How can we communicate with aliens?

In their final series of Tweets, Marcus Chown and Govert Schilling ask how we can talk to extraterrestrials

In 19th century, scientists proposed communicating with Martians by planting trees in geometric shapes, or lighting big fires in Sahara.

In 1959 Nature article, Giuseppe Cocconi and Philip Morrison suggested 21-cm radio waves is best choice for interstellar communication.

One year later, Frank Drake started Project Ozma. He tuned in to stars Epsilon Eridani and Tau Ceti to search for artificial radio signals.

Since 1960, the search for extra-terrestrial intelligence, or SETI, has become ever more sensitive. But still no ET signal detected.

We’ve also sent messages on spacecraft – Pioneer Plaque and Voyager Interstellar Record – and coded radio messages to other stars.

Also, radio & TV broadcasts have turned Earth into a ‘natural’ emitter of strong artificial radio waves. These could be picked up by aliens.