The Search for Extraterrestrial Intelligence (SETI)

Learning Objectives

Define the goals of the Search for Extraterrestrial Intelligence (SETI).

Explain the Drake Equation as a probabilistic framework for estimating the number of detectable civilizations.

Describe the Fermi Paradox and some of its proposed solutions.

Identify the types of signals SETI experiments search for.

Are We Alone?

The Search for Extraterrestrial Intelligence (SETI) is a scientific field whose goal is to detect intelligent life elsewhere in the universe. SETI does not involve searching for UFOs; instead, it uses scientific methods to search for evidence of technology created by alien civilizations.

The Main Strategy: Listening for Signals

The primary strategy of SETI is to use large radio telescopes to listen for artificial signals coming from space. The reasoning is that any technological civilization would likely discover radio waves and use them for communication and radar, just as we have. Some of these signals might "leak" into space, or a civilization might intentionally broadcast a powerful signal.

SETI experiments scan the sky for narrow-band signals, which are not produced by any known natural astrophysical process. They also look for complex, patterned signals that would indicate an intelligent origin.

The Drake Equation

The Drake Equation, formulated by astronomer Frank Drake in 1961, is not a rigorous physical equation but a tool to stimulate scientific dialogue. It provides an estimate for the number (N) of active, communicative civilizations in our galaxy.

N = R* ⋅ fₑ ⋅ nₑ ⋅ fₗ ⋅ fᵢ ⋅ fₑ ⋅ L

R* = Average rate of star formation

fₑ = Fraction of those stars with planets

nₑ = Average number of planets that can potentially support life

fₗ = Fraction of those planets that actually go on to develop life

fᵢ = Fraction of life-bearing planets that develop intelligent life

fₑ = Fraction of civilizations that develop a technology that releases detectable signs of their existence

L = The length of time for which such civilizations release detectable signals

While we have good estimates for the first few terms, the last few are almost complete speculation, making the final answer for N highly uncertain.

The Fermi Paradox: Where Is Everybody?

Given the vast number of stars in our galaxy, the Drake Equation suggests there should be at least some other intelligent civilizations. This leads to the Fermi Paradox, first posed by physicist Enrico Fermi: If the galaxy is teeming with alien civilizations, why have we found no evidence of them?

Proposed solutions to the paradox range from:

We are alone (or the first): Intelligent life is exceedingly rare.

Great Filter: There is some barrier (a 'Great Filter') that is very difficult for life to overcome, such as the leap to multicellularity or the development of technology that doesn't lead to self-destruction.

They are hiding: Advanced civilizations may choose not to be detected for safety reasons (the 'dark forest' hypothesis).

We aren't looking correctly: We may be searching for the wrong kinds of signals or in the wrong places.

Key Terms

SETI: The Search for Extraterrestrial Intelligence, a field of science that searches for evidence of intelligent life beyond Earth.
Drake Equation: A probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.
Fermi Paradox: The apparent contradiction between the lack of evidence for extraterrestrial civilizations and high estimates for their probable existence.
Radio Telescope: A specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. It is the primary tool for SETI.
Narrow-band Signal: A radio signal that occupies a very small range of frequencies. Such signals are not known to be produced by natural phenomena and are a primary target for SETI searches.