Does all intelligent life face a Great Filter?

The Universe should be full of alien civilizations. Why can’t we find any?

“Where is everybody!?”

It was around 1950. UFO mania had recently ramped up across the world, with dozens of reported sightings of strange flying machines fueling rampant speculation regarding their origins.

The eminent physicist Enrico Fermi was visiting his colleagues at Los Alamos National Laboratory in New Mexico that summer, and the mealtime conversation turned to the subject of UFOs. Very quickly, the assembled physicists realized that if UFOs were alien machines, that meant it was possible to travel faster than the speed of light. Otherwise, those alien craft would have never made it here.

At first, Fermi boisterously participated in the conversation, offering his usual keen insights. But soon, he fell silent, withdrawing into his own ruminations. The conversation drifted to other subjects, but Fermi stayed quiet.

Sometime later, long after the group had largely forgotten about the issue of UFOs, Fermi sat up and blurted out: “But where is everybody!?”

Every scientist at that table immediately knew what he meant.

That central question—where is everybody?—is now known as the Fermi Paradox in his honor. And while he wasn't the first person to wonder about the nature of other intelligent civilizations, he was the first to give the idea a modern spin.

The paradox arises from a seemingly innocent line of rational thinking that leads to an incorrect conclusion. It goes like this. We’re not special. We live on just another rocky planet around a ho-hum star in an unspectacular arm of an average spiral galaxy. There’s nothing incredibly unique or exotic about our physical circumstances.

And here we are. Alive. Intelligent. (Almost) spacefaring.

Nature tends not to do things just once. If we’re here and on the cusp of exploding into space, and there are hundreds of billions of stars in the galaxy, trillions of galaxies in the Universe, and billions of years to play around with, the Universe should be teeming with alien civilizations. We should see evidence for them everywhere we look, the same way we see any other common process played out again and again in the cosmos.

And yet we’ve got nothing. No artificial signals in our radio receivers. No mega-engineering projects reshaping other solar systems. No artifacts embedded in our planet or any others. As far as we can tell, we are alone. To the limits of our observations, we are the only living creatures, the only intelligent civilization, to inhabit the cosmos.

So what gives? Something must be wrong with the chain of reasoning. Even though every step sounds sane and reasonable, it leads to a conclusion that flies in the face of the evidence. Hence, a paradox.

Doomsaying with Robin

Decades later, the economist Robin Hanson was mulling over Fermi’s paradox and came to an uncomfortable conclusion. Maybe we’re alone because essentially nobody ever makes it. Maybe there’s some unavoidable barrier between the origin of intelligent life and said life setting off to explore the galaxy.

The position of this Great Filter, as he named it, is critically important as we contemplate the future of humanity. If the Filter is behind us, in our past, then we are one of the few lucky ones to survive in an otherwise lonely universe. If it’s in front of us, in our future, then we likely do not have much time left as a species (at least, a spacefaring one).

The key point of the Great Filter argument is that Fermi’s Paradox is not really a paradox. Instead, there is a false assumption baked into the chain of reasoning. The assumption is that because life is probably common, intelligent, spacefaring life is also common. Take that assumption away and Fermi’s argument breaks down, resolving the paradox. But how do we square the claim that intelligent life is rare with the solid argument that there’s nothing special happening here on Earth?

It comes down to a game of numbers. Intelligent life doesn’t have to be outright impossible. In fact, it can’t be—we’re here, and we’ve already begun taking our first steps into space. It just has to be so deeply rare that we shouldn’t expect to see any evidence for it elsewhere, despite decades of searching. So the game is to start with the plausible assumption of abundant life, then find a way to whittle that down to as close to zero as we can get it.

Hanson pointed out that life requires many steps to reach spacefaring status—especially the kind of spacefaring status that would get you noticed by nascent astronomers across the Universe. First off, life needs a place to call home. As far as we know, that means a rocky world with lots of liquid water, a decent atmosphere, and a stable star.

Second, life has to… well, become alive. There’s some secret sauce that turns an odd collection of prebiotic compounds and molecules into self-reproducing structures that undergo Darwinian evolution.

Once established, life has to go through a series of steps where it increases in complexity. On the Earth, this progression likely started with simple reproductive molecules like RNA. Life then figured out how to encapsulate itself as single-celled microbes. It figured out sex and exploded into a variety of multicellular forms. Some billions of years later, some of that multicellular life figured out how to be reasonably smart and start using tools to manipulate and control its environment. In our case, the birds figured it out first, but then the primates took it to another level.

Those smart, tool-using creatures then conceived of machines to take them into the edge of space. The last step is for that space-faring species to really go for it, sending themselves or their robotic emissaries far and wide, colonizing every available corner of the galaxy, and if they are sufficiently motivated, the Universe. Or, if they’re not quite in the mood for galactic colonization, then at least making some other signs of their presence, like blasting out galaxy-wide radio transmissions, modifying every star they come across, or engaging in a fair bit of mega-engineering.

For the Great Filter to work, one or more of these steps must be incredibly hard. It could be just one step, a cataclysmic cliff that species have an impossible time getting around. Or it could be a series of Lesser Filters that, taken together, create a labyrinth of steps that species can’t escape. No matter what, though, getting to the galactic stage has to be hard. So hard that any wannabe starfarers get snuffed out in the cradle.

So where is the Filter? Is it early on, with the development of life itself? Is it somewhere in the middle, on the long march to intelligence? Is it at the end, when going from simple orbital jaunts leads to lengthy interstellar excursions?

Considering that our own species is right at the very edge, at the last stage before galactic explosion, the question of the Great Filter takes on an existential edge. Have we already gotten through it safely, or are we counting down to the ending of our species?

Waltzing with microbes

To decide on the location of the Great Filter (or collection of Lesser Filters), we don’t have a lot of evidence to go on. Just us, our evolutionary heritage, and our meager astronomical observations. But even those slim lines can give us some insights.

We know from observations that the basic ingredients of life are ridiculously common. The Universe is perfectly capable of producing oxygen, carbon, and water in great abundance and then combining them into basic biochemicals. And as we continue to expand our catalog of exoplanets, we’re beginning to learn that potentially life-bearing worlds are a dime a dozen. Heck, even our nearest neighbor star, Proxima Centauri, plays host to a small terrestrial world in its habitable zone.

So when it comes to the very first step on the list of potential roadblocks, we can probably cross that off the list. Homes for life, if not life itself, are very popular.

But what about the magic of abiogenesis? We only have one known example of this happening in the entire Universe, but there is something interesting about the timing. Life on Earth appeared pretty much as soon as it could, right after (astronomically speaking) the crust cooled and the oceans formed. So we can reasonably argue that once the conditions are right, life starts to do its thing.

The next series of steps, going from basic life that has just figured out how to reproduce to complex life that can launch itself into space, is a different story. Again, going from our one single example of life on Earth, we see that it took a really, really long time for that to happen.

In fact, humanity probably represents the Earth’s last shot. In just a few hundred million years, the Sun will grow too hot. Our oceans will boil, and we will turn into another Venus. So while life got started in Earth’s first chapter, intelligent life didn’t appear until its last.

So maybe that’s it. That’s the Great Filter: achieving intelligence. In that case, woohoo! We made it! Pop the champagne. We’re one of the extremely rare, lucky species that survived the Filter, and we have nothing but the stars in our future.

Flirting with disaster

Or not. We really don’t know. We only have evidence for life on one planet. If we ever see signs of microbes in the dust of Mars or buried under the ice sheets of the outer moons, that might be a hopeful sign that we’ve made it through—that life is common but intelligence is not.

But the Great Filter may not be done with us yet. For the foreseeable future, humanity lives balanced on the edge of a knife. To go and spread among the stars, we must develop the technology to acquire, store, and harness vast amounts of energy. But that same technological need carries with it existential risk; the same harnessed energies that can propel our species to the stars can grind us into the dust.

Already, we are haunted by the specters of nuclear warfare and uncontrollable climate change. It might be that neither would kill us completely (hopefully), but they would definitely put a long-term damper on our space ambitions.

The more we expand and establish ourselves on other worlds, the safer we’ll be, simply because of the numbers. At some stage, we’ll reach a tipping point, where the long-term survival of our species is all but guaranteed. The loss of a planet would be a tragedy, but it wouldn't be the end of our entire civilization. The path to getting there, however, is a treacherous one we’ll have to tread carefully.

Surviving with ourselves

But “everybody is going to die” isn’t necessarily the end result of the Great Filter, even if it does lie in our future. We see no evidence for any intelligent civilizations in the Universe. But that doesn’t mean they don’t exist. One possible resolution to Fermi’s Paradox is that aliens really are out there, but they're invisible to us.

Maybe the Universe could be teeming with advanced aliens who just… prefer to stay home. Maybe our current phase of accelerated technological growth is just that: a phase. It doesn’t necessarily end with a galaxy-spanning civilization. Not every culture on Earth, either throughout history or even right now, values constant expansion, consumption, exploration, and innovation. Maybe we’ll touch the edge of space and decide that our home planet is just fine, thank you very much. In that case, we shouldn’t expect to see evidence for aliens, because stay-at-homers don’t really make themselves noticeable.

Maybe colonizing space is exceptionally difficult, far more difficult than we could possibly imagine. It seems that the speed of light really is the ultimate limit, and no bit of clever physics can ever get around it. Other stars are painfully far away, and the energies needed to travel to them in any meaningful way are beyond even our wildest reckoning. We could work to advance our technologies for thousands of years and still barely make our presence known in the wider galactic scene.

In fact, the filter might simply be detection. A super-advanced civilization could persist for millions of years and spread to a bubble a hundred light-years across… and we may not have the technology to find it.

There are, of course, arguments that we should still see evidence for aliens somewhere. All it takes is one hyper-expansionist species to litter the whole galaxy with their technological detritus. Ultimately, given the current lack of evidence in any direction, the resolution to Fermi’s Paradox, and our response to the argument of the Great Filter, is one of personal preference.

If you think humanity is headed on a dark path, then you’re probably inclined to believe that the Filter is in our (near) future. If you think we have a shot at redemption and harmony, then you might think the Filter is safely behind us. If you look up at the night sky and don’t want to be alone in the Universe, you likely believe there’s some other resolution to Fermi’s famous question.

But no matter what, as we continue to scan the silent heavens above us, we are forced to ask the same question again and again. Where is everybody?

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