Fermi’s Paradox (the fact that we never detected any sign of aliens even though, conceptually, life could be relatively common in the universe) has already been discussed in this blog, as new results come in about the rarity of life bearing planets, the discovery of new Earth-like planets, or even the detection of possible signs of aliens.
There are a number of possible explanations for Fermi’s Paradox and one of them is exactly that sufficiently advanced civilizations could retreat into their own planets, or star systems, exploring the vastness of the nano-world, becoming digital minds.
A very interesting concept related with Fermi’s Paradox is the Great Filter theory, which states, basically, that if intelligent civilizations do not exist in the galaxy we, as a civilization, are either rare, first, or doomed. As this post very clearly describes, one of these three explanations has to be true, if no other civilizations exist.
The Great Filter theory is based on Robin Hanson’s argument that the failure to find any extraterrestrial civilizations in the observable universe has to be explained by the fact that somewhere, in the sequence of steps that leads from planet formation to the creation of technological civilizations, there has to be an extremely unlikely event, which he called the Great Filter.
This Great Filter may be behind us, in the process that led from inorganic compounds to humans. That means that we, intelligent beings, are rare in the universe. Maybe the conditions that lead to life are extremely rare, either due to the instability of planetary systems, or to the low probability that life gets started in the first place, or to some other phenomenon that we were lucky enough to overcome.
It can also happen that conditions that make possible the existence of life are relatively recent in the universe. That would mean that conditions for life only became common in the universe (or the galaxy) in the last few billions years. In that case, we may not be rare, but we would be the first, or among the first, planets to develop intelligent life.
The final explanation is that the Great Filter is not behind us, but ahead of us. That would mean that many technological civilizations develop but, in the end, they all collapse, due to unknown factors (some of them we can guess). In this case, we are doomed, like all other civilizations that, presumably, existed.
There is, of course, another group of explanations, which states that advanced civilizations do exist in the galaxy, but we are simply too dumb to contact or to observe them. Actually, many people believe that we should not even be trying to contact them, by broadcasting radio-signals into space, advertising that we are here. It may, simply, be too dangerous.
Image by the Bureau of Land Management, available at Wikimedia Commons