Semantic Coherence: SETI Institute – Signal Evidence & AI Readability

Your browser does not support the video tag.

[H1]
Are we alone?Let’s find out.
Leading the search for life beyond Earth.

The SETI Institute is a 501(c)(3) nonprofit dedicated to finding out.

News
[H5]
Disclosure Needs Data

SETI Institute leads research into possible signs of life and technology beyond Earth. SkyMapper's global network helps create a reliable record of unusual events. Together, they strengthen our ability to validate future discoveries.

News
[H5]
Beyond Disclosure Day: The Real-World Protocols

The IAA SETI Committee announced last week updated rules for evaluating and revealing the detection of extraterrestrial intelligence. A University of Manchester astronomer has led a major international overhaul of the rules that would govern how scientists announce evidence of extraterrestrial intelligence to the world.

News
[H5]
SETI Institute Awards $1 Million in STRIDE Grants to Advance Astrobiology, Exoplanet Science, and Public Engagement

The SETI Institute established the STRIDE fund to support SETI Institute researchers and EOC (Education, Outreach, and Communications) professionals in developing innovative research and education proposals.

News
[H5]
SETI Institute Looks for Signs of Technology in Interstellar Visitor 3I/ATLAS

Scientists at the SETI Institute searched for technological signals from 3I/ATLAS, the third interstellar object observed in our Solar System.

News
[H2] In search of what’s news?

The latest industry news, interviews, technologies, and resources.

VIEW ALL News

[IMG: Community College Internship Program Celebrates Fourth Year of Student Research Achievement]

Jun 17, 2026 • Community College Internship • Education • Lauren Sgro • Unistellar • Uma Gorti • Douglas Caldwell • Pamela Harman • Simon Steel
[H5]
Community College Internship Program Celebrates Fourth Year of Student Research Achievement

[IMG: Planetary Picture of the Day - Week of June 8, 2026]

Jun 16, 2026 • PPOD
[H5]
Planetary Picture of the Day - Week of June 8, 2026

[IMG: Not Planets, Not Stars: NASA Volunteers Doubled the Number of Known Brown Dwarfs]

Jun 12, 2026 • Blog • Lauren Sgro
[H5]
Not Planets, Not Stars: NASA Volunteers Doubled the Number of Known Brown Dwarfs

[IMG: Community College Internship Program Celebrates Fourth Year of Student Research Achievement]

Jun 17, 2026 • Community College Internship • Education • Lauren Sgro • Unistellar • Uma Gorti • Douglas Caldwell • Pamela Harman • Simon Steel
[H5]
Community College Internship Program Celebrates Fourth Year of Student Research Achievement

[IMG: Planetary Picture of the Day - Week of June 8, 2026]

Jun 16, 2026 • PPOD
[H5]
Planetary Picture of the Day - Week of June 8, 2026

[IMG: Not Planets, Not Stars: NASA Volunteers Doubled the Number of Known Brown Dwarfs]

Jun 12, 2026 • Blog • Lauren Sgro
[H5]
Not Planets, Not Stars: NASA Volunteers Doubled the Number of Known Brown Dwarfs

VIEW ALL News

SETI Institute
[H2]
Postdoctoral Fellowship Opportunities

Learn More & Apply

About
[H2]
What We Do
At the SETI Institute, we explore the origins of life, the search for extraterrestrial intelligence, and the mysteries of the cosmos. Through cutting-edge research, education programs, and public outreach, we strive to expand humanity's understanding of the universe.

[H2]

[H4] Research
We lead scientific investigations into planetary environments, astrobiology, and the search for life beyond Earth.
[H4] Education
Our educational initiatives inspire and train the next generation of scientists, fostering curiosity and innovation.
[H4] Outreach
We engage the public with space science and discovery, making complex scientific ideas accessible and exciting for all.

Careers
[H2]
Searching for lifeis a team sport.Want to join?

Career Center

[H5] Research

[H2] Featured Projects

VIEW ALL

[IMG: Featured Image]

ATA • Radio Astronomy • Hat Creek Radio Observatory • HCRO

[H5] The Allen Telescope Array (ATA)

The ATA is the first radio telescope designed from the ground up to be used for SETI searches.
#ATA
#Radio Astronomy
#Hat Creek Radio Observatory
#HCRO

[IMG: Featured Image]

COSMIC • VLA

[H5] COSMIC at the Very Large Array

The Commensal Open-Source Multimode Interferometer Cluster (COSMIC) is a new commensal Ethernet-based digital signal processing backend and computer cluster on the VLA in New Mexico operated through the NRAO.
#COSMIC
#VLA

[IMG: Featured Image]

Discovery and Futures Lab

[H5] Discovery and Futures Lab

What happens if life beyond Earth is discovered? The Discovery and Futures Lab at the SETI Institute fosters novel and anticipatory research at the intersection of science, society, our planet, and the search for life beyond Earth.
#Discovery and Futures Lab

Mar 24, 2025
[H5]
The Allen Telescope Array (ATA)

The ATA is the first radio telescope designed from the ground up to be used for SETI searches.
#ATA
#Radio Astronomy
#Hat Creek Radio Observatory
#HCRO

Mar 24, 2025
[H5]
COSMIC at the Very Large Array

The Commensal Open-Source Multimode Interferometer Cluster (COSMIC) is a new commensal Ethernet-based digital signal processing backend and computer cluster on the VLA in New Mexico operated through the NRAO.
#COSMIC
#VLA

Apr 16, 2026
[H5]
Discovery and Futures Lab

What happens if life beyond Earth is discovered? The Discovery and Futures Lab at the SETI Institute fosters novel and anticipatory research at the intersection of science, society, our planet, and the search for life beyond Earth.
#Discovery and Futures Lab

VIEW ALL

Research
[H2]
Key ResearchConcepts
What is the Drake Equation and how is it used in our research?
How are we searching for life in the universe?
What will we do if we detect a signal?
How do we know the best places to look for life?

Video
[H2]
How SETI Designed A Telescope To Look For Extraterrestrial Civilizations
In 2025, we invited Scott Manley for a tour of the Allen Telescope Array in Hat Creek, CA. Take an in-depth look of the facilities and research happening at the Hat Creek Radio Observatory!

Watch on YouTube

Support Us
[H2]
Support theSETI Institute
Scientists are getting closer in their search for life beyond earth. But with limited federal funding for the search for extraterrestrial intelligence, supporters are the reason cutting-edge scientists can keep their eyes on the sky.

DONATE NOW

[H5] News

[H1] Beyond Disclosure Day: The Real-World Protocols
Read More

[H5] News

[H1] Disclosure Needs Data
Read More

[H5] News

[H1] SETI Institute Awards $1 Million in STRIDE Grants to Advance Astrobiology, Exoplanet Science, and Public Engagement
Read More

[H5] News

[H1] SETI Institute Looks for Signs of Technology in Interstellar Visitor 3I/ATLAS
Read More

Previous

Next

All News

More

Oops! Something went wrong.
We’re sorry, but something didn’t work as expected.

Oops! Nothing was found.
We’re sorry, but there are no results matching your search.

[IMG: loading]

Support Us
[H2]
Support theSETI Institute
Scientists are getting closer in their search for life beyond earth. But with limited federal funding for the search for extraterrestrial intelligence, supporters are the reason cutting-edge scientists can keep their eyes on the sky.

DONATE NOW

[H5] Events

[H1] SETI Live: Humanity's Plan for First Contact: Who Speaks for Earth?
Read More

[H5] Events

[H1] In-Depth Svalbard Cruise
Read More

[H5] Events

[H1] Take Two: Ori Gersht, Laurent Grasso, Dawn Ng, Erin O'Keefe
Read More

[H5] Events

[H1] Iceland to Greenland Cruise
Read More

Previous

Next

Show Previous Events

All Events

More

Oops! Something went wrong.
We’re sorry, but something didn’t work as expected.

Oops! Nothing was found.
We’re sorry, but there are no results matching your search.

[IMG: loading]

Support Us
[H2]
Support theSETI Institute
Scientists are getting closer in their search for life beyond earth. But with limited federal funding for the search for extraterrestrial intelligence, supporters are the reason cutting-edge scientists can keep their eyes on the sky.

DONATE NOW

SETI 101: Core Concepts & Projects
[H1]
The Drake Equation

[H2] At a Glance
What It Is: A probabilistic formula, devised by Dr. Frank Drake in 1961, to estimate the number of communicative civilizations in the Milky Way.
Purpose: Serves as a conceptual roadmap for astrobiology and the search for extraterrestrial intelligence (SETI).
Key Variables Include:- R*: Rate of star formation- fₚ: Fraction of stars with planets- nₑ: Planets per star capable of supporting life- fₗ, fᵢ, fc: Fractions relating to life, intelligence, and detectable communication- L: Longevity of civilizations
Why It Matters: Guides scientific inquiry into extraterrestrial life—driving research on exoplanets, astrobiology, and SETI efforts.
https://www.seti.org/research/seti-101/drake-equation/

The Drake Equation has been called the second most important equation in science. How many alien societies exist and are detectable? The Drake Equation is a step toward the answer. Frank Drake created the equation to be part of the agenda for a meeting of experts held in West Virginia in 1961 and it serves as a roadmap for astrobiology to this day.
It’s a probabilistic equation for estimating the number of extraterrestrial civilizations in the Milky Way galaxy with technology that can be detected by humans. It estimates N, the number of transmitting societies in the Milky Way galaxy. The terms are defined as follows:

[H2] N:
[H5] The number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable.
[H2] R*
[H5] The rate of formation of stars suitable for the development of intelligent life (number per year).
[H2] fp
[H5] The fraction of those stars with planetary systems.
[H2] ne
[H5] The number of planets, per solar system, with an environment suitable for life.
[H2] fl
[H5] The fraction of suitable planets on which life actually appears.
[H2] fi
[H5] The fraction of life bearing planets on which intelligent life emerges.
[H2] fc
[H5] The fraction of civilizations that develop a technology that produces detectable signs of their existence.
[H2] L
[H5] The average length of time such civilizations produce such signs (years).

[H2] Understanding the Drake Equation
This simple formulation is generally agreed to be the “second most-famous equation in science (after E= mc2),” and you can find it in nearly every astronomy textbook.
The Drake Equation was cooked up by astronomer Frank Drake in 1961 to serve as the agenda for the first meeting on the topic of SETI. In 1960, Drake had conducted a pioneering search for extraterrestrial signals – a several-week long effort he named Project Ozma. Somewhat unexpectedly, this modest experiment attracted a great deal of attention, and Drake was encouraged by J.P.T. Pearman, a staff officer at the National Academy of Sciences, to organize an informal gathering of accomplished researchers and engineers to discuss the prospects for finding a signal. Was listening for radio signals a worthy endeavor or not?
Approximately a dozen people attended this informal meeting, and they all were eminent. Among them was biochemist Melvin Calvin (who received a call during the meeting notifying him that he had just won the Nobel Prize), biologist Joshua Lederberg, physicist Philip Morrison, and planetary astronomer Carl Sagan, as well as Peter Pearman and self-invited guest Barney Oliver, a highly accomplished radio engineer. The conference took place at the Green Bank Observatory – the site of Project Ozma – in November 1961.
While planning the event, Drake chose to organize the discussion around a simple formula he concocted that estimated a critical number for SETI, namely the estimated count of transmitting worlds in the Galaxy. His equation is comprised of seven factors which, when multiplied together, yield the number of societies that are now broadcasting signals one might conceivably pick up. The factors are listed and defined above.
As Drake himself has noted, his simple formula can be likened to how you might estimate the number of students at a university. All you need to do is consider the number of new students (freshmen) entering each year and multiply that by the average number of years the students will spend at the school (four years.) Voila, you have a good estimate of the total number of undergraduate students.
The Drake Equation is constructed with similar logic. The first six terms, when multiplied together, yield the average number of new technologically transmitting societies that come on-line in the Milky Way galaxy each year. This “freshman” rate is then multiplied by the equation’s last term, L: the average lifetime they stay on the air. The result is N, the average number of transmitting societies in the Galaxy now. Clearly, if this number is very small, then the chances of a signal detection by SETI are also small. Conversely, a large value of N would be incentive to press the search.
At the time of the meeting, essentially none of the seven factors in the equation was known excepting the first, the production rate of stars. Nonetheless, the attendees bandied about their best guesses for the other terms, concluding that the “freshman” rate was on the order of one. In other words, new transmitting societies appear once a year somewhere in the Milky Way. All that remains is to multiply this by the lifetime of such a broadcasting civilization.
This last term, L, is obviously dependent on alien behavior. It’s not a factor we can quantify with studies in astronomy or biology. Our own experience also doesn’t help much. We’ve been transmitting on a widescale basis, and at frequencies and powers that might conceivably be picked up by someone in another solar system, for less than a century. How long will we continue to do this? Some people think that humanity is hellbent on self-annihilation, and the value of L for Homo sapienswill be merely a century or two. Others are less dramatic and more optimistic. But obviously we have little basis for estimating L.
Due to such uncertainties, estimates for N have ranged from 1 (Earth houses the only galactic society that is transmitting) to several million, Drake himself currently suggests that N = 10,000 (the consequence of assuming that new transmitting societies are produced at intervals of one per year and enjoy an average lifetime of 10,000 years).
It has been sixty years since the Drake Equation was conceived. Have we nailed down more of the terms than the single one known in 1961? Sadly, no. In fact, we’ve made little progress in this regard apart from the terms giving the fraction of new stars sporting planets, and (to a lesser degree) the average number of planets per solar system suitable for complex life. The attendees of the 1961 Green Bank meeting thought it likely that the former was close to 100 percent, and the latter was approximately one. Both guesses are within a factor of two or three of modern estimates, based on the discoveries of thousands of exoplanets since 1995.
It’s worth noting that many people have suggested amendments to the Drake Equation, adding terms to account for facts that don’t seem to be part of the original formula, such as colonization of other star systems by ambitious societies. Others have proffered changes to the math, replacing single terms with mathematical distributions. But according to Drake, none of these refinements is necessary nor do they alter the equation in any essential and substantive way.
While the Drake Equation cannot be “solved” or even accurately calculated, it retains considerable utility for discussions about extraterrestrial life and intelligence. And that, after all, was the reason for its invention. It’s also noteworthy that this famous formulation encompasses all the research activities of the SETI Institute, from our efforts to probe the harsh landscapes of Mars to our extremely high-tech searches for alien signals. It is the scaffolding upon which the Institute has been built.
-Seth Shostak, updated: July 2021.

[H3] Gain a deeper understanding of the Drake Equation—download our eBook, The Drake Equation
Sign up to receive our latest ebook delivered free directly to your inbox.