The contradiction between the size and age of the galaxy and the complete absence of confirmed evidence that anyone else is in it. It is named for a remark Enrico Fermi made over lunch at Los Alamos in the summer of 1950, in a conversation with Edward Teller, Emil Konopinski and Herbert York; the version usually quoted, “Where is everybody?”, is Teller’s recollection thirty-five years later (Konopinski remembered “but where is everybody?”, York “Don’t you ever wonder where everybody is?”), so the wording is a reconstruction and the question is not.
Why it is a paradox and not a shortage of data
The galaxy is roughly a hundred thousand light-years across and about thirteen billion years old. A probe that breaks no known physics, travelling at a small fraction of light speed and replicating at its destination, crosses it in a few million years, which is a rounding error against the age of the disc. So the expected state of a galaxy containing even one expansionist civilisation of any age is a galaxy visibly occupied, and that is not what is observed.
The families of answer, and what each costs
They are few and each pays a price. Nobody else exists, which requires a filter somewhere behind us or ahead of us that is close to total. They exist and are silent, which is the Dark Forest Hypothesis and requires unanimity that a single defector ends. They existed and are gone, which pushes the problem onto the lifetime term of the Drake Equation. They are here and the evidence is not of a kind the search was built to catch, which is the position from which the UAP literature argues and which Avi Loeb and the Galileo Project have tried to make instrumented rather than testimonial. The one thing the silence does not do is choose between them.
Connections
Drake Equation · SETI · Dark Forest Hypothesis · Kardashev Scale · Wow! Signal · Silurian Hypothesis · Avi Loeb