The **Fermi paradox** is the alleged inconsistency between the lack of evidence of extraterrestrial intelligence and the various high estimates for its probability. The solution to the paradox has implications for the long-term future, in particular the feasibility of interstellar colonization and the likelihood of extinction.

The expected number of interstellar civilizations observable from Earth is the product of the number of planets in the galaxy and the following three probabilities:

- The probability that any given planet produces intelligent life
- The probability that any given intelligent species develops an interstellar civilization
- The probability that any given interstellar civilization would be detectable from Earth at this time

Since the number of planets in the galaxy is in the hundreds of billions, even very small values of the three probabilities would imply that we should see many interstellar civilizations. However, we observe none. This means that at least one of the probabilities must be *extremely* low....

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