pattern he could find, and Soren was good at finding patterns. That was the part that bothered him.

"It's the flippers," he said finally. "They're loose. They wobble."

"So tighten them."

So they tightened them. They glued the foam down so it could not shift. They marked the exact starting spot with a pencil line and rested the marble against a ruler so it left from the same place every single time, to the width of the pencil mark.

"Now," said Maya, "it has to do the same thing."

"It has to," said Soren. "There's nowhere left for the difference to come from."

They dropped it ten times.

Left, right, left, left, right, left, right, right, left, right.

Maya laughed, which she did sometimes when something refused to behave. "Okay. Okay. What's different each time?"

"Nothing I can measure."

"Then nothing is different."

"No," said Soren slowly. "Something is different. I just can't measure it."

Maya stopped laughing. "Say that again."

"The marble isn't exactly the same temperature. My hand shakes a tiny bit. There's dust. The pencil mark is wide. The differences are too small to see."

"Too small to see," Maya said, "but the marble can feel them."

They looked at the run.

"Try this," said Maya. She took the marble and held it a hair to the left of the pencil line. Not enough to call it a different spot. Enough that she knew. She let go. Right cup.

Then she held it a hair to the right. Let go. Left cup.

"A hair," she said. "A hair changes everything."

Soren picked up the second flipper and turned it in his fingers. "It's because of the flippers. Each one takes a tiny difference and makes it bigger. The marble comes in barely to the left, the flipper sends it noticeably to the left, then the next flipper sends it way to the left."

"It multiplies," said Maya.

"It multiplies. Every flipper. So by the bottom, a difference you couldn't even see at the top has grown into the whole answer."

Maya was quiet. Then she said, "How many flippers would we need so we could never, ever predict it. Even if we measured the start perfectly."

"You can't measure the start perfectly," said Soren. "There's always one more decimal place. The width of an atom. Smaller."

"So there's always a difference too small to catch."

"And enough flippers turn it into everything."

They sat with that on the cold garage floor.

"Wait," said Maya. "The weather is flippers."

Soren looked up.

"The air," she said, talking faster now. "It's all bumping into itself. Every bump is a flipper. A tiny warm spot here grows into a bigger thing, grows into a bigger thing, and you can't measure the warm spot small enough, ever, so nobody can know the weather way ahead. Not because they're bad at it. Because it's a marble run that never stops."

"That's why the forecast only goes about a week," Soren said, and he said it like something had clicked into place that he had wondered about for years. "It's not that they're lazy. It's that the difference too small to measure on Monday is the whole storm by Friday."

"A butterfly," said Maya.

"What?"

"A butterfly flaps its wings and the air bumps and it grows and grows. People say that like it's a poem. It's not a poem. It's our marble."

Soren reached for his notebook. His hand moved before he decided anything. He drew the run, then the three flippers, then beside them a row of more flippers trailing off the edge of the page where he ran out of room. At the bottom he wrote: too small to see is not the same as not there.

"Here's the part I can't hold," he said. "It's not random. The marble follows the rules every time. If you knew the start perfectly, you'd know the end. The rules are perfect."

"But you can never know the start perfectly."

"Never. So a thing can follow perfect rules and still be impossible to predict. Forever. Not because we're not smart enough yet. Because of what it is."

Maya pressed her palms against the sides of her head like she was keeping it from coming open. "So somewhere out there a system is running right now, following rules exactly, and nobody who ever lives will be able to say where it ends up."

"Lots of them," said Soren. "Most of them, probably."

Maya thought about the kids at school who said math was just answers in the back of the book. She thought about how this was math too, this exact thing, the most certain thing in the world wearing a coat that you could never see under.

"Do it again," she said softly.

Soren set the marble against the ruler, on the pencil line, the same as every time.

"You know it won't tell us anything," he said.

"I know. Do it anyway."

He let go.

The marble rattled down, kissed the first flipper, the second, the third, and the small differences nobody in the world could have measured grew and grew and grew along the foam, and the marble dropped, and the two of them leaned in over the cups to see which way an unmeasurable thing had decided to become the answer.