Fermi Paradox
The contradiction between the high probability of extraterrestrial life and the lack of evidence for it, generating numerous proposed solutions from the Great Filter to the Zoo Hypothesis.
OVERVIEW
The Fermi Paradox is the apparent contradiction between the high probability estimates for the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations. The paradox is named after physicist Enrico Fermi, who reportedly asked 'But where is everybody?' during a 1950 conversation about UFOs. The Milky Way galaxy is estimated to contain 100–400 billion stars, many with Earth-like planets. The Drake Equation estimates the number of communicative civilizations. Solutions to the paradox include: the Great Filter (some step in evolution is extremely difficult), the Zoo Hypothesis (ETIs are deliberately avoiding contact), the Dark Forest (civilizations hide to avoid destruction), and the possibility that we are alone.
KNOWN FACTS
The Drake Equation estimates N (communicative civilizations) could be in the thousands
Kepler and TESS have confirmed thousands of exoplanets, many in habitable zones
The age of the galaxy (13.6 billion years) allows time for civilizations to spread
No confirmed evidence of extraterrestrial technology has been found
SETI has been searching for 60+ years with no confirmed signal
CLAIMS
The galaxy should be teeming with civilizations, yet we see no evidence
The Great Filter may be ahead of us (we are doomed) or behind us (we are rare)
The Zoo Hypothesis suggests ETIs are observing us without interfering
The Dark Forest theory suggests civilizations hide to avoid destruction
The lack of evidence may be because we haven't looked properly
EVIDENCE FOR
The Drake Equation estimates N (communicative civilizations) could be in the thousands
Kepler and TESS have confirmed thousands of exoplanets, many in habitable zones
The age of the galaxy (13.6 billion years) allows time for civilizations to spread
No confirmed evidence of extraterrestrial technology has been found
SETI has been searching for 60+ years with no confirmed signal
EVIDENCE AGAINST
The Drake Equation's variables are highly uncertain; N could be 1 (us)
The galaxy may be too large for civilizations to have reached us
Technological civilizations may be short-lived (self-destruction hypothesis)
We may not recognize ETI signals or technology
The paradox assumes ETIs would want to expand and communicate
OPEN QUESTIONS
No open questions recorded.
SOURCES
TIMELINE
Fermi poses the paradox during a lunch conversation
Drake formulates the Drake Equation
Hart publishes formal analysis of the paradox
Exoplanet discoveries sharpen the paradox