How the Fermi Paradox Works

What's the Paradox, Fermi?

enrico fermi
The Fermi Paradox has evolved way beyond its namesake's original question. CORBIS/Corbis via Getty Images

The story goes that in 1950, famous physicist Enrico Fermi was enjoying a pleasant luncheon with some fellow geniuses in the Los Alamos Jet Propulsion Lab cafeteria while idly flipping through a "New Yorker" magazine. Between bites of Waldorf salad (or possibly a fluffernutter sandwich), Fermi pointed to a cartoon of aliens unloading some New York City garbage bins they'd collected from a foray to earth. Casually, Fermi asked, "Where is everybody?"

What he was actually referring to, according to his colleagues, was the question of whether interstellar travel was at all possible [source: Gray]. At the time we hadn't even managed to leave our own atmosphere, and the moon landing was still 19 years off, so it was a fair question. Actually, it still is. We might be talking about sending a manned mission to Mars in the coming decades, but that's child's play next to visiting other solar systems.


Using current rocket technology, we should be able to get to Mars in about six months. By contrast, the nearest star, Proxima Centauri, is 4.25 light years away. That doesn't sound so bad, except that a light year is, as the name suggests, how far light can travel in a year, and even when we hit the turbo boost in our fastest rockets, we're crawling compared to that. Hustling at our top speed, it would take us 73,000 years to go next door, cosmically speaking [source: NASA].

Anyway, that's what Fermi was evidently getting at with his offhand lunchtime remark. But as the years elapsed, his question evolved as it was filtered through other scientists' ideas. In 1975, the astronomer Michael Hart alleged that the reason there aren't any aliens here is because they don't exist. If they did, he reasoned, they inevitably would have colonized the galaxy by now. Then, in 1977, an astrophysicist named David G. Stephenson said that Hart's statement could answer Fermi's question, which he officially dubbed "Fermi's Paradox." The Fermi Paradox as it's known today goes something like this: Our universe could, quite possibly, have billions of Earth-like planets teeming with intelligent life. If that's true, how come we haven't heard or seen a single, solitary speck of evidence of said life? [source: Gray]

Even if Enrico Fermi didn't actually pose this question, it's still an interesting one, and there are loads of possible answers. When the question is asked, usually something called the Drake equation is invoked. In the 1960s, an American astronomer named Frank Drake came up with an equation that would help us calculate how many alien civilizations there might be in our galaxy. The results of the equation can vary according to the numbers you plug in, but, by even the most skeptical estimates, our galaxy alone probably has at least 2 billion habitable planets. By "habitable," astronomers mean planets in the so-called "Goldilocks zone" — not too big, not too small, not too close to their star, not too far away from it, but juuuust right.

Of course, just because they're habitable doesn't necessarily mean they're inhabited. Life might or might not be likely under the right conditions. We just don't know. Let's say it's not, let's say it's extremely rare. In fact, let's say only one-half of 1 percent of suitable orbs feature some kind of life form — that's still 100 million planets!

Of course, the next question is, how many of those potentially life-bearing planets evolve species capable of developing the technology necessary for communication and travel? This is a hotly contested question — are techno-capable species an inevitable outgrowth of evolution? Or are Earth's humans unique? Let's say, for the sake of argument, that the answer lies somewhere in between — species like our own are uncommon but not unlikely. Even if there's only one-half of 1 percent chance of life evolving technologically savvy populations, that would mean there should be 500,000 other civilizations in our galaxy alone. And if you multiply that number by the quantity of galaxies thought to be spinning around in the known universe (about 150 billion) you get a whole lot of smart aliens [source: BBC]. So, as Fermi said, where is everybody?