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This is relatively brief on a geological scale, let alone a cosmological one.
Since there are many stars older than the Sun, and since intelligent life might have evolved earlier elsewhere, the question then becomes why the galaxy has not been colonized already.
Even if colonization is impractical or undesirable to all alien civilizations, large-scale exploration of the galaxy could be possible by probes.
The fundamental problem is that the last four terms are completely unknown, rendering statistical estimates impossible.
According to a 1985 paper by Robert Freitas, when recast as a statement in modal logic, the paradox no longer exists, and carries no probative value.
facetiously blaming the disappearance of municipal trashcans on marauding aliens. Teller remembers, "The result of his question was general laughter because of the strange fact that in spite of Fermi's question coming from the clear blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life." Herbert York recalls that Fermi followed up on his comment with a series of calculations on the probability of Earth-like planets, the probability of life, the likely rise and duration of high technology, etc., and concluded that we ought to have been visited long ago and many times over.
For distant galaxies, travel times may well explain the lack of alien visits to Earth, but a sufficiently advanced civilization could potentially be observable over a significant fraction of the size of the observable universe.
Even if such civilizations are rare, the scale argument indicates they should exist somewhere at some point during the history of the universe, and since they could be detected from far away over a considerable period of time, many more potential sites for their origin are within range of our observation.