Listening for someone else

In the spring of 1960 a young astronomer called Frank Drake pointed a radio dish 26 metres wide at two nearby stars and listened. He was not listening for words. He was looking for one thing nature does not make: a radio signal squeezed into a single sharp note. Stars and gas clouds hiss across a wide band. A transmitter sings on one pitch. For six hours a day, for three months, Drake heard only hiss.

The nearest thing to a hit came on 15 August 1977. A telescope in Ohio recorded a signal that rose and fell over exactly 72 seconds, just as a steady source in the sky would as the turning Earth swept the telescope past it. It was on one pitch, and at its peak it was 30 times stronger than the background. The astronomer who found it on the printout wrote “Wow!” in the margin. It has been looked for many times since, and never heard again. Nobody knows what it was.

People have also spoken. On 16 November 1974 the great dish at Arecibo in Puerto Rico sent a message of 1,679 ons and offs, which make a simple picture when set out in rows. It lasted less than three minutes. It was aimed at this cluster of stars, called M13, which is 25,000 light years away. The message is still on its way. It will arrive in about 25,000 years, and any answer would take as long again to come back.

Two messages are travelling more slowly. Each of the two Voyager spacecraft, launched in 1977, carries a gold-plated record. This real photograph shows one of them, with its cover, a few days before launch. On it are 115 pictures, greetings in 55 languages, 90 minutes of music, and the sounds of surf, thunder, birds and whales. It was made for whoever might one day find it.

The silence so far means less than it seems. Strong radio signals have been leaving the Earth for only about a hundred years, so they have reached only the stars within about a hundred light years. The galaxy is a thousand times wider than that. On a map of the Milky Way, everywhere our voice has reached is a dot too small to see. And the listening has barely begun.

Still, there is a puzzle, first put by the physicist Enrico Fermi in 1950. The galaxy is very old, and has billions of stars like the Sun. If clever life arose often, some of it should have spread or shouted long ago. “Where is everybody?” he asked. Perhaps life is rare. Perhaps civilisations do not last. Perhaps they are quiet, or we are listening in the wrong way. Nobody knows, and listening is the only way to find out.