What if the aliens are actually machines?

“If you look at the timescales for the development of technology, at some point you invent radio and then you go on the air and then we have a chance of finding you,” he told BBC News.

“But within a few hundred years of inventing radio – at least if we’re any example – you invent thinking machines; we’re probably going to do that in this century.

“So you’ve invented your successors and only for a few hundred years are you… a ‘biological’ intelligence.”