The Push You Can't See

The radio in the garage only got one clear station, and it was playing a documentary about two spacecraft that were lost.

"Not lost," Maya said. "They know exactly where they are. That's the whole problem."

"How is knowing where they are a problem?" Soren asked. He was holding a little finned block of aluminum his grandfather had labeled HEAT SINK in marker that had gone brown.

The narrator said it again, slower, like he wanted them to feel it. The Pioneer probes were drifting out past the planets, faster than anything people had ever thrown, and they were slowing down. Not much. A tiny, tiny tug, always toward the Sun. Smaller than the weight of a feather spread across a whole spacecraft. But it was there for thirty years, and nobody could find the hand doing the pushing.

"Okay, that's a good problem," Soren admitted.

"Something's grabbing them." Maya pulled her knees up onto the workbench stool. "Or there's extra gravity out there nobody mapped. Or the rules are different that far out."

"Or the math is wrong."

"The math is never wrong by exactly the same amount for thirty years, Soren."

He thought about that. It was a fair point. Wrong math wiggled. This thing didn't wiggle. It just leaned, gently, forever, in one direction.

The documentary was an old one. The narrator listed all the ideas scientists had thrown at it. Dust slowing them like a headwind. A new force of nature. Dark matter clumped where no one expected. Each idea got knocked down. Each time, the tiny push stayed exactly the same size.

"I keep thinking it's something far away," Maya said. "Out in the dark. Reaching in."

Soren turned the heat sink over in his hands. The fins caught the bare garage bulb. "Why far away?"

"Because the spacecraft is just metal. It's tiny. The push has to come from something big."

"Does it, though?" He set the heat sink on the bench. "Feel this."

Maya touched it. "It's cold. It's a piece of metal in a garage."

"Right. But when Grandpa's amplifier ran, this thing got hot on one side. And the fins on the other side were for throwing the heat off. The whole point of it was to push heat out one direction."

"Heat doesn't push."

Soren stopped. "Light pushes," he said. "You told me that. The solar sail thing. Light hits a sail and the sail gets shoved, because light carries a little push."

"A really little push."

"And heat is light. Sort of. The invisible kind. Infrared. When something warm glows in the dark, it's glowing, it's just glowing in a color we can't see."

Maya went quiet, and then she leaned all the way forward over the bench. "Say that again."

"When something warm sits in the dark, it's throwing off invisible light. And if it throws more of it off one side than the other —"

"It pushes itself." Maya grabbed the heat sink out of his hands. "It pushes itself the other way. Like letting go of a balloon."

"A balloon made of light you can't see."

They looked at the radio. The narrator was still listing failed ideas, dust and dark matter and new physics, none of them knowing what the two of them now had between them on a workbench.

"The spacecraft has a power source," Maya said fast. "It has to. It's been flying for decades, way past the Sun, way out where solar panels would be useless. So it makes its own heat. All the time. A little nuclear hand-warmer."

"And it's shaped weird," Soren said, catching up and pulling ahead at the same time. "A big dish on one side. Instruments on the other. It can't be even. One side runs warmer than the other."

"So it glows brighter on one side."

"In infrared. In the color of warm."

Maya pressed the cold fins against her cheek like she could feel the idea through them. "So the push isn't coming from far away at all."

"It's coming from the spacecraft." Soren said it slowly because it was almost too good to say at normal speed. "It's pushing on itself. The whole time. The thing everyone was looking for in the dark was sitting inside the thing they were measuring."

"That's why it never wiggled," Maya said. "That's why it leaned the same way for thirty years. The heat always came off the same side. It always glowed brightest where the hot stuff was. So the push was always the same direction, always the same size."

They sat with it. Outside a car went by. The radio crackled.

"Wait," Maya said. "But how warm is warm enough to push a whole spacecraft?"

"Barely warm," Soren said. "That's what gets me. It glows like a person glows. Like you glow. You're throwing off invisible light right now. So am I. So is the radio."

Maya held her own hand up in front of the bulb, fingers spread, and stared at it like she had never seen it before.

"I'm glowing."

"In a color nobody can see. All the time. In every direction."

"And if I were in space, in the dark, with nothing else around —"

"You'd push yourself," Soren said. "A tiny bit. Toward wherever you were coldest."

The documentary ended. A different narrator came on, a newer voice, recorded years later, and said that a team had finally done the calculation. They had built a careful model of the heat coming off the spacecraft, every warm part of it, and added up the gentle shove of all that invisible light. The number came out exactly the size of the mystery. The hand that had been pushing the Pioneers across the solar system for thirty years was the Pioneers themselves.

Maya didn't say anything. She kept her hand up in the light.

"You knew it was something nobody could see," Soren said. "You were right. It just wasn't far away."

Maya turned her hand slowly in the bulb's glow, watching the back of it, then the palm, as if waiting to catch the light leaving.