The Longest Broom in the World

The gym smelled like floor wax and old balloons. Everyone had gone home except Maya and Soren, who had volunteered to stack the poles because the robotics team had left forty of them lying across the free-throw line.

"These are the long ones," Maya said. She held one up. It was aluminum, thin, and nearly as tall as the basketball hoop. "Watch. If I push this end, the other end moves. Instantly."

"It doesn't," Soren said.

"It does. Look." She jabbed her end. The far end twitched.

"It looks instant. That's not the same as instant."

Maya lowered the pole and squinted at him. This was her favorite thing about Soren. He would argue with a pole.

"Okay," she said. "Prove it."

"I can't with one pole. It's too short. The delay's too small to see." He looked at the pile. "How long would the pole have to be?"

"To see the delay? I don't know. Long."

"Longer than long." Soren crouched by the poles. "Imagine one pole that stretched from here to the Moon. You push your end. When does my end move?"

"Right away."

"That's what everybody thinks. But nothing goes faster than light. Not even a push."

Maya stopped. She set her end of the pole against the floor and held very still.

"Say that again."

"A push is information. My end has to find out your end moved. And no information beats light. Light takes over a second to reach the Moon." He tapped the aluminum. "So if this went to the Moon and you shoved it, my end would sit there for a second doing nothing."

"That's stupid," Maya said, delighted. "It's a solid pole. It's one thing."

"It feels like one thing. It's actually a huge crowd of atoms holding hands."

Maya picked the pole back up. She looked down its length like she was aiming it. "So when I push, I don't push the whole pole."

"You push the first atom. It bumps the next one. That one bumps the next."

"Like people in a line getting shoved from behind."

"Exactly like that. A bunch of tiny bumps traveling down." Soren pulled his notebook out of his back pocket and drew a row of dots and a little arrow shoving the first one. "The bump moves as a wave. Down the whole pole."

Maya was already nodding, faster than he was drawing. "So the pole squishes. A little. Every time."

"It has to. If your end moved and my end didn't, for even a tiny moment, the pole got shorter. Just for that moment."

"So there's no such thing as a stiff pole."

"There's no such thing as a perfectly stiff anything. It can't exist. If it existed, the push would arrive everywhere at once, and that's faster than light, and you can't."

Maya laughed out loud in the empty gym. The sound bounced off the bleachers.

"How fast does the bump go?" she asked.

"The speed of sound. In the metal, not the air. Way faster than sound in air." Soren chewed his pen. "In aluminum, thousands of meters a second. Fast. But not light-fast. Not close."

"So we could measure it," Maya said. "With the poles."

"They're too short. I already said."

"No. Listen." She was pointing at the pile now, tapping the air over it. "You don't measure how fast the pole moves. You measure the sound. Tap the end. The bump travels down the metal to my ear at metal-speed, and also through the air to my ear at air-speed. Two different times. Same tap."

Soren stopped drawing.

"Put your ear on one end," Maya said. "I tap the other. You hear it twice. The metal one first, because the bump wins."

"The bump wins," Soren repeated, slowly, like he was tasting it.

They grabbed the longest pole and stretched it across the whole gym floor. Soren lay down and pressed his ear flat against the cold aluminum. Maya stood at the far end with a quarter from her pocket.

"Ready?"

"Do it."

She cracked the coin against the metal.

Soren heard it come twice.

A small hard click, right against his ear, arriving through the metal. And then, a hair later, the same tap again, softer, floating across the open gym through the air.

He sat up so fast the pole rolled.

"Twice," he said. "I heard it twice."

"You heard it twice." Maya was already walking down the pole toward him. "The metal one came first."

"Because the bump is faster in the metal." He was talking with his whole hands now. "The exact same tap. Split into two arrivals. Because the push takes time."

"It always takes time." Maya sat down on the floor beside him. "Even in the pole. Even in the floor. Even in you."

Soren looked at his own hand on the aluminum.

"If I push anything," he said, "the far side finds out late."

"Everything finds out late." Maya knocked her knuckle on the gym floor. A dull sound. "That's a wave too. Slow one."

Soren pressed his palm to the pole again and did not push it. He just held it, feeling how solid it felt, knowing now that the solid was a story the atoms told fast enough to fool him.

"There's no rigid thing anywhere," he said. "In the whole universe."

"Not one," said Maya. "Everything's a little squishy. Even the Moon pole."

"Especially the Moon pole."

They sat with that. The gym ticked as it cooled.

Then Maya reached for the quarter, walked back down to the far end, and pressed her own ear to the aluminum.

"Your turn to tap," she called. "I want to hear the bump win."

Soren picked up the coin, aimed at the cold far end of a pole that was suddenly not one thing but a line of a trillion atoms waiting to pass the news along, and knocked.