A summing up of many ideas, google translated
In ancient times, the hypothesis of extra-terrestrial life was put forward in Greece, as early as 600 BC. AD
In the context of a universe centered on the earth, until the Copernican revolution, the question could not be asked.
Until the 19th century, man was too self-centered, too much of himself as the sole master of creation, to seriously think that there could be other intelligent creatures.
Advances in astronomy and astrophysics have shown that there could be around 300 million and up to 500 million habitable planets in the galaxy. Out of a total of 6 billion. Even if not all of them actually support life, the probability that some do is high.
But if the galaxy contains a significant number of intelligent species, why haven’t we found a trace of them? This is the Fermi paradox.
Several possible explanations, including:
-Even though many planets are home to life, intelligent life is more complicated.
-life is one thing, intelligence is another and few planets are home to intelligent life.
- problem of concordance in time: the universe is 4 billion years old. Man intelligent enough to develop technology only appeared a few tens of thousands of years ago and there is no guarantee that our civilization will last. There may have been intelligent extra-terrestrial races which could have disappeared, or whose civilization could have abandoned space travel.
-strangeness: these races may have totally different reasoning from ours, be neither interested in space travel, nor in contact with other intelligent species, or by existing and manifesting themselves in such a way that we cannot perceive them.
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impossibility of encounter: the rarity of intelligent races and the distances are such that the probability of an encounter is low, travel at a speed greater than that of light is impossible. We know that there are probably habitable planets around twenty light years away. Sending enough people to create a viable colony no matter what is not easy. The journey would take centuries, given the technological means available, and it would be a journey of no return, with a strong dose of the unknown.
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the zoo or asylum hypothesis: humans are dangerous madmen and should be avoided.
Some claim that in fact our planet has already been visited by extra-terrestrials. The evidence is considered unconvincing.