There are several ways that we can view our situation (characteristics, geography, ...) in the universe. Before Copernicus, we thought that the Earth was at the centre of the universe, i.e. that the Earth had a privileged situation. In contrast to this opinion, we could also think that the Earth doesn't have anything special, and that intelligent life exists on all planets. From the first point of view, we can not conclude anything about the rest of the universe. Therefore it is useless. But we have also noticed that the second point of view isn't correct, because we haven't detected any extraterrestrial life in the solar system. Between these two extremes, the anthropic principle stipulates that our situation in the universe is unremarkable, compared to all the observers. It is especially interesting if we include extraterrestrial intelligences within this group of observers.
Application examples :
- Our position in space is atypical (on the surface of a planet with an atmosphere, close to a yellow star, etc, ...), whereas the most probable position by chance is in empty space, which is predominant in the universe. However, life is impossible in the vacuum, and the surface of a planet seems necessary for life to appear. That's why we live where we are. It is not a coincidence, it is a necessity (on earth there are less people in the deserts than on the coasts).
- Our position in time is particular : we appeared when the universe was between 10 and 20 billion years old (since the Big-Bang). Why not sooner or later?
Life requires "heavy" chemical elements (carbon, oxygen, ...). These elements did not form during the Big-Bang, but in the massive stars of the galaxy (first generation). These stars became supernovae that ejected the elements in the interstellar medium, thus allowing new solar systems to form with planets. Therefore, some stellar generations were necessary before our emergence (a few billion years ago), but not too many (too few suitable stars for the emergence of life would be present with numerous generations of stars).
NB: the first generation of stars probably didn't contain planets!
It is surprising to notice that life appeared on earth 4.5 billion years ago, that is at half of the life of the Sun. Why is this date comparable to the life span of the Sun? Let us consider three suppositions :
1) Intelligent life appears in general much more rapidly than the life span of the Sun
2) Intelligent life emerges at a time comparable to the half-life of the Sun
3) Intelligent life appears in general much more slowly than the life span of the Sun (i.e. it is unlikely)
Which of these assumptions is the most probable, knowing the date of our appearance?
The second would be surprising, because we a priori do not understand the link between the emergence of intelligent life and the length of the life of a star. And it only represents a particular case. We can reject it because it is very unlikely. If the first assumption were true, we would probably have appeared much sooner in the history of the Earth. Therefore it is also unlikely. If the third supposition is true, it is not surprising that life didn't emerge beyond the death of the Sun, since it is not possible (it is a selection effect). We could only appear before, at a date comparable to the life span of the Sun. So it is the most probable case.
We deduce from this reasoning that intelligent life is rare in the universe, and it would be difficult to detect it. Indeed, the SETIproject had not begun yet.
We can deduce interesting conclusions from this metaphysical principle. However, we must use it with caution, since it can lead to wrong conclusions if the observer is in an exceptional situation. Some research on this subject is in progress.