“If the Universe Is Teeming with Aliens… Where Is Everybody?” –Stephen Webb
It’s one of the biggest conundrums in the Universe, known as the Fermi Paradox: if the Universe is so conducive to life, and if there are so many opportunities for it within our galaxy alone, why isn’t there any evidence (outside of the History Channel) of extraterrestrial life?
Moreover, why haven’t we been visited by some extraterrestrial intelligence? After all, given the fact that our Universe is nearly 14 billion years old, while our galaxy itself is only a hundred-thousand light years from end-to-end, shouldn’t all of the potentially habitable planets have been visited, and colonized, by now?
A couple of years ago, I went through the estimates — both conservatively and liberally — of how rare or common intelligent life in the Universe is. But what I wanted to focus on, today, are the difficulties in even intelligently speaking about this question. First off, let’s start with the good news.
The good news is, we’re here. That means we have at least one example in the Universe where things worked out in favor of intelligent life. As far as we can tell, this means the following things have happened:
- A star was born with a planet that orbits it at the right distance for it to be potentially habitable.
- That planet had the right mix of elements on it — particularly carbon, nitrogen, oxygen, hydrogen and phosphorus — in order to create life-as-we-know-it.
- Life actually begun, at some point. This means, at the very least, a self-replicating complex molecule encoded with information that isn’t necessarily fixed from one iteration to the next, found a way to replicate itself. (The “information-encoded” clause distinguishes DNA/RNA-based life from, say, a non-living crystal.)
- Life succeeded for a long enough time that it evolved not only beyond this simple, primitive state, but into complex, multi-cellular, highly differentiated organisms.
- At least one of these organisms developed what we consider “intelligence” to be, and used it to learn about — and in some ways, master and manipulate — their environment.
- Eventually, before going extinct, these organisms — and it’s the great hope our our civilization — managed to leave their own world, and set out to explore, and possibly colonize and/or inhabit some of the other ones in our galaxy.
Now, there’s no doubt that all but the last step, above, has already happened here on Earth. And there are huge contingents of our society striving to achieve that last one, which — despite everything — I’m still extremely optimistic about our chances of successfully accomplishing.
But the big question is this: how common should this occurrence actually be? Should there really be tens-of-thousands of civilizations just like (or even more advanced than) us out there right now, in our galaxy alone? Or are we the only ones like us, in the entire history of the observable Universe, who’ve ever existed?
Of course, we don’t know. But the fact of the matter is this: we have no right to expect that just because it happened once, here, that any of these steps are at all common!
Thankfully, we do have some evidence to tell us that some of the things that happened are, in fact, common.
The stars, for example. We know there are somewhere between about 200 billion and 400 billion stars in our galaxy. The vast majority of them — about 95% — live as long or longer than our Sun, meaning that they do have, at least potentially, plenty of time for evolution to take place.
We’ve also measured the metallicity of these stars, or how abundant heavy elements (i.e., not Hydrogen or Helium) are, and we find that our Sun is pretty run-of-the-mill. In other words, the heavy elements that we need for life on Earth are common and abundant all throughout our galaxy. In fact, the largest star thus far discovered in our galaxy, VY Canis Majoris (below), was found to have all the atoms and molecules — including phosphorous — in the outermost layers of its atmosphere!
And we’ve also learned, through our exoplanet hunts, that perhaps as many as 20% of the stars in our galaxy have planets within their habitable zones.
In other words, we’re quite confident that, within our galaxy alone, there are literally billions of stars with rocky planets orbiting at the right distance for life to exist, and they’re loaded up with the right kinds and amounts of elements to possibly give rise to life.
Which is amazing! But, it’s not everything. For example, we don’t know how hard it is for life to actually begin. We’d like to think that it’s easy — after all, the laws of physics and chemistry are the same everywhere in the Universe — so that life would exist in many different places. But it’s possible that to take that leap — from non-life to life — is actually an extraordinarily rare process.
The fact that life appeared at least 3.8 billion years ago on Earth is the only evidence we have, at this point, of life existing anywhere in the Universe. While we don’t think the conditions on Earth were all that rare or special, it is possible that the emergence of life here was rare and special. We’ll continue to search for life on Earth that doesn’t share a universal common ancestor with other known forms of life (because evidence that it happened here twice independently would be amazing), and for life — both past and present — on other worlds, as well as continuing to make life-from-non-life in the lab. But until we succeed somewhere, we won’t have any real quantitative idea of how rare or common life in the Universe actually is.
The development part is another great unknown. For nearly 3 billion years, life on Earth was no more complex than single-celled, asexually reproducing organisms. Yes, this includes some extraordinary one-celled creatures, like corals and sponges, but still single-celled organisms, nonetheless. But at some point, evolution permitted complex, highly differentiated, multicellular animals to arise.
Is that a commonplace occurrence in the Universe? Or is that an extraordinary rarity? Again, we have no quantitative information except this one instance here on Earth. Until we do, we’re really just playing a guess-timation game.
Finally, here we are! The most intelligent species — as far as we know — ever to exist! Even given the existence of life and the evolution of highly complex, highly differentiated creatures, how likely or rare is the evolution of a human-level of intelligence? Our brain-to-body-size ratio dwarfs that of our nearest competitor, the dolphin, by nearly a factor of two, and the next-nearest great ape by a factor of about three. A chimpanzee society might not have the intelligence to understand or explore the Universe, but we certainly are capable!
How rare is this level of intelligence? As far as we know, we’re the only species to attain it in the history of Earth, and we have no idea how common this is in the Universe. It could be extremely common, or we could be the only ones.
And finally, what about spacefaring civilizations? What about the colonization of other worlds? The fact that we haven’t either contacted or been contacted by another species very likely tells us that all of these things haven’t happened abundantly in our galaxy, but how rare or common is it?
The evidence, at this point, points to, at best, not that common. (And, at worst, we might be the first-and-only, and even that’s only if we can get our act together!)
But the fact of the matter is, I’ve never thought of the Fermi paradox as very much of a paradox. It’s not at all hard to imagine that the answer to, “Where is everybody?” is that there isn’t anybody else.
But there could be, and so we have to look. Either way, what is here is remarkable, and I want to know everything I can about it.