william18 wrote:I think that looking for aliens can help shed light on The Fermi Paradox, which is the contradiction between the high probability of of life, but the reality that there isn't any evidence. This leads scientists to believe there is a Great Filter, which most if not all life forms failed to surpass.
I'm convinced that it is behind us, but not all that far behind us. Single cell life is probably pretty common in the universe (on a galactic scale that is). Our single cell life, which is in fact a combination of two different single cell organisms (the second being mitochondria within the cell) is probably much rarer. Single cells that can survive poisonous oxygen rich environments are even rarer.
Remember that all life on earth started out as anaerobic. Such life still exists deep within the bottom of the ocean, near volcanic vents where the poisonous oxygen supply is nonexistent. They can form multicellular organisms but you aren't going to see them build space craft any time soon. I've seen a lot of speculation on the ancestor of chloroplasts, (the things in plant cells that makes lots of energy and all that evil poisonous oxygen), the cyanobacteria.
Let's just pause here. The single cell organism on earth was invaded by two major life forms, which became mitochondria and chloroplasts within the cell, a process that almost destroyed all previously existing life on the planet! And one of them may have come from OUTER SPACE! (Argument based on the fact that the optimum frequencies for cyanobacteria are more commonly found at a distance from the sun more at the range of Jupiter than Earth.)
Now it's going to be a long way to go from these cells to ALIENS FROM OUTER SPACE! First of all, electronics is going to be a problem for anyone who remains in a liquid H2O medium (just ask the dolphins). So you have to get life out of the seas and onto the surface. Probably not going to happen unless you have tidal forces creating places where water comes and goes. Also without a Magnetosphere you are going to loose atmosphere to the solar wind, especially hydrogen which is an annoying element in water, so you start to quickly dry out the planet. (Hello Mars.)
So based on this notion I believe that the probability of life living on solid surfaces capable of achieving intelligence is so small that they are effectively apart from each other. Homo Sapiens is what, 200,000 years old? That's the diameter of the Milky Way Galaxy! Now there are 8 potential hospitable worlds within 50 light years of the planet (a rough idea of how long we have been able to leave the Earth's atmosphere) but whether they contain life at all is questionable and probably not much evolved beyond simple organisms.
Once we get to space faring races the problems grow astronomically (no pun intended). Let's say that a race started seriously exploring stars 10,000 years ago (well we can't but I'm going to ignore relativity for the moment). At 10 psol, they would need to be closer than 100,000 light years in order to reach us, assuming that they actually by sheer luck and strange behavior made a direct journey to our general direction. If the timelines are even tighter the distances become even smaller. So if there were, let's say, four major potentially space faring races in the galaxy they would probably never see each other EVER.