If aliens could see us what would they see?

We put Omkar's question to Judith Croston... Judith - Well the answer to that is yes. So the questioner is right. If we think about aliens that were say living in the Andromeda galaxy. So the Andromeda galaxy is our nearest galaxy and it is a few million light years away. And so what that means is that any signal that those aliens could measure, it would have to travel at the speed of light to get there. So it would have taken some number of millions of years to get there so what they would be seeing would be prehistoric creatures wandering around on the Earth.

Chris - Doesn't that presuppose that the light hasn't spread out so far that they wouldn't be able to see a dinosaur?

Judith - There are a couple of problems with that. One of them is that, obviously, the sorts of telescopes we have are nothing like good enough to be able to see the prehistoric creatures on planets in the Andromeda galaxy - so that's one problem.

所以当我们瘦k about trying to hunt for aliens, we try to think about the sorts of radio signals, we think about technological society. So the earth has been only sending out signals that are likely to be detectable for sort of a hundred years so that's one thing and that's if we think about aliens in a different galaxy.

But I think perhaps what the questioner has missed, or at least isn't in the question, is whether the aliens would have to millions of light years away. Because, in fact, we know of plenty of planets that are a lot closer than that. There are actually, I think, about seventy five known planets within fifty light years. So if there were aliens on one of those planets, then maybe they would be listening in to signals we were sending out, fifty light years away they might be listening to the Beatles on the BBC or something.

Chris - They've got reasonable taste then?

Judith - We don't really know. This light travel time thing is a really important thing to take into account if you're trying to figure out what we can see from alien civilizations and what they might be watching.

Chris - So in summary: the light is spreading out, it's spreading out at the speed of light and, in theory, they could pick up that light, they could interpret what that light came from and the would, effectively, be seeing prehistoric Earth, if they were that far away. But they may be much closer and they may even be watching Neighbours?

Judith - Yes, absolutely.