Are We Alone?

Hey, Horace here! Are we alone in the universe? It's a question humanity has been asking ever since we first realized that those twinkling points of light in the night sky are stars, some of which might be very similar to our own sun. You'd be forgiven for thinking this is simply one of life's great unanswerables. Who could possibly know whether or not we're alone in the endless reaches of space?

Perhaps the US government? They're definitely hiding something in Area 51, and it's either aliens or the top-secret KFC recipe. For now, we can't say for sure one way or the other whether there are other advanced civilizations out there just waiting for us to point our telescopes at just the right spot in the sky. But science has long been trying to figure out the chances that we will ever run into our galactic neighbors and whether they exist at all.

It all started in 1961 with an American astronomer named Frank Drake. He organized a meeting to discuss humanity's search for extraterrestrial intelligence and invited everyone he knew who was interested in the subject— all 12 of them. Despite the small size of the meeting, it was attended by some seriously important people, including future Nobel Prize winner Melvin Calvin and a man who would go on to become one of the most famous figures in the search for extraterrestrial life, Carl Sagan.

But there was a problem: Drake was so busy focusing on bringing together all these brilliant minds that he totally forgot to come up with an agenda for the meeting. With just a few days to spare, the stressed astronomer sat down to thrash out something so that his illustrious friends would have something to talk about. Now, when faced with this kind of situation, you or I might spend a couple of hours coming up with fun icebreakers or putting together impressive-looking PowerPoint presentations. Not Frank Drake. He sat down, scratched his head for a moment, then jotted down a truly unique starter: a short calculation that could be used for estimating the number of intelligent civilizations in the galaxy.

Today, almost 60 years later, some would argue that the Drake Equation, as it has become known, is the second most famous formula in the world of science after Einstein's iconic E=mc². Unfortunately, it never garnered the same level of popularity because, well, it doesn't fit nicely on a t-shirt. The aim of the equation—to calculate the number of intelligent civilizations in the galaxy—is almost comically ambitious. But the equation itself is actually very simple, consisting of seven variables multiplied together:

1. The average rate of star formation in our galaxy

2. The fraction of those stars that have orbiting planets

3. The average number of planets per star that might be habitable

4. The fraction of these habitable planets that actually go on to develop life

5. The fraction of planets on which life develops that go on to develop intelligent life

6. The fraction of these intelligent life forms that produce detectable signs of their existence

7. The length of time one of these detectable civilizations might stick around for, releasing signals into space

Some of the variables in Drake's equation we understand well, like the average rate of star formation in our galaxy and the fraction of those stars that have planets. Some, like the likelihood of life arising on a habitable planet or the chances of that life developing intelligence, not so much. But here's the thing: our galaxy is big. Let me rephrase that—our galaxy is mind-bogglingly, pun-wittingly, Nebo-kingly, astonishingly gigantic. There are thought to be as many as 400 billion stars in our galaxy, the Milky Way. That sounds like a big number, right? Well, try this one on for size: recent studies suggest that there may be two trillion galaxies out there, possibly more, in the observable universe. That means the number of star systems out there containing potentially habitable planets is too large a number to fit on this screen.

But why am I telling you all this? Well, when the numbers you're dealing with are this ridiculously huge, it doesn't really matter which values we choose for the Drake Equation. A recent Kepler space mission concluded that there might be as many as 40 billion Earth-sized planets orbiting sun-like stars in the Milky Way alone. Forty billion chances for life to catch hold just in our galaxy. And yes, that's 40 billion planets within their system's habitable zone, or Goldilocks zone—planets whose orbits maintain a precisely sufficient proximity from the host star to contain liquid water on the surface. And where there's water, there's life.

According to Drake himself, having played around with his brand-spanking-new equation in that meeting back in 1961, he and his illustrious friends concluded that there were probably between 1,000 and 100 million detectable civilizations in our galaxy. In our galaxy, which is just one of over two trillion. Our universe is simply so huge that even if the probability of life arising is minuscule, even if the chances of that life developing intelligence are vanishingly small, there should still be hundreds of thousands, if not hundreds of millions, of civilizations knocking around out there, probably far more.

And yet, we've looked. The SETI Institute has been at it for decades, and we've found nothing. Diddly squat. Not a single peep from our insecure magnetic neighbors.

Now all of this begs a rather obvious question: if probability suggests there are so many intelligent civilizations out there busily colonizing the universe, where the hell are they?

Perhaps the first person to ponder this conundrum was Italian-American physicist and Nobel Prize winner Enrico Fermi some ten years before Drake jotted down his now-famous equation. Fermi was in his late forties at the time and already a world-renowned scientist, a prominent member of the Manhattan Project. He is perhaps most famous today for being the creator of the world's first nuclear reactor, which is why he's sometimes known as the architect of the atomic bomb.

Back in 1950, Fermi was eating lunch with a couple of his physicist buddies. They were discussing the feasibility of faster-than-light travel, as you do when you're a physicist, when Fermi suddenly blurted out, "But where is everybody?" Despite the apparent randomness of the comment, Fermi's lunch buddies knew exactly what he was talking about: aliens. Fermi had realized that the sheer scale of our universe ought to suggest that life, even intelligent life, should be commonplace. He even carried out his own series of calculations soon after that were similar to the Drake Equation, and he came to that very same conclusion: aliens should be everywhere. Although interstellar travel is a slow process, Fermi reasoned, within 10 million years or so, an advanced civilization should be able to make a pretty good go of populating a galaxy. That may sound like a long time to us humans, who have been here for more like 200,000 years, but remember, right here on Earth, we've had species live much, much longer than that. Modern crocodiles have been around for about 85 million years, and certain species of sharks are thought to have existed for 175 million years or more, relatively unchanged. Advanced species with that kind of longevity, Fermi reasoned, should have had the time to do some serious intergalactic exploring. Hence his utterance of the immortal line, "But where is everybody?"—which forms the basis of what is today known as the Fermi Paradox, the apparent contradiction between the perceived likelihood of other intelligent civilizations existing in the universe and our complete lack of evidence for them.

So today, 70 years on, do we have an answer to Fermi's famous question? The short answer is no. No, we don't. The long answer? Well, that's much more interesting. Unless governments around the world have already detected or even made contact with alien races and are covering up the discovery for some reason, there can only be two possible explanations for Fermi's paradox: either we aren't seeing evidence of alien civilizations because there are none, or there are just as many intelligent species out there as we think there should be, but for one reason or another, we aren't able to detect them.

Let's take a look at the first option: that we haven't bumped into our neighbors yet because we don't have any, that we are alone in the never-ending expanse of space. This idea is probably either very reassuring or extremely depressing, depending on your appetite for living out your Star Trek fantasies. But since our current understanding suggests that there should be many advanced civilizations out there, there must be something wrong with that understanding in the first place—some variable we're either dramatically misjudging or are unaware of entirely. Suggestions for what this variable might be are collectively known as the Great Filter. And for various reasons, whatever this thing is, it seems to be preventing aliens from becoming advanced enough to be detectable by us humans here on Earth or from detecting and visiting us.

There are many suggestions for what the Great Filter might be, any one of which might hold the key to explaining the Fermi Paradox. It may simply be the case that the creation of life from nonliving matter—a process known as abiogenesis—is the Great Filter. It took around 1 billion years to happen here on Earth, after all. Or maybe it's the evolutionary leap from cells of the simple prokaryotic variety to the complex eukaryotic cells that make up all higher forms of life today. That took almost 2 billion years on Earth. If one of these steps is indeed the Great Filter, we should probably all be feeling a little relieved right now. Why? Because all these things have already happened to life on Earth, meaning the Great Filter is already behind humanity. If this is the case, we've beaten ridiculous odds to become perhaps the only intelligent species in the whole universe.

The alternative is that the Great Filter is ahead of us. Life on Earth has managed to make it this far relatively unscathed, but something in our future is set to cut our story short and put an end to humanity before we ever get to go trekking among the stars. While this may sound a bit too much like the plot of a disaster movie for comfort, it is far from out of the question. It could even be that the very thing that makes a civilization detectable—radio signals, for example—is the thing that dooms it. Maybe other advanced species across the galaxy have been busy broadcasting their existence and have drawn attention to themselves from some sort of intergalactic predator. Maybe they've blown themselves up by building bigger and bigger nuclear arsenals or have made their home planets uninhabitable through pollution and climate change. Perhaps the universe itself is just too hostile a place for long-term survival, thanks to the regularity with which it tends to fling asteroids at planets and erupt in catastrophic supernovae.

But what about the other possibility: that alien civilizations are out there but, for one reason or another, we simply aren't seeing them? As unlikely as this may sound, there are quite a few different ways this could be true.

First, they might be ignoring us. I mean, it shouldn't be all that difficult to believe that aliens would find us boring, right? They're probably not interested in our cat memes or our latest TikTok challenges. Or maybe they're just following the Prime Directive, which states that you shouldn't mess with developing civilizations. They might be watching us like we're some kind of cosmic reality TV show, tuning in every week to see what ridiculous thing we humans will do next. Or they might be here already, blending in and observing us in the form of humans or animals. Who knows, your cat might actually be an alien spy!

Alternatively, maybe they simply can't reach us. Our galaxy is a pretty big place, and even if there are other advanced civilizations out there, they might be so far away that our signals haven't reached them yet, or vice versa. The nearest star system, Alpha Centauri, is over four light years away. That's roughly 24 trillion miles. It's one thing to send a signal; it's quite another to travel that kind of distance.

Another possibility is that they don't want to reach us. Maybe they're deliberately avoiding contact with us because they've seen what happens when more technologically advanced civilizations come into contact with less advanced ones. Historically, it doesn't end well for the less advanced civilization. Perhaps they're waiting for us to reach a certain level of technological maturity before making contact. Or maybe they just think we're a bit too primitive and violent at the moment.

There are also more speculative ideas out there, like the possibility that we're living in a simulation and the aliens are just part of the simulation or that they're using some kind of technology that we simply can't detect with our current capabilities.

In the end, the truth is we just don't know. But the search for extraterrestrial intelligence continues, and with new technologies and missions, like the James Webb Space Telescope, we might one day get an answer to Fermi's paradox. Until then, keep your eyes on the skies and your cat on a short leash. You never know what they might be up to.