For those who want to understand why we haven't found any space aliens, the Fermi Paradox is as popular as cheeseburgers. First proposed by physicist Enrico Fermi in 1950, this perennial head-scratcher rests on the idea that it would take only a few tens of millions of years for an advanced civilization to colonize the Milky Way — leaving their mark on every last star system in the galaxy.
So why hasn't some ambitious race of aliens done that? After all, the Milky Way is three times older than Earth, so they've had plenty of opportunity to finish the project. We should see outposts of someone's galactic empire in every direction. Why don't we?
As Fermi put it, "Where is everybody?"
A Russian physicist named A.A. Berezin recently addressed this cosmic conundrum in a short paper. He thinks he knows why we haven't espied aliens. Mind you, he's not the first. The Fermi Paradox has prompted dozens if not hundreds of explanations. One possibility is that colonizing the galaxy is simply too costly. Or maybe alien societies are out there, but we lack the instruments to find them. Others favor the idea that extraterrestrials find Homo sapiens inconsequential and juvenile — so they keep a low profile and avoid us.
Berezin suggests something else. He presumes that at some point in the 13.8 billion years since the Big Bang, an extraterrestrial civilization managed to develop the capability to travel between the stars. Soon thereafter, they embarked on a project to spread out. But as they — or their robot underlings — took over the galaxy, they eradicated everyone else. Some of this might have been inadvertent, in the same way that construction crews mindlessly obliterate ants.
Does this sound like a variation on Douglas Adams' "Hitchhiker's Guide to the Galaxy," in which Earth is unintentionally destroyed to make way for a hyperspace bypass? Well, it's the same basic idea. But unlike Adams' story, Berezin's doesn't make much sense. To begin with, it's unclear how this suggestion really differs from the original paradox. If some ancient society of Galactans took over our galaxy (and maybe all the nearby galaxies too — there's been time enough), why don't we see evidence of that?
By 200 A.D., the Roman Empire had infested nearly all the lands edging the Mediterranean. If you were living within the empire, you'd definitely know it — you could find fluted architecture just about everywhere. So if the Galactans have been all over the place, why don't we notice? In addition, these hypothesized alien colonists couldn't just sweep through the Milky Way once and leave it at that. A new species — such as Homo sapiens — might arise at any time, offering a new challenge to imperial dominance and forcing the Galactans to clean house again.
Keeping control of the galaxy would be an endless project, and one that couldn't be managed from some central "headquarters." Even at the speed of light, it takes tens of thousands of years to get from one random spot in the Milky Way to another. Compare that to the response time for Rome — the time between learning that there was trouble afoot and getting their armies in place to confront it. That was typically weeks, not tens of thousands of years.
Ask yourself: Would the Roman Empire have existed if the legions took centuries or more to trudge to Germania every time the troublesome Alemanni crossed the Rhine? Germania would cease being Roman before you could say "barbarian."
It seems clear that Galactans would have to adopt the Roman strategy: Station some defensive infrastructure throughout the Milky Way so it's possible to deal with problems quickly. Sounds easy, but it would present a difficult logistical problem. How do you adequately maintain and update such a massive network when travel times are measured in millennia?
Berezin's idea of how to resolve the puzzle presented by the Fermi Paradox seems neither more convincing nor more plausible than many of the others. It replaces one paradox with another by arguing that the galaxy is, indeed, inhabited everywhere by a pervasive culture that presumably sprang up billions of years ago but somehow manages to evade all our detection efforts.
The paradox continues to fuel many lunchtime conversations, which at least is a nice diversion from gossip or politics. But if we someday find a signal from space, Fermi's question will become nothing more than an historical curiosity — a bit of misplaced musing that confounded Homo sapiens for a few decades.
Meanwhile the aliens — and who could doubt they exist? — keep their own company.
Dr. Seth Shostak is senior astronomer at the SETI Institute in Mountain View, California and host of the "Big Picture Science" podcast.
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