It was summer, 1950.
Hidden from the blazing Los Alamos sun, a group of America’s finest minds sat in a canteen, deep in conversation.
Just seven years earlier, some of these men had built the first atomic weapon at this very site. This time, they’d returned to make something even-more powerful: a hydrogen bomb.
But it wasn’t nuclear yield those gods of science were discussing that lunchtime… but a simple cartoon.
A strip in that week’s New Yorker showed two aliens flying away with some NYC trashcans – a reference to both a recent story about missing trashcans in the Big Apple, and the flying saucer craze then gripping America.
Now the men were discussing how likely it was such aliens really existed. Given the size and age of the universe, there had to be little green men out there somewhere, right?
It was at this point that Enrico Fermi spoke up.
A mild-looking Italian, Fermi was one of the brightest men alive. He’d designed the first nuclear reactor, been key to the Manhattan Project.
And he had a question for his fellow brainboxes. One that would still haunt humankind three quarters of a century later.
“Don’t you ever wonder,” he asked, “where everybody is?”.
Welcome to the Fermi Paradox. A simple question with answers so profound, they could change everything we know about the universe.
When you look up at the sky on a clear night, you’re probably struck by the sheer number of stars out there.
Yet what you can see from Earth is only the tiniest fraction of our galaxy’s true size.
It’s estimated the Milky Way is home to anywhere between 100 and 400 billion stars, at least 5% of which are thought to be like our own sun.
If fewer than a quarter of these sun-like stars are orbited by Earth-like planets, that shakes out to over a billion habitable worlds.
Given so many planets, and the 10 billion plus years our galaxy has existed, it seems likely intelligent life should’ve arisen on countless worlds by now.
Yet train our most-powerful telescopes on the night sky, and you won’t see distant Dyson spheres, or hear the chatter of alien radio hosts.
Instead, you’ll hear nothing. A nothing so profound, it’s been called the Great Silence.
As far as we can tell, humankind is hurtling through the void of space on its pale blue dot, alone in all the cosmos.
For astrophysicist Michael Hart, this was the basis for something he called Fact A.
One of the first people to seriously work on the Fermi Paradox, Hart realized that even if just a tiny fraction of habitable worlds give rise to life, and even if just a fraction of that life became intelligent, aliens should’ve colonized the galaxy by now.
Since aliens don’t walk among us today, handing out spare copies of To Serve Man, they therefore must not exist.
If that’s true, then all we can try and do is figure out why. Why are humans all alone?
The potential answers all share one thing in common. They’re existentially terrifying.
The most-popular – and likely most-depressing – is something called the Great Filter.
First proposed in 1996, the Great Filter basically does for evolving species what a speeding windshield does for june bugs.
The idea is that there’s something on the evolutionary path that almost no species ever clears.
This something could be abiogenesis – the creation of life itself, something we’ve only ever witnessed here on Earth.
Or it could be that extremely simple Prokaryotic life is common, but becoming more-complex is the one in a billion-billion chance.
Or maybe the galaxy is stuffed with all sorts of weird-ass animals, and it’s the jump to intelligence that’s the june bug smashing barrier.
Of course, all these examples assume the Great Filter is something humanity already cleared. That we’ve already won the universe’s impossible lottery.
But there’s nothing to say this barrier doesn’t lie ahead of us. That countless alien civilizations haven’t reached our level before, only to be wiped out in an unavoidable cataclysm.
In that case, humanity is no more special than a bug blissfully buzzing towards the oncoming windshield, convinced it’ll survive where all its brethren have gone splat.
If it’s in our future, this filter could be anything.
Maybe, as any given species expands its technological knowledge, the chance of it wiping itself out rises to nearly 100%.
To return to our opening, Enrico Fermi was at Los Alamos working on the hydrogen bomb because he hoped to prove such a destructive weapon was physically impossible.
When Ivy Mike successfully detonated in 1952, he glumly wondered if it might not be the answer to the paradox he’d posed two years earlier.
Yet the Great Filter doesn’t necessarily come from within.
Maybe every now and then our galaxy simply presses the reset switch, wiping out any intelligences to have arisen.
One mechanism could be outbursting, the black hole equivalent of you burping at the end of a meal – only this time your burp is more powerful than multiple supernovas.
We’ve detected outbursting events from supermassive black holes that have laid waste to entire galactic clusters.
If our galaxy’s own supermassive black hole gave a big enough belch often enough, it could effectively wipe the slate clean.
OK, so far, we’ve looked at scenarios that assume humankind really is all alone.
But how likely actually is that?
The Drake Equation – first formulated in 1961 – was an early attempt to put real numbers onto the Fermi Paradox.
By estimating things like number of habitable worlds, number of species that become intelligent, and length the average civilization lasts, it lets us see how likely it is that we share the Milky Way with others.
Turns out, it’s pretty damn likely.
Even using super-conservative figures, and giving spacefaring civilizations a maximum lifespan of a hundred years, the Drake Equation suggests there should be at least ten other advanced species in the galaxy right now.
So why don’t we hear from them?
It’s here we tiptoe into sci-fi territory.
Theories that assume aliens exist but we haven’t yet seen them tend to come in two flavors: plain weird, or flat-out horrifying.
It’s into the latter category that the predator theories fall.
Popularized by Cixin Liu in his novel of the same name, the Dark Forest theory compares the universe to a forest in the dead of night, with every species a hunter tiptoeing through.
The hunter’s goal is to not draw attention to himself, lest that result in a predator attacking him.
But it’s also to play the predator himself. To kill anyone who exposes their location to him, lest they turn out to be a threat.
In this scenario, the galaxy is silent because every other civilization is hunkered down in the dark, ready to shoot anyone who makes a noise and draws the predators closer.
And here’s humankind, beaming out radio signals like an overweight guy crashing through the undergrowth in tiger country, shouting “here, kitty, kitty…”
A more succinct version of this is the Superpredator Theory.
Many aeons ago, one civilization in the Milky Way reached its full potential before any others.
Realizing letting another species reach its level would be an existential threat, this first civilization started destroying all others to reach a certain technological threshold.
That means we don’t hear anyone because there’s only one other advanced society to hear. And they’ll annihilate us the moment we live long enough to challenge their dominance.
See what we mean about horrifying?
Into the plain weird category fall most of the other major theories.
Theories that say we’re the galactic equivalent of those isolated human tribes it’s forbidden to contact. That we’re living in a computer simulation into which aliens were never programmed.
That the aliens are living in a computer simulation, one that’s a digital paradise compared to the pain and misery of the physical world.
But, in the rest of this video, we’re going to zero in on one relatively new theory: the Aurora Effect.
For our money, it may be the most-fascinating explanation of all.
In 2019, a team including NASA scientist Caleb Scharf published the results of a computer simulation they’d designed.
Using a 3D model of a starfield that moves as our galaxy does, they tested how alien civilizations might spread if a portion of so-called habitable worlds were not actually settleable.
In other words, if planets that could technically support life frequently had something about them that made it too expensive or difficult to maintain a colony.
What they found had some exciting implications.
Because stars – at a local level – move like particles in a gas, their positions are far from fixed.
Just in our own local neighborhood, the small red dwarf star Ross 248 is expected to overtake Proxima Centauri as our nearest companion in 37,000 years.
That means the routes for any civilization to expand along are constantly shifting; as settleable planets worth colonizing bunch together, or drift too far away to be explored.
Therefore certain clusters of desirable worlds would statistically wind up coming together and being colonized again and again by successive waves of aliens.
For native inhabitants of these planets, interactions with aliens would be a fact of life. For them, their Fact A would be completely the opposite of Hart’s.
Other worlds, though, would fall out of favor as they drifted away. The time between waves of colonization would stretch out, until they halted all together.
Eventually, a civilization might arise on one of these forgotten worlds. A civilization that one day produced a physicist who asked “don’t you ever wonder where everybody is?”.
In this solution, Earth isn’t some fluke world no alien species has ever visited.
It could be that short-lived colonies were established here tens of millions of years ago, colonies that were dismantled as our world drifted too far from the settleable clusters.
At such time scales, scientists like Adam Frank and Gavin Schmidt have shown no traces of such a colony would remain – bar some chemical anomalies we’d have to be pretty lucky to stumble across.
Humankind, then, may have simply arisen at a time when no-one is interested in settling our world; like one of those Pacific islands settled and abandoned in waves by ancient polynesians.
Finding signs of intelligent life, in that case, might be as simple as identifying likely clusters of settleable planets and listening in.
So that’s the Fermi Paradox explored in a nutshell: one of the enduring mysteries of our galaxy. If life is out there, why haven’t we found it yet? And what might that mean?
To answer that, we’ll leave you with the words of the great science fiction author, Arthur C. Clarke:
“Two possibilities exist: Either we are alone in the universe or we are not.
Both are equally terrifying.”
Fun, in-depth essay from Wait But Why: https://waitbutwhy.com/2014/05/fermi-paradox.html
Scientific American, why isolated worlds may not be rare in a Milky Way full of life: https://www.scientificamerican.com/article/alone-in-a-crowded-milky-way/
Scientific American, the Fermi Paradox is not Fermi’s: https://blogs.scientificamerican.com/guest-blog/the-fermi-paradox-is-not-fermi-s-and-it-is-not-a-paradox/
Britannica, the Fermi Paradox and Drake Equation: https://www.britannica.com/story/the-fermi-paradox-where-are-all-the-aliens
NY Times, why aliens must be out there: https://www.nytimes.com/2021/02/11/opinion/aliens-extraterrestrial-life.html
Sky at Night, the Fermi Paradox: https://www.skyatnightmagazine.com/space-science/fermi-paradox-search-extra-terrestrial-intelligence/
Atlantic on the digital alien answer: https://www.theatlantic.com/technology/archive/2018/06/but-seriously-where-is-everybody/563498/
Universe Today, the Great Filter: https://www.universetoday.com/145512/beyond-the-fermi-paradox-iii-what-is-the-great-filter/
Supermassive black hole outbursts: https://www.nytimes.com/2020/03/06/science/black-hole-cosmos-astrophysics.html
The last milky way outburst: https://astronomy.com/news/2019/10/the-milky-ways-supermassive-black-hole-erupted-with-a-violent-flare-a-few-million-years-ago Biographics on Enrico Fermi: https://www.youtube.com/watch?v=-TlFyLqlyLo