How do we know if we’ve made contact?

Gerry Harp, director of the Center for SETI Research, reveals what we would expect to see

What are we looking for?

We’re looking for signals that are clearly artificial. We have lots of ways we communicate using radio on Earth, and in principle ET might use the same. But the interstellar medium – the gas between stars – messes up signals, so we look for narrow-frequency bandwidth signals.

How would we verify a signal is real?

The signal has to persist. You have to look away and back with telescopes several times, until there have been five cycles. This tells you if it really is from that direction in the sky, or just scattered radiation.

Have we ever had a promising signal?

There has only been one that passed all our automated testing. That was in 2015, and it was exciting. We’re always very sceptical, but we took a look and it was a very interesting signal almost designed to fool our system. It had all the right properties to look like something from outer space, but it was interference.