Is there some evidence suggesting that humans are the Galaxy’s only intelligent species? Enrico Fermi thought so – and he was a pretty smart guy. Could he be right?
In 1950, the famous physicist made a seemingly innocuous lunchtime remark that has caught and held the attention of every SETI researcher since. The utterance came while Fermi was discussing with his mealtime mates the reasonable possibility that many sophisticated societies populate the Galaxy. But somewhere between one sentence and the next, Fermi’s supple brain realized that if this was true, it implied something profound. If there are really a lot of alien societies, then some of them might have spread out.
Fermi grasped that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonize the entire Galaxy. Within a few tens of millions of years, every star system could be brought under the wing of empire. Tens of millions of years may sound like a long project, but in fact it’s quite short compared to the age of the Galaxy, which is roughly a thousand times more.
So what Fermi immediately recognized was that the aliens have had more than enough time to pepper the Galaxy with their presence. But looking around, we don’t see any clear indication that they’re out and about. We don’t see any obvious evidence of a galactic empire or a United Federation of Planets.
This prompted Fermi to ask what was (to him) an obvious question: “where is everybody?” In a galaxy assumed to be filled with clever beings, why don’t we see any? This dissonance is known as the Fermi Paradox.
A lot of folks have given this a lot of thought. The first thing they note is that the Fermi Paradox is a remarkably strong argument. You can quibble about the speed of alien spacecraft, whether it’s 1% the speed of light or 10% the speed of light. It doesn’t matter. You can argue about how long it would take for a new star colony to spawn colonies of its own. It still doesn’t matter. Any halfway reasonable assumptions about how fast colonization could take place still end up with time scales that are enormously shorter than the age of the Galaxy. It’s like discussing whether Spanish ships of the 16th century could heave along at two knots or twenty. Either way they could speedily colonize the Americas.
Consequently, scientists in and out of the SETI community have conjured up other arguments to deal with this conflict between the idea that aliens should be everywhere and the circumstance that we see them nowhere. In the 1980s, dozens of papers were published that considered technical and sociological explanations for why the aliens weren’t hanging out nearby.
One possible explanation is that interstellar travel is just too costly. Consider sending a small rocket to Proxima Centauri for example: Not a matchbook-size device as envisioned by the Breakthrough Starshot project, but a craft able to house a crew and all the necessities for sustaining them in flight. Maybe something the size of the Mayflower. If your intention is to get this modest interstellar ark to our nearest stellar neighbor in 50 years, you’ll require about 150 billion billion joules of energy. We’re not sure what aliens pay for energy, but here on Earth the going rate is about ten cents a kilowatt-hour. So the transportation bill per Pilgrim would be $40 billion.
That’s a lot of cash, a lot more than it takes to buy each emigrant a few thousand six-bedroom palaces and set them up for life. The fact that the trip is costly, in whatever currency, is reason enough to deter any alien society from trying to settle distant real estate. With far less expenditure, the extraterrestrials could pursue the good life at home.
But even if the aliens have hefty incomes that make colonization affordable, maybe they haven’t got the stamina to see it through. Subduing the Galaxy takes more than sending a ship full of restless nomads to the next star. The nomads have to settle that star, and then spawn pilgrims of their own. And those émigrés have to produce yet more settlers. And so on. If each and every colony eventually founds two daughter settlements (a pretty handsome accomplishment), then 38 generations of colonists are required to bring the entire Galaxy under control. Even the Polynesians, who swept across the western Pacific domesticating one island after another, didn’t manage this. Maybe the aliens can’t do it either.
An alternative suggestion that would explain our apparent solitude is that the Galaxy is urbanized, but we just happen to have the bad luck to reside in a dullsville suburb. Yet another proposal is that we’ve been singled out for special treatment: we are an exhibit for alien tourists or sociologists. Our world may be known to the extraterrestrials, but they observe us through a sophisticated type of one-way mirror.
These explanations, and a bushel-basket more, have been offered to deal with the Fermi Paradox. Any of them might be true. Nonetheless, there are scientists who deem them all too contrived and unlikely to work in every case. Will all the aliens really find colonization too costly? Will they all run out of empirical steam? Are we so special that someone has really gone to the trouble to put us in a cosmic zoo? While answers to the puzzle put forth by Fermi are as plentiful as crabgrass, we still have no idea which, if any, is correct. Perhaps the universe is teeming with subtle societies we can’t find, or haven’t found yet. Then again, maybe the explanation is simpler: we’re alone.
But keep this in mind: The Fermi Paradox is a very large extrapolation from a very local observation. You might just as well look out your window and conclude that bears, as a species, couldn’t possibly exist because you don’t see any. This, despite the fact that, in the history of North America, the bears have had plenty of time to shamble into your yard.
SETI experiments at least offer the promise of relegating the Fermi Paradox to the dustbin of historical curiosities by proving that other intelligence is out there. In science, speculation is essential, but experiment is definitive.