The Fermi Paradox is one of the major unanswered questions in astrobiology. It started with physicist Enrico Fermi, who in 1950 asked his co-workers over lunch: “Where are they?” What he meant was intelligent extraterrestrials. If there are billions and billions of stars and probably even more planets, why have we not already been in contact with extraterrestrial (ET) civilizations?
This is even more puzzling since our Sun and Earth are relatively young, meaning that life could have originated on other worlds long before it did here, and intelligent beings on those planets could easily be millions of years ahead of us.
There are two principle answers to the paradox: The alien civilizations are (1) present but for some reason we can’t detect them, or (2) they simply are not there, or at least not in our vicinity. In regard to the first option, Star Trek’s prime directive comes to mind, or perhaps a scientific variation of the Zoo hypothesis (aliens don’t interfere with us because we are an unstable emerging civilization).
There are in fact so many possible solutions to the Fermi Paradox that whole books are written about it. Nevertheless, assuming aliens are around us, shouldn’t there be some evidence? Well, not necessarily. Carl Sagan pointed out that if an ET civilization is far ahead of us, their actions would appear to us as magic. Just imagine us flying a spy drone over our Stone Age ancestors!
What about the so-called UFO sightings that we astrobiologists are sometimes asked about (see, for example, the top UFO cases of 2012 as judged by the Mutual UFO Network). On one hand, I believe that we scientists are sometimes too dismissive of eyewitness reports, and too quick to rationalize them away as natural phenomena or hallucinations. On the other hand, we rely on the scientific method, and reported sightings are not reproducible events that we can test in the laboratory.
What about the second answer—that aliens simply do not exist? The Drake equation, which is usually used to estimate the number of extraterrestrial civilizations in the universe, includes a term for the probability of life originating on a planet. If this term is near zero, the number of expected ET civilizations is near zero. Usually we assume that life on Earth was not a singular event, and that it would have happened elsewhere under similar conditions, but we don’t know for sure. We still don’t know how life occurred on Earth, and what ingredients were needed. It’s possible that the rise of intelligent civilizations is such a rare event that the next civilization with our kind of technology is thousands or even millions of light years away.
Or perhaps there’s another possibility, which I raised once at a SETI meeting when we were examining the question of why we haven’t had a positive detection yet. Imagine using a walkie-talkie in modern New York, and wondering why no one responds on your frequency. It’s because everyone is on Facebook or Twitter! So it might be with ET, who may be using technology well advanced of our own.