The Smallest Stretch

The detector was broken, and that was why they were allowed inside it.

Not broken broken. Decommissioned. Which meant the same thing, Maya figured, except with a plaque. The long concrete tunnel stretched ahead of them, four kilometers of perfectly straight tube disappearing into Louisiana pine forest, and the air inside smelled like dust and old purpose.

"They measured something smaller than a proton with this," Soren said. He was walking slowly, one hand on the wall. "Less than a thousandth the diameter of a proton. That's what the stretch was."

"I know."

"I'm not telling you. I'm telling myself. I need to say it out loud because it doesn't make sense yet."

Maya liked that about him. She stopped walking and looked down the tunnel. It was just concrete and steel and a pipe that used to hold a laser beam. Nothing impressive to look at. That was the part that bothered her.

Dr. Voss, the site manager who had met them at the gate, was already back in the control building eating her lunch. She had walked them to the tunnel entrance, pointed, and said, "Don't touch the mirrors at the end, they're worth more than my house, back by one, I have a meeting." She had not seemed particularly interested in the fact that they had won a national competition to be here.

"Soren. The two black holes that made the first signal. They were one point three billion light-years away."

"Yeah."

"And the wave traveled for one point three billion years to get here."

"Right."

"And when it arrived, after crossing a billion years of space, it moved this tunnel by less than a thousandth of a proton."

Soren pulled out his notebook. Not to write anything. Just to hold it. Maya had seen him do this before, when the inside of his head was getting crowded.

"That's the part I can't get," he said. "Not the big part. Not the black holes or the billion years. The small part. How do you build something that notices a change that small?"

Maya started walking again. "You split a laser beam. Send half down each arm. When the arms are exactly the same length, the light cancels itself out when it comes back together. Destructive interference. Silence."

"And when a gravitational wave passes through, it stretches one arm and squeezes the other."

"By less than a thousandth of a proton. And the silence breaks."

They walked without talking. The tunnel had a slight hum, not from any machine, just from being long and hollow and full of still air. Their footsteps sounded small.

"Maya."

"Yeah."

"That means silence was the detector. Not the laser, not the mirrors. The silence. They built a machine to listen to nothing, and when the nothing changed, that was the signal."

She stopped.

She had read about LIGO for two years. She had read the papers. She had watched the press conference where the scientists played the chirp, the rising tone of two black holes spiraling into each other, converted into sound. She had understood, she thought, what the machine did.

But she had not understood it like that.

"Say that again," she said.

"They didn't detect a signal. They detected the failure of silence."

Something turned over in her chest. The tunnel felt different now. She was standing inside an instrument so sensitive that it could register disturbances smaller than the smallest component of an atom. And the way it worked was not by seeing something, or hearing something, or measuring something. It worked by being so perfectly still that any disturbance, no matter how impossibly small, announced itself.

The whole machine was a held breath.

"Soren. The wave that hit this detector on September fourteenth, twenty fifteen. Two black holes, each about thirty times the mass of the sun, spinning around each other, getting closer and closer, moving at half the speed of light."

"And then they touched."

"And spacetime rang like a bell. And the ring spread outward at the speed of light, in every direction, for over a billion years. And by the time it got to Louisiana, to this tunnel, it was so faint that it moved these walls by a distance so small that there is literally no human experience to compare it to. Nothing you have ever seen or held or imagined is that small."

"And they caught it."

"Because they built a silence and they waited."

Soren opened his notebook now. He wrote for a long time. Maya didn't watch. She walked further down the tunnel, running her hand along the wall, feeling the concrete under her fingers. She thought about what it meant that the universe was full of signals like this. Black holes colliding. Neutron stars tearing each other apart. Events so violent they shook the fabric of space itself, and the shaking traveled for billions of years, and arrived here as a whisper smaller than a proton, and someone had thought to listen.

Not someone loud. Not someone who shouted over the noise. Someone who had thought that if you could build a quiet enough room, the universe would speak.

There were gravitational waves passing through her body right now. Right this second. Waves from collisions so far away the light from them wouldn't arrive for millions of years. Spacetime was stretching and squeezing, everywhere, all the time, by amounts so small they passed through everything, through buildings and mountains and planets, without anything noticing.

Except here. In this tunnel. For a few years, something had noticed.

She turned back. Soren was sitting cross-legged on the tunnel floor, notebook on his knee, pencil still.

"They're building new ones," he said. "Bigger. More sensitive. Space-based. LISA. Three satellites, two and a half million kilometers apart, flying in formation, making a triangle of laser beams in orbit around the sun."

"A silence that big," Maya said.

"Imagine what it'll hear."

She sat down next to him. The tunnel stretched in both directions, vanishing into points of light at each end. Somewhere above them, through the concrete, the Louisiana sun was beating down on pine trees and asphalt and a site manager finishing her sandwich.

Below all of that, beneath every sound and every motion, spacetime was ringing with collisions they had no instrument quiet enough to catch.

Not yet.

Maya pressed her palm flat against the tunnel floor and held absolutely still.