The Ones That Knock Twice

The dish on the trailer roof was the size of a satellite TV antenna, and it heard the whole sky at once.

Maya's mother had gone to sleep at midnight. She studied bridges, not stars. The receiver was a favor for a friend, left running to collect data while they camped in the dark reserve where no phone tower could muddy the signal.

That left Maya, Soren, and the companion.

The companion was a small flat screen that watched the data stream and learned its shape. It did not talk much. It put dots on a map of the sky and labeled the ones it had seen before.

"Thousands a day," Soren read from the screen. "It says fast radio bursts hit the whole Earth thousands of times a day. Each one lasts about a thousandth of a second."

"From where?" Maya asked.

"Everywhere. Other galaxies. Really far." He scrolled. "Most of them flash once and never again. One spark, gone."

Maya watched the dots bloom and fade on the map. She pulled her knees up and watched for a while without talking. That was how she watched things.

"There," she said.

"Where?"

"The bottom left one. It came back."

Soren leaned in. The companion had drawn a small circle, then a question mark beside it. Two flashes from the exact same spot, eleven minutes apart.

"Could be two different sources," Soren said. "Close together by accident."

"Wait for it."

They waited. The trailer ticked as it cooled. Forty minutes later the same spot lit a third time, and the companion turned the question mark into something it almost never showed: NO CATALOG MATCH.

Soren sat back. "It doesn't know this one. Nobody's logged it."

"How many does it know?"

He checked. "Tens of thousands of single bursts. But repeaters are rare. Only a small number of sources have ever been caught flashing more than once." He looked up. "This isn't on the list."

Maya was already counting on her fingers. "Eleven minutes. Then forty. That's not even."

"Repeaters aren't always even," Soren said. "Some go quiet for months. Some come in clusters and then nothing." He opened his notebook and wrote down the times. "I want the gaps. If there's a pattern in the gaps, that means something is keeping time. A spinning star, maybe. Something heavy and fast."

They caught the fourth at two minutes past the third. The fifth at nineteen. The gaps would not line up into anything Soren could draw.

"It's not a clock," he said quietly. "Or if it is, I can't read it."

Maya pressed her thumb against the screen, over the little circle. "Why do most of them only knock once?"

"Nobody knows," Soren said. He said it plainly, the way he said true things. "Honestly. The one-offs and the repeaters might be two different kinds of thing. Or they might be the same thing, and we only catch the repeaters knocking twice. Maybe everything repeats and we mostly look away too soon."

That landed on Maya strangely. She thought about all the single dots she had watched bloom and die over the last hour. Thousands of them, across the whole sky, every day. Each one written down as a one-time event. Gone forever.

"What if they're not one-offs," she said. "What if we just weren't pointed at them the second time."

Soren stopped writing.

"We assume they only knocked once," Maya said, faster now, "because we only heard once. But the dish can only really listen hard at a tiny piece of sky. The rest of it, we kind of guess. So a thing could knock and knock and knock, and if nobody happened to be facing it, it goes in the book as a single spark."

"Most of the universe is in the part we're guessing about," Soren said slowly.

"Yeah."

He looked at his own list of uneven gaps. Two minutes. Eleven. Nineteen. Forty. Numbers that refused to be a pattern. And then he understood why they refused, and the understanding made his skin go cold and bright at the same time.

"The source might be perfectly regular," he said. "Knocking like a heartbeat. We're only catching the knocks that happen to land while we're listening. The pattern's there. We're seeing it through a fence."

Maya turned and looked at him, and grinned, because he had gotten somewhere she hadn't.

The companion flashed the circle a sixth time.

Soren wrote down the time. Then he set the pencil down and did something he almost never did mid-problem. He stopped writing.

"This source has been flashing for as long as there's been a sky," he said. "Millions of years, maybe. Knocking the whole time. And the first creature in the history of everything to write it down is us. Tonight. In a trailer."

"Because nobody asked it why it repeats," Maya said.

"Because nobody was facing it."

Maya stood up. She unlatched the trailer door and pushed it open. The cold poured in. Above the dark reserve the real sky was thick with stars, more than either of them ever saw at home, and somewhere in the lower left of it, far past anything her eyes could reach, a thing they had no name for was sending out its sixth flash, and its seventh, whether or not the dish was pointed the right way.

She found the spot. Low, near the ridge of black pines. She lifted her hand and held two fingers out at it, the way you'd block a streetlight, marking it so she wouldn't lose it.

Soren came and stood in the doorway behind her, notebook against his chest, looking where she was pointing.

"It's still going," he said. "Right now. We just can't hear this one."

Maya kept her two fingers up against the dark, holding the place where the knocking was, and did not put her arm down.