Fermi Paradox
The apparent contradiction between high probabilistic estimates for the existence of extraterrestrial civilizations and the complete lack of physical evidence or contact.
Governing formula
Enrico Fermi: 'Where is everybody?'
SI unit
Astronomical Paradox
In depth
Posed by Enrico Fermi in 1950. Proposed solutions include the Great Filter hypothesis (civilizations self-destruct or face technological bottlenecks), rare Earth hypothesis, or vast interstellar distances.
Examples in the real world
SETI radio astronomy searches scanning the water hole frequency spectrum without detecting alien signals.