A Signal From Space Lasted 72 Seconds. Someone Answered It.

On the night of August 15, 1977, a radio telescope in Ohio called the Big Ear was doing what it always did. Listening to the sky and printing numbers.¹

The numbers were usually boring. Ones and twos and spaces. Background noise from space. Volunteers checked the printouts by hand, days later, looking for anything that rose above the hiss.

A few days later, an astronomer named Jerry Ehman was reading through the printouts at his kitchen table when he found a column that did not look like noise. Six characters, rising and then falling: 6EQUJ5. A signal, much stronger than the background, that lasted about 72 seconds, the exact length of time the telescope's beam would take to sweep across a single point in space.²

Ehman circled the characters in red pen. In the margin he wrote one word.

Wow!

The signal came from the direction of the constellation Sagittarius.³ It was close to a frequency that scientists had long argued an alien civilization might use to get attention, the natural frequency of hydrogen, the most common element in the universe.⁴ Astronomers pointed telescopes back at the same spot dozens of times over the following decades.⁵

They never heard it again.

Nadia Brenner's grandfather, Walt, had been one of the volunteers at the Big Ear. He talked about the Wow! signal at every family Thanksgiving until everyone begged him to stop. When he died at ninety-one, the family left the garage for Nadia, because she was the only one who had ever asked him questions.

The garage held a workbench, a cot, and a radio transmitter he had built himself. A real one. Old tubes and new circuits, soldered together over forty years. A cable ran from it, out a hole in the wall, to a homemade dish antenna on the roof, pointed low toward the southern sky.

Toward Sagittarius.

There was a notebook on the cot. On the first page, in her grandfather's careful handwriting:

It wasn't a message. It was a question. 72 seconds is the length of a knock.

And on the last page, underlined so hard the pencil had torn the paper:

Do not answer from the place you live.

Walt had spent forty years working out what the signal meant. The notebook explained it, page by page, in math Nadia mostly could not follow, and in plain sentences she could.

The signal was not "hello." Hello would have repeated. The signal was a single pulse, sent once, aimed at a star system, the way a person knocks once on a door at night to see if anyone is home. Whoever sent it was not trying to talk. They were trying to find out where the listeners were.

A knock does not need an answer, he had written. A knock only needs you to move toward the door. The light comes on. Then they know which house.

In the margins, near the end, his handwriting got shakier.

They have been knocking on every star for longer than we have had language. Most stars never answer. Quiet stars are safe stars.

The ones that answer get visited.