AOriginally posted by twhitehead
Just to be clear, there is a big difference between life, and intelligent life. It is likely that the former is far more common than the latter.
I also do not expect aliens to attack at all. Once a civilization gets to the point that it can space travel across vast distances, there is nothing worth attacking earth for that they wouldn't already have.
...[text shortened]... re we encounter biological life. Or we might be replaced by machines before any contact is made.
I think the real reason we don't find traces of extra-terrestrial life is because civilizations on that level come and go, as we see our civilizations come and go.
The problem there is the wavefront of radiation that would leave a planet indicative of advanced life, always assuming such advanced life develops like we do, finding out about electricity and later about radio waves and using them. Our wavefront now covers a sphere about 200 light years across, 100 light year radius. So that means a civilization say 10,000 light years away won't get our signal for over 9000 years.
But 9000 years is a long time for intelligent life, at least like ours. We have had countless civilizations come and go in all that time.
For instance, the ancient Babylonians apparently had electricity but their civilization died before it could advance further.
So 9000 years from now, some civilization picks up or signal but we are say, 2000 years gone by then, or we stop using radio and go to something much more advanced, like neutrino beams or something we can't even think about today.
The net result is those signals only last for 2000 years and if they send one back to us, another 9000 odd years goes by so now it is 20,000 years into our future and there is a signal if only we had the equipment to detect it.
Chances are not very good that 20,000 years from now, we will still have radio telescopes.
So it would be like ships passing in the night, neither one seeing the other, never knowing about them.