Stephen Hawking says that we shouldn’t try to contact aliens, lest they come and attack us for our resources:
Hawking believes that contact with such a species could be devastating for humanity.
He suggests that aliens might simply raid Earth for its resources and then move on: "We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet. I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonise whatever planets they can reach."
He concludes that trying to make contact with alien races is "a little too risky". He said: "If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn't turn out very well for the Native Americans."
I can’t get too worried about this. It seems to me that any alien civilization with the technology to get here and attack us would also have the technology to search telescopically for planets with useful resources. We’ll probably be able to do a decent job on that ourselves within the next decade or two. To be specific, we’ll be able to do spectroscopy on the atmospheres of lots of planets, which would give us a good idea of which ones to go to and mine — if only we could get there.
For anyone who wants to find us, get to us, and exploit us, finding will be by far the easiest step, so this doesn’t strike me as a good argument for hiding.
Of course, there may be other reasons for not broadcasting our presence to aliens, the most obvious being that it’s a poor use of resources. It all comes down to a cost-benefit analysis: Hawking doesn’t want to do it because of the potential cost (alien attack); I’m more concerned about the (overwhelmingly likely) lack of benefit.