Sixty years ago, the physicist Enrico Fermi famously asked his lunch companions at the Los Alamos National Laboratory, "Where is everybody?" They had been discussing the odds of finding signs of extraterrestrial life, when a paradox occurred to Fermi: On the one hand, many technologically advanced ET civilizations should exist, given the size and age of the Milky Way galaxy; on the other, there's no evidence of them.
Our galaxy contains somewhere between 100 and 400 billion stars. The best estimate for the number of earth-like planets in the galaxy is at least 100 billion, based on models of stellar evolution and analysis of the several hundred extra-solar planets found in the past two decades. With numbers like these, you need only a tiny fraction of benign planets to spawn life that evolves to intelligence to end up with a galaxy thick with intelligent beings.
And not just intelligent-as-we-know-it, either, but beings, on average, far more advanced than us. Consider the timing. Earth formed 4.6 Gyr (Giga-year = 1,000,000,000 years) ago, rocks began to appear 3.9 Gyr ago, and life emerged a few million years after that. The Milky Way is about three times the age of the Earth, so most stars and planets are much older than the sun and Earth -- an average Earth-like planet in our galaxy is 1.6 Gyr older than Earth, according to one well-reasoned estimate. In other words, if intelligent life does emerge from time to time on Earth-like planets, the odds are that they're way more advanced than we are.
That's a big "if." One response to the paradox is to assume that Earth is special -- that life is rare, and intelligent life even rarer. The numbers suggest otherwise, however. With 100 billion Earth-like planets in our galaxy, most of them much older than our own, you'd think -- wouldn't you? -- that at least some of them would have birthed curious, expansionary, radio-using civilizations. (And I'm confining the discussion to the Milky Way, our local galaxy -- throw in a few gazillion other galaxies and the numbers are, as they say, astronomical.)
The Fermi Paradox, then, lays out the likelihood of a bustling, life-filled galaxy and asks, "Why haven't we heard from them?" or for that matter, "Why haven't they colonized us?" -- since it wouldn't take more than a few million years to colonize the entire galaxy, by most estimates. That's a mere blip in the lifetime of the Milky Way.
It's easy to come up with answers: They consider us too primitive to worry about; they've quarantined us; they live in oceans and aren't aware of the night sky; they don't use radio; they don't want to be found; they destroyed themselves through warfare or ecological collapse; they prefer a sensuous life of virtual reality on their home planet ... the list goes on. I bet you can come up with a few.
The problem with all the "they" responses is that we're talking about an awful of "theys." How come none of the putative intelligent species out there have given themselves away? We don't see one anomalous star as evidence of stellar engineering (such as you might expect from an advanced civilization needing to capture vast amounts of energy from its parent star). We haven't detected a single unambiguous "LGM" (Little Green Men) radio signal. Not one tale of alien sighting, or artifact, or abduction, has gotten beyond the criteria for hard evidence demanded by the National Enquirer. If civilizations like ours were all over the galaxy, as the numbers suggest, we could not but be aware of them: At the very least, we should be awash in radio waves from ETs.
As the search for extraterrestrial intelligence (SETI) continues to draw a blank, the Great Silence only grows louder. A paradox like Fermi's tells us our assumptions are wrong. But which ones?
Barry Evans' (firstname.lastname@example.org) gets lonely at night wishing he could chat with more than one species. He lives in Old Town Eureka.
CAPTION: The three-minute "Arecibo message" consists of 1679 binary bits of information that, when laid out in a 23 x 73 grid, presents a highly stylized picture of the elements and form of DNA, a human figure, the solar system, the Arecibo radio telescope, and much else. The tongue-in-cheek (sort of) message was transmitted to -- who knows? -- by the telescope in 1974. (Color added for clarity.)