Hm, one could write a whole book about this. I´ll try to be a little more brief
1. If I would be offered a job on a space mission?
Where do I have to sign?
2. What´s out there?
Well, to quote Jody Fosters father in Contact: "If we are alone in the universe, it is an awfull waste of space".
Seriously, I do belive that there is/was life out there.
This being said, the chances that two sentient races develop in the same neighbourood (say, some couple thousand ly)
at the same time are rather remote. The time part is the important one IMO. The universe (as far as we know) is roughly 15 billion years old. Because there were no elements beside Hydrogen and Helim in the beginning, lets say the first sentient race appeared 10 billion years ago.
Lets assume 1 in 1000 stars is of a type to support life(1), then 1 in 1000 of those has planets where they can develop life(2) and on 1 of 1000 of those life becomes sentient(3). This gives us a 1 in 1 billion chance for a star to be home of a sentient race. As there are some 100 to 200 billion stars alone in our galaxy, this would leave us with 100 to 200 alien races in the milkyway.
However, those 200 races would be spread over 10 billion years. If each race would stay communicative (starts with radios, ends with, who knows) those races would cover only 200 million years. So the chance for 2 of those to be communicative simultaneously(sp) is 1/50 or 2%.
Yes, I know, a lot of assumption.
Ralph Hoenig, Germany
(1)
This number is actually somewhat backed up by data we have.
Most of the stars in the milkyway are brown dwarfs, who simply don´t have the radiation-spectrum to support life.
A lot of stars are part of a binary/trinary system, which either blows orbits of planets to hell (if they are orbiting only one of the stars) or forces the plaets too far away, if they orbit the common center of gravity of the two/three stars.
All stars above, say, double sunmass are simply too short lived to develop life.
(2)
The actual number is just a guess, but it took a lot of coincidents to make earth habitable to human life (A Jupiter big enough to sweep most asteroids away, so we have only a big impact every 30 million years or so, instead of every 300.000 years, the giant moon to stabilise the earth-axis and so on)
(3) Simply a wild guess and using the 1000 for three times just looks too good
