By Carl Sagan, The Cosmic Evangelist

When I was five years old, I went to the 1939 World's Fair in New York and saw the future. By the time I was ten, I had decided to spend my life answering one question: is there anyone else out there?

I spent the next fifty-two years working on that question. I helped design instruments for the Viking landers that searched for life on Mars. I co-founded the Planetary Society to advocate for the search. I defended SETI against ridicule when it was unfashionable to take seriously. I wrote Contact, a novel about what it would actually mean to hear a signal from another civilization.

I died in 1996 without an answer. But I died knowing the question was the right one. And I died knowing that the math was on our side.

The Drake Equation

In 1961, a radio astronomer named Frank Drake gathered ten scientists in Green Bank, West Virginia, to talk about searching for extraterrestrial intelligence. I was one of them. We called ourselves the Order of the Dolphin (because one of us studied dolphin communication, and if we could not understand dolphins, what chance did we have with aliens?).

Drake wrote an equation on the blackboard. It looked like this:

N = R x fp x ne x fl x fi x fc x L*

Each variable represents something we need to know:

• R* = How many stars form in our galaxy each year?
• fp = What fraction of those stars have planets?
• ne = Of those planets, how many could support life?
• fl = On how many of those does life actually arise?
• fi = How many develop intelligent life?
• fc = How many develop technology that sends detectable signals?
• L = How long do those civilizations last?

The equation is not a calculator. It is a way of organizing our ignorance. In 1961, we did not know the answer to a single one of those variables. We were guessing from the first term to the last.

What We Know Now

Sixty-five years later, the first three variables are no longer guesses.

R*: about one to three new stars form in the Milky Way each year. Measured.

fp: virtually every star has planets. The Kepler space telescope and its successors have confirmed over 5,000 exoplanets and estimated that there are more planets than stars in our galaxy. Not a guess. A census. There are roughly 200 billion stars in the Milky Way. There are at least as many planets.

ne: current estimates suggest that roughly one in five Sun-like stars has an Earth-sized planet in the habitable zone, the distance where liquid water could exist on the surface. That gives us approximately 20 billion potentially habitable worlds in our galaxy alone. Not the universe. One galaxy. There are two trillion galaxies.

The first three terms of the Drake Equation, the ones we used to guess, are now grounded in data. And the data is staggering. The raw material for life is everywhere.

The Great Silence

And yet.

Five thousand exoplanets confirmed. Twenty billion potentially habitable worlds in our galaxy. Instruments sensitive enough to detect a television broadcast from the nearest star system. The James Webb Space Telescope reading the atmospheres of worlds forty light-years away.

And we have heard nothing. Seen nothing. Detected no signal, no artifact, no beacon, no structure, no waste heat, no sign that anyone else is out there doing what we are doing.

This is what Enrico Fermi called the paradox that bears his name. If life is common, and intelligence is even modestly likely, and civilizations last for any significant time, then the galaxy should be teeming. Where is everybody?

Three Possible Answers

The silence permits three broad interpretations. Each one is profound. None is comforting.

ONE: We are early. The universe is 13.8 billion years old. Our Sun is 4.6 billion years old. It is possible that the conditions for complex life have only recently become widespread, that heavy elements needed for rocky planets and complex chemistry took billions of years to accumulate from successive generations of stellar explosions. We may be among the first civilizations to arise. The galaxy is not empty. It is young.

TWO: Life is common, intelligence is rare. Life may arise wherever the conditions permit. But the step from microbe to mind may be fantastically improbable. Earth had single-celled life for three billion years before anything multicellular appeared. The evolution of intelligence may require a sequence of accidents so unlikely that it has happened only once, here, in this galaxy. The universe may be full of bacteria and empty of conversation.

THREE: Civilizations do not last. This is the one that kept me up at night. Perhaps intelligence arises frequently. Perhaps civilizations develop technology frequently. And perhaps they destroy themselves before they can send a signal that lasts long enough to be detected. Nuclear weapons. Climate collapse. Engineered pandemics. Self-replicating machines that consume their creators' resources. The window between developing radio and developing the means of self-destruction may be terrifyingly narrow. And most civilizations may not make it through.

This is the variable L in the Drake Equation. The lifetime of a communicating civilization. Everything else can be large. If L is small, N is small. If civilizations routinely destroy themselves within a few centuries of developing technology, the galaxy could produce millions of civilizations and still be silent, because they are all dead before their signals reach us.

Why This Matters to You

You are reading this on a device that would not exist without the same physics that makes nuclear weapons possible. The same electromagnetic spectrum that carries your WiFi signal carries SETI's search for extraterrestrial intelligence. The same rocket technology that could send humans to Mars could deliver a warhead to any city on Earth.

We are in the window. Right now. The window between developing the power to destroy ourselves and developing the wisdom not to.

The Drake Equation does not tell us whether aliens exist. It tells us what KIND of universe we live in. And the answer depends, more than anything, on whether civilizations like ours can survive their own adolescence.

That is not an abstract question. It is the most practical question there is. Because the answer determines whether the night sky stays silent or, one day, a signal arrives that says: you are not alone, and you made it through.

Where to Look Next

Want to explore the data yourself?

The NASA Exoplanet Archive catalogs every confirmed exoplanet: exoplanetarchive.ipac.caltech.edu

The SETI Institute runs the most comprehensive search for extraterrestrial signals: seti.org

The UFORIC database contains over 160,000 reported sightings, archived and categorized since 1977. Most have mundane explanations. The ones that do not are the interesting ones.

The question remains open. The instruments are listening. And somewhere out there, the photons are still arriving.

Read Richard Feynman's take: What a Physicist Sees in the Numbers. The Drake Equation as honest uncertainty.

Read Buckminster Fuller's take: What an Architect Sees in the Silence. If Spaceship Earth is the only crewed vessel, the safety factor is zero.

"The universe is a pretty big place. If it's just us, seems like an awful waste of space."

Carl Sagan, The Cosmic Evangelist