There is a pervasive belief that there must be extraterrestrial life and in particular intelligent life in the universe. In fact, that is usually presumed to be so true that the only question people tend to ask is why haven’t we heard from anyone yet. The celebrated Drake Equation, which tried to estimate the number of civilizations in the galaxy that we might be able to communicate with ended up with the number 10. One of the factors in the Drake equation is the fraction of earth like planets that eventually develop life. In 1961, when Drake wrote down the equation he assumed this fraction was one. After all, the earth is teeming with life, so it must be easy to develop life anywhere, right?
Actually, we have no idea what the probability of forming life is. We do not know how life developed on earth. There are several competing theories but we have no empirical evidence to support any of them. The probability of forming life could be very high or it could be close to zero. It is as equally likely that there are lots of civilizations out there to talk to or there are none. The possibility that there is no life whatsoever in the visible universe beyond earth is as likely as any other hypothesis. We really could be alone in the universe.
An argument that there must be life out there is based on the Copernican Principle, which is that we are just a typical planet in a typical solar system in a typical galaxy. Thus, anything that happens here probably happens on any other typical planet. So the fact that we exist means that life must be widespread in the universe. The Copernican Principle may have been very useful for making astrophysical predictions but I will argue that it cannot give us any conclusion about the prevalence of life in the universe.
Ironically, it is recent developments in String theory and cosmology that have led me to this belief. From what I understand, string theorists and the like currently believe that there are many many possible types of universes known as the string theory landscape (an oft quoted result is that there are of them, which might as well be infinity), they all basically exist (either have existed or will exist but that doesn’t really matter), and we are in one of them. Actually, I like this idea because it means we don’t have to worry about fine tuning theories or dynamics or anything to make a universe, we just try everything. I bet that eventually string theory will come to the conclusion that basically all computations (a countable infinite number) can exist and we are one of these computations (See here for a previous post related to this).
Hence, if we apply the Copernican Principle to the string theory landscape, then it could be that there is one civilization or one planet with life in a typical universe. Thus the prior probability that there are multiple civilizations or just one is equally likely using the Copernican Principle. In fact, this even suggests that the most plausible possibility is that there are either lots of civilizations as in a Star Trek or Star Wars universe or we’re all alone. If that is true then I would say that since we haven’t found any evidence of extraterrestrial intelligence in nearly 50 years of searching, then they are either really good at hiding or there is no one out there.