Silence. Complete, unnerving silence. Despite decades of searches for any form of life, intelligent or otherwise, out there in the cosmos, the Universe has but one message for us: No one is answering.
But that solitude is not a curse. The great expanse of the empty heavens above us does not carry with it an impossible burden of loneliness. It begets a freedom—a freedom to explore, to be curious, to wonder, to expand.
The Universe is ours for the taking.
The great silence
According to physics legend, in the 1950s, the great scientist Enrico Fermi put it bluntly during a casual conversation with a friend: “Where is everybody?”
The logic behind the question is simple. Modern cosmology is built on the Copernican principle, or what I call the "Principle of We’re Not Special." The Milky Way is an average, run-of-the-mill galaxy, one of hundreds of billions, if not trillions, in the observable volume of the cosmos. Our Sun is about as average as you can get for a star: middle-aged and middle-sized.
The Earth? OK, it's somewhat special. There’s liquid water on the surface and a nice—but not too chokingly thick—atmosphere. Other worlds in the Solar System boast liquid water, too—it’s just underground. And water is the most abundant chemical compound in the entire Universe, so we shouldn’t be that surprised that it gets to be liquid here and there.
But even given that the Earth is pretty good, we’re still not special. There’s nothing that’s obviously, triumphantly remarkable about the Earth, the appearance of life on it, or the eventual evolution of intelligent life. It happened here; it can happen anywhere. And given that the Universe is creeping on 14 billion years of age, life is bound to have arisen elsewhere.
But all those billions of years is more than enough time for some civilization to become extremely technically competent and send either themselves or their robotic emissaries throughout the galaxy, exploring if not outright colonizing every planet they wish. It’s not like the Milky Way is that big. It’s just 100,000 light-years across, so billions of years is plenty of time for someone to explore every little nook and cranny, even if they have to do it the slow way. Given these assumptions, evidence for alien civilizations should be obvious and manifest.
So we have a paradox: Where is everybody?
Search patterns
One answer is that we haven’t looked hard enough. Obviously, intelligent life isn’t super-duper common, considering that we’re the only intelligent critters to arise in our own Solar System, and not every planet around every star will have the right conditions for life. So if intelligent civilizations aren’t going to come calling, maybe we need to actively hunt for them.
In response to Fermi’s paradox and at the urging of several prominent scientists like radio astronomy pioneer Frank Drake, SETI was born: the Search for Extraterrestrial Intelligence. The thinking behind SETI is that while intelligent life may be relatively rare in the cosmos, it would be exceptionally loud. Consider our own species as an example. As soon as we figured out the basics of electromagnetism and hit upon the concept of using radio waves to transmit information, we started blasting, generating radio emissions powerful enough to encircle the globe. And those radio emissions were truly omnidirectional, meaning that for every Earth-to-Earth transmission we generate, some of those radio waves make their way out into the vastness of space.
If we lowly humans, who just climbed the first rung or two of technological progression, figured out how to signal our existence to the wider Universe, imagine what an older, more sophisticated civilization could accomplish. All we have to do is start looking and listening, and we should eventually see evidence of their existence.
Most SETI searches focus on radio signals, because (a) radio astronomy was the hot new thing when SETI was getting started, (b) radio waves are really good at sailing through interstellar dust clouds, which tend to scatter and absorb just about everything else, and (c) radio makes sense to us as a deep-space means of communication because you can get decent, broadly directional signals for not a lot of energy input.
SETI searches began in earnest in the 1960s. Teams of professional and amateur astronomers have spent tens of millions of dollars scanning the radio skies, looking for any signal that might be interpreted as artificial. On top of that, other astronomers have gone looking for technosignatures, signs of mega-engineering projects like Dyson spheres or intentional alterations of stellar chemistry.
And after six decades of searches looking for any technological hint of an alien civilization, we’ve come up empty.
The dispirit of radio
To be fair, we’ve barely scratched the surface of potential signals, something that SETI proponents point out when faced with the string of null results. The search space is huge, comprising all the radio frequencies available across the entire sky, throughout the depth of the galaxy, at all moments in time. If some alien civilization collected all their resources to build a mega-transmitter that blasted out a great big "HOWDY" that could be detectable throughout the entire galaxy, but they only managed to do it once, we likely missed it entirely. Even if another civilization generated a series of signals at a frequency that made sense to them, our telescopes could have been tuned to a different range when those signals washed over us. If so, we’re out of luck.
Our own mastery of radio has only come relatively recently, especially compared to the depths of galactic time available to other civilizations. And our search has been pretty limited. Mainstream astronomy generally looks down on the practice as a waste of time, so SETI efforts largely rely on private funding. As a result, we have scanned only the tiniest fraction of where alien signals might be hiding. So other civilizations might well and truly be out there, thriving and abundant, but our searches are too underfunded, too unfocused, and too unsophisticated to catch them.
And there’s now reason to think that radio might not be the best place to look. Less than a century in, and humanity is already moving away from globe-spanning blasts. We’re starting to prefer lower power, more localized forms of wireless communications. We’re also introducing a lot of encryption, which tends to make signals look very noisy. An alien civilization may not pick up on that, mistaking it for mere galactic fuzz. Radio transmissions seem sexy to us, but they may be as old-hat as stone hand axes to sufficiently ancient alien civilizations. So maybe the search space is even larger, encompassing modes of communication that we can’t even fathom because we do not yet possess the necessary grasp of the fundamental physics.
The Great Filter
Another, more troubling explanation is that something catastrophic happens to intelligent civilizations before they can reach the great galactic kumbaya stage. Known generally as the Great Filter, many hypothesized means of intelligence-ending cataclysms exist.
Perhaps intelligent life is just really hard to get to in the first place. Consider the Earth as an example. Life appeared pretty much as soon as it could, right when the oceans formed and our planet stopped being a molten hellscape. That might have been as early as 3.7 billion years ago. But intelligent life appeared basically yesterday—what we identify as anatomically modern humans arose about 120,000 years ago. In about 500 million years, our Sun will become too hot to sustain liquid water on our planet. The oceans will boil and the atmosphere will evaporate.
In other words, life arose at the very first opportunity that it could, less than 500 million years after the formation of the planet itself. But intelligence only came around with less than 500 million years to spare. It’s only one data point, but it’s a potentially informative one: Life is common, but intelligence is not.
And then there’s the whole matter of nuclear bombs and climate change. Again, we only have this one data point to work with, but it appears that technologically advanced civilizations have the ability, if not the propensity, to kill themselves and/or their homeworld. We exist in a very fragile time, simultaneously developing the tools to become an interplanetary species but not yet sophisticated enough to guarantee our own survival.
Perhaps intelligent civilizations just wipe themselves out with alarming frequency.
And then there are the cosmic accidents. Asteroids. Volcanic eruptions. Natural climate change. Massive solar flares. Supernovas. Gamma-ray bursts. The galaxy is a dangerous place, unfriendly to life and especially unfriendly to life that can’t get its day started without a triple vanilla cold brew with cold foam.
Ultimately, we don’t know. We don’t know how common life is. We don’t know how common intelligence is. Attempts to quantify our uncertainty, such as the Drake equation, are just window dressing for our ignorance: We don’t know. And until we find somebody—anybody—we can’t know.
The great emptiness
There’s another way to interpret this silence, and it’s my personal preference. The Universe is just way too big. Even our galaxy—all 100,000 light-years of it—is just far bigger than our puny human brains are capable of dealing with.
Consider this. Let’s pretend there’s an intelligent, advanced alien civilization on Proxima b, the nearest known exoplanet, sitting just a few light-years down the road. Our most powerful radio broadcasts have been washing over that system for decades. But even at that relatively minuscule distance, our radio signals are so weak that they are essentially indistinguishable from noise. There’s almost no way to tease out our artificially generated signals from the general background radio hum of the galaxy at large.
Or consider this. Our most distant spacecraft is the Voyager 1 probe. Launched in 1977, it’s now hurtling through the void of interstellar space at over 38,000 miles per hour. If it were aimed at Proxima b (and it’s not), it would reach that system in…75,000 years.
This is the pinnacle of our civilization’s greatest technological triumphs: the ability to communicate across the globe and send mechanical probes into the reaches of space. This is the stuff that our ancestors could only fantasize about. But compared to the enormity of truly galactic scales, we might as well not even be here.
Even incredibly advanced civilizations may not make more than a minor dent in the Milky Way. The scales, both in time and space, are just too great. A civilization that endures for millions of years is just a blink of an eye compared to the age of our galaxy. A species that expands and explores dozens, even hundreds, of systems doesn’t even register compared to the hundreds of billions of stars that call our galaxy home.
There’s no need for filters. The great machines of time and space simply do their work. Our cosmic insignificance is the only barrier we need to explain Fermi’s great puzzle. We’re not equipped to deal with the astronomically large numbers that our galaxy casually throws around, so what appears at first glance to be a paradox is really our inability to handle truly cosmic scales. Our galaxy could be teeming with life. There could be dozens, hundreds, or even thousands of intelligent species in our galaxy right now, but the vast gulfs of nothingness that surround them make us interstellar islands.
We can imagine all sorts of technological leaps and fantastic physics that would render these extreme divides of time and space insignificant. But just because we can imagine them doesn't mean the Universe permits them. Maybe the limitation of the speed of light really is the iron law that it appears to be. Maybe there is a limit to how much energy a civilization can collect, harness, and put to fruitful means. Maybe what we know is all we’ll ever know.
That’s even assuming that technology continues to advance and that “advancement” is a good thing. The extreme growth of technology has only been a feature of our species for, what, a hundred years? Who says it has to continue like this forever? And we’re kind of hurting our planet in our quest for more sophistication, so what’s stopping our descendants from deciding that this whole thing was a big mistake?
No filters. No maliciousness. No cosmic accidents. Just space and time and energy. Perhaps other civilizations are out there, staring into their night sky, minds full of curiosity and wonder, but they, too, are effectively alone.
The great freedom
What do we do in an effectively empty galaxy? Well, with great power comes great responsibility. There won’t be anybody to come save us from our own destruction. If we want to survive as a species, we’ll have to figure out climate change and civilization-ending weaponry on our own. We’ll have to navigate a galaxy fraught with hostile planets and rampaging stars without any outside help. There will be no gifts of seemingly magical technology. No secrets of the cosmos except what we divine ourselves.
But alongside that responsibility comes a certain liberation. An effectively empty galaxy is, frankly, pretty great. Yes, aliens may be out there somewhere, but they are so far removed from us in time and space that we are effectively alone. So there’s no need to worry about invasions (which is frankly silly anyway, as there’s literally nothing on the Earth that can’t be found in a million other places for far less energy expenditure). We don’t have to worry about interstellar wars over space or resources. There is plenty of space and plenty of stuff to go around for everybody. We can grow, expand, and explore as much as our hearts desire without worrying about any friction.
And despite the lack of success of SETI, we can still SETL: Search for Extraterrestrial Life. We can hunt for biosignatures in the atmospheres of exoplanets or search for microbial material in the worlds of the Solar System. While we can’t exactly have a heart-to-heart with a single-celled alien critter, we can at least take comfort in the fact that life in the galaxy is hardy and robust.
By all indications, intelligent life is exceedingly rare—we may well be alone in our galaxy. But that rarity is counterbalanced by the unimaginable enormity of the number of galaxies, stars, and planets in the Universe (not to mention all the imaginable and unimaginable forms that life may take).
We can look out on a clear dark night and watch as thousands of stars dance across the heavens. We can take comfort in the fact that we are not truly alone, that by all reasonable likelihoods there are other intelligent civilizations out there, maybe like us and maybe not, perhaps just as curious as we are. But the magnitude of space and time that separates us means that we won’t ever have to meet them. For all intents and purposes, every star we see is our own.
Perhaps Fermi was wrong. We are special! There is something unique about our star, our planet, and our circumstances. That there is no other nearby place in the Universe where a collection of chemicals has arranged themselves in such a fashion as to call themselves conscious beings. We are remarkable, and we are precious. And that cosmic emptiness calls us to be responsible stewards of our home planet, and it urges us to continue to explore its mysteries.
