Next week, Denis Villeneuve’s Arrival will give its answer, but what’s the plan in the real world?
Well, there has been a lot of thought about it. “There’s a big debate within the whole community over whether we should respond or not,” says Dr John Elliot, joint coordinator for the UK Search for Extraterrestrial Intelligence (Seti) Research Network – although he admits that it may not be a choice that any single body gets to make.
Seti’s policy, once it is fairly sure that an intercepted signal is an alien message, will be to share it openly to allow people all over the world to try to understand it.
Thereafter, it will be hard to stop anyone from answering. “I would have thought that there would be a reply made in some form at some point round the globe by someone with the required equipment,” Elliot says.
This scenario imagines receiving a transmission, but not alien visitors themselves.The chance of that is considered remote, even by those who expect aliens to exist, because even aliens are presumed to work within the laws of physics. In summary: our galaxy is a flattish disc about 100,000 light years across, and our planet is right at a sparse edge of it; only a small proportion of the planets in our galaxy are within, say, 1,000 light years of us.
As a result, even a craft from such a nearby planet travelling directly towards us at half of light speed – which may be impossibly fast for a machine with life inside it – would take 2,000 years to get here.
Why would it come? And what are the chances of it arriving exactly now?
Alien transmissions could be passing by all the time, however. Much of the problem is a matter of linguistics.
Part of Elliot’s work is to devise and understand languages that might make sense to an intelligent alien – so-called xenolinguistics.
“It’s the same as if you’re in the Amazon rainforest and you encounter a lost tribe,” he says. “You’d include their message, or part of it, as an echo.
But you’d need to have something simple at the front and build upon it.” A good part of the work could be done by sending simple binary images – perhaps a picture of a rock – and starting to establish words from there.
The trouble is, it won’t happen quickly. If the aliens are only 100 light years away, their message will be 100 years old by the time we get it – as will our reply by the time it turns up.
“It’s a 200-year turnaround even just to say hello,” says Elliot. We might improve on this by imagining a technology that can send messages faster than light, but then we have overturned a fundamental law of physics, which leaves yet more explaining to do.