> So if you were aliens and you didn't want to 'alarm' or 'damage' humans, what would you use as a signal that it was probably a good time to say "Hello" ?
You don't say Hello if you are at 10 000 lights years away and have no means of traveling faster than light, because 10 000 years ago humans had no means of Radio communication anyway.
And for all we know, even if intelligent life arises everywhere in the cosmos, there is absolutely no model that says that intelligent life is supposed to evolve in the direction of mankind. After all, the Earth is full of intelligent life, well spread among numerous species (dolphins, crows, mammals, apes, etc...) yet only one species among them has been able to move to the next level. There could be many things happening in the meantime to prevent such a single species domination, and indeed for most of the time mankind was just another ape fighting for survival.
So, for me that's the big missing factor in Fermi's equation: species may not end up evolving in the same way that we did, and I can well imagine planets swarming with life yet having no mankind-like species that would be reaching out in the first place.
Well, and not only that, but some other fairly simple things that make our Earth truly rare indeed. For example, our solar system is "inside out" with the big fat gas giants on the outside. This is proving to be pretty unusual. We've modeled that there used to be another Saturn or Neptune that ended up causing our gas giants to move outward in their orbits. This might make it extremely easier for Earth-like conditions to form, where life could evolve beyond some kind of muck living off chemicals in environments with immense pressure and heat. Can't build a factory there. And we've got a moon. A fairly large moon, at that. Perhaps Jupiter and our own moon have been absorbing the space debris for some billion years, allowing conditions on Earth to become better still. I think the Rare Earth theory doesn't make our Earth rare enough.
You don't say Hello if you are at 10 000 lights years away and have no means of traveling faster than light, because 10 000 years ago humans had no means of Radio communication anyway.
And for all we know, even if intelligent life arises everywhere in the cosmos, there is absolutely no model that says that intelligent life is supposed to evolve in the direction of mankind. After all, the Earth is full of intelligent life, well spread among numerous species (dolphins, crows, mammals, apes, etc...) yet only one species among them has been able to move to the next level. There could be many things happening in the meantime to prevent such a single species domination, and indeed for most of the time mankind was just another ape fighting for survival.
So, for me that's the big missing factor in Fermi's equation: species may not end up evolving in the same way that we did, and I can well imagine planets swarming with life yet having no mankind-like species that would be reaching out in the first place.