In outline, the argument runs like this:

1. The Milky Way contains hundreds of billions of stars, and billions of them are similar to the sun.
2. It is highly likely that some of these stars will have planets that are similar to Earth.
3. If we assume – via the Copernican principle – that Earth is not particularly special, then intelligent life should also exist on some fraction of these Earth-like planets.
4. Some of these intelligent life-forms might develop advanced technology, and even interstellar travel.
5. Interstellar travel would take a long time, but as there are many sun-like stars that are billions of years older, there has been plenty of time for such travel to have occurred.
6. Given all this, why haven’t we met or seen any trace of aliens? Where is everybody?

According to Herbert York, who was present at the lunch, Fermi reportedly backed this argument up with some rough calculations, but he never pursued the question seriously. That task was left to the astrophysicist Michael Hart, who ran some more rigorous numbers in a paper published in 1975.

Though the Fermi Paradox is the accepted name for the argument, some argue that it more properly belongs to Hart.

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