For its 60th anniversary I hosted a sci-if movie night at the Roxie cinema in San Francisco of the 1951 classic Forbidden Planet. It was delightful to see it on the big screen with the Roxie’s gorgeous projection system and hear that crazy soundtrack through their audio system.

After the show, I broke with my usual tradition of discussing any of the interfaces (after all, I’d already reviewed all of them on the blog years ago, and recently discussed the film in depth with the guys at Decipher Sci-fi) and instead discussed an idea that’s present in the film. In this handful of posts, I’m going to represent that content, but also add some additional content that there just wasn’t time for before we had to leave the cinema to make way for the next show.

Necessary Spoilers: In Forbidden Planet, a platoon travels to Altair IV to figure out why a 19-year old colony of scientists has gone silent. They meet Morbius, the only survivor of the original colony, and his daughter Altaira. They learn that Morbius has discovered the complete knowledge and technological remains of a long-dead, highly advanced civilization called the Krell. Through the Krell’s still-working machines Morbius has greatly enhanced his intelligence, but unwittingly unleashed an invisible “monster from the id” that has violently destroyed everyone but him and his family. Morbius refuses to return to Earth with Captain Adams, and so Adams grounds the mission while his crew uses parts of the ship to construct a communication device to ask for orders. During the downtime, with some truly wince-worthy 1950s slut-shaming courtship, Captain Adams somehow wins the heart of Altaira, who falls for it and defies her father and becomes engaged to the captain. Crushed by the betrayal, Morbius’ control of the monster wanes, and it attacks and defeats him. His world asunder, Morbius decides in his dying moments to scuttle the entire planet, including all traces of the Krell. From a safe observing distance in space, Adams, Altaira, and the surviving crew watch the explosion before heading back to Earth.

The movie presents one answer to a long-standing astronomical question,

“With 400 billion stars in our galaxy, and 400 billion galaxies in the observable universe, and billions of years of time since the start of the universe, even if only a very small fraction of stars produced advanced civilizations, where the hell is everybody? Why does space seem so devoid of life?”

This question is commonly known as the Fermi Paradox. It’s not really a paradox in the logical sense, so it works better in discussions to call it the Fermi question.

In this post I’m going to review a categorical deconstruction of the many hypotheses that seek to answer the question. If you’re already familiar with the topic, feel free to skip this post and check out the next one, in which I look at which of the hypotheses that screen sci-fi likes to talk about. If you’re not familiar with these hypotheses, would like a refresher, or want to check how I did in my explanation, read on.

The question poses is deeply troubling for a tribal species who came into being on a planet just lousy with life. Many people have put forth many theories, but by my reckoning they break down categorically into two broad supergroups five subgroups. But before we talk about those hypotheses, we should take a look at an equation.

Drake’s buzzkill

About eleven years after Fermi asked his question, an astrophysicist named Frank Drake created a probabilistic equation by which we could come up with some answer about how many civilizations we might expect to find out there in the void. Now this isn’t like most equations for which you expect to get a single answer. This equation hopes to get us at least in the ballpark of the right right answer, say to an order of magnitude within the actual one. Just so we can know whether we’re talking about 10, 100, or 1,000 civilizations, etc. Drake’s equation has seven variables which we multiply together to produce an answer.

1. How often are stars created?
2. What fraction of these stars have planets?
3. What fraction of these planets could support life?
4. What fraction of these planets do support life?
5. What fraction of these develop symbolic intelligence?
6. What fraction of these produce detectable signs?
7. How long do these civilizations last (in a detectable way)?

Since each of these are an estimate, with only statistically insignificant observed data to inform them, the equation ultimately produces a broad range of numbers. With the variables filled in, Drake got a low number of 1,000. At the high end, with the dials of the components each turned up to a scientifically plausible maximum, there can be upwards of 100 million civilizations. More modern estimates put it much higher, with a high estimate of 280 million civilizations out there—right now—that we should be able to detect. Many of which should be much, much older and presumably much more advanced than ours, and spread throughout the universe.

That puts the question into some context by which we can think about it, and begin to understand some of the hypotheses.

Life is rare

The first group of hypotheses wrap around components 2–6 of the Drake equation, by asserting that each of these things are prohibitively rare. Even with more and more exoplanets being found all the time, theories go that having all the conditions is too fantastic of a proposition. We are biased to think we’re likely because we exist, but we must consider that our sun is just the right size, our planet is just the right kind of rocky and orbits in just the right zone around our star for favorable temperatures, we have just the right mix of chemicals present back in the day, we have a silent and invisible bodyguard in the solar system called Jupiter, &c &c. If any one of these links in the long chain that led to us was off, we wouldn’t be here. And for that reason, it’s just us.

There is another subcategory of why there are no aliens out there, but first let’s look at the reasons why they might be there, and being silent.

There are aliens out there, they’re just silent

There are a lot of these, but you can think of them as being a result of danger, policy, or logistics.

It’s Dangerous

It’s possible that there exist untold dangers in space, and so it is wise to hide. This could be because a superpredator species patrols the darkness, ensuring its place atop the space food chain by destroying any others it comes into contact with, and so surviving species have learned to hide themselves. Broadcasting our presence on earth is like banging a drum and shouting while walking through a jungle teeming with hungry tigers.

Another of these explanations is that messages themselves might be dangerous. The threat comes from the ideas that are conveyed between the stars, that drive receiving species insane or trick them into self-destruction. Even if there’s no hope of bodily confronting aliens, you can’t even risk their information. So they go quiet.

It’s Policy

It might be that—even if there’s no inherent risk in contact—after thinking about it long and hard, alien civilizations have decided as a whole that they’re not interested in communication with us. It could be an isolationist policy derived from philosophical principles or part of intergalactic agreements. Maybe we’ve already been violating that Treaty of Silence. It would be a terrifically weird first encounter to receive our first verifiable message from alien intelligence, only to have it read, “Hush, you.”

It could be that advanced civilizations agree not to unduly influence or “pollute” primitive ones. Maybe there’s an endangered planets law in effect that keeps aliens at bay from this mudball of monkeys. Or maybe we have some technological or ethical threshold to cross before they’re allowed contact. The threshold could be surpassing the speed of light, picking up the giant phone they left orbiting Jupiter for us, or demonstrating ethical maturity by demonstrating compassion for all living beings instead of you know, slaughtering and subjugating everything in sight for profit, sport, or lunch. I’m lovin’ it.

It could be that we are mistaken about the nature of our reality. Maybe we live in a carefully constructed zoo or farm, and zookeepers/ranchers only let visitors observe our habitat from a distance or under careful concealment, or when some of us are being harvested for the deep fryers. Perhaps some vast distance from the center of our sun is a spherical planetarium display on which an empty universe glows very, very convincingly for us (and behind which the aliens watch). Or maybe we’re wrong about the nature of everything, and we are part of a virtual reality program, only as self-aware as we have been programmed to be.

It’s Logistics

Or maybe it’s not deliberate at all. It’s just the way things work. Perhaps they are all shouting their greetings, but that communication comes as a rare beam that sweeps around its source every million years or so. If we just missed the last passes, we might just need to wait a few million years for the next broadcast. Maybe they’re like space cicadas, and their life cycles have them out of the ground (and detectable) only once every several thousand years.

Or maybe they are happy to answer our greeting, but we’re just impatient, because it will take our messages a very very long time to reach them? In this phone’s still ringing hypothesis, if they’re pretty far away, or think on a much slower time scale, we may need to keep saying hello for thousands or millions of years before they realize it isn’t just a weird blip on the radar.

It could be that we were just unlucky enough to have evolved in Hicksville, far away from the big city lights. As a result our messages dilute too much before it reaches them. Or maybe we’re uninteresting. Sure, they heard us, and we’re just an annoyance, messing with their reception of the latest episode of Game of Space Thrones.

It could be that space is bigger than we think, and the numbers of civilizations are lower than we thought, and the distribution is too great between us. We are seafarers living on very distant islands. One theory complicates the problem even further by saying it’s not just distribution in space but across dimensions, too.

It could be that we’re shouting through a tin can and they’re talking on cell phones. Maybe we just don’t have the right technology yet to detect their messages, and they’re not listening to our primitive electromagnetic waves. Do you spend time at a lake looking for messages in the ripples on the water? I don’t.

It could be (and this is my favorite) they’re just too alien for us to conceive. We expect them to be some variation of life on our planet, but they’re nothing like what our brains can handle. Think of how difficult it would be for ants to conceive of humans, or to understand our efforts at communication. They don’t have the eardrums to receive the signals as we do, or the memories to keep the first part of the sentence in place while we get to the end. If we bothered to figure out their chemical language, the best we could hope is to convey “done/not done,” “Protect the queen!” or some such. Even if we expose ourselves physically to them, if they can conceive of us at all, we would be “threat” or “large mammal” at best, or possibly even “vast, warmish, irregular, hairy landscapes.”  Why would they think to communicate with the landscape? Why would we? We don’t have anything to say with ants, and we evolved on and live on the same planet.

That analogy helps explain another hypothesis. We are a species struggling with our own set of problems like feeding ourselves, caring for our young, avoiding wars, battling our worst natures, wondering about the meaning of life, enjoying life’s pleasures and getting there early enough for good seats at the sci-if movie. Our resources are already tapped out. How much money and effort do we want to spend trying to communicate with ants? It might just be too expensive in resources or effort to either colonize or communicate. They have other things on their minds.

A number of other hypothesis entail the last idea, that they are here, all around us, and they’re just well-hidden, perhaps for one of the other reasons we’ve already discussed.

So that is an overview of the reasons why they might be there but apparently silent from where we float. Let’s return now to the other (and darker) hypothesis about why they may not be out there at all.

Life doesn’t last

This last subcategory concerns the very last component of Drake’s equation, and that is how long civilizations tend to last in the universe, because of external or internal forces.

The universe is a chaotic place. Could it be that random natural disasters of an incalculable scale just occasionally happen? Like lightning that just seems to sort of appear out of nowhere and scar the ground as it leaps up to the skies, maybe events wipe out sections of the universe every billion years or so, and any civilizations that have evolved since then just go poof, caught up like ants unfortunate enough to have built their bed on the hill where the lightning is brewing. Consider that our sun has an estimated life span, and we only have about 5 billion years before it undergoes a transition to become a red giant, ballooning to eventually swallow Mars, Venus, and yes, Earth. To the best of our science, that’s a long way off but coming, and we have no idea how we would survive it.

Or maybe it’s some unfortunate fact about civilizations themselves. What if the sophistication that gives rise to civilization always entails self-destruction? Yes, we have sent Voyager into space, eradicated smallpox, and greatly eased life for some subset of humanity. But we’ve also trashed the planet, dumped millions of tons of plastic into our oceans that have broken down into a toxic slurry gyre that is poisoning our entire food chain. We’ve overfished the oceans and seas to the brink of extinction. We’ve relied on nuclear power that, when it fails, it maims and kills people in horrific ways and renders parts of the earth uninhabitable for millions of years. We continue to deplete limited natural resources for short term gains. We’ve overpopulated the planet to its breaking point, and anthropogenically warmed the atmosphere so that our glaciers are crumbling and weather patterns are swerving between greater and greater extremes. We are constantly at war with each other somewhere. We have poisoned, burned, gassed, and dropped fucking nuclear bombs on people. We’ve very nearly come to global thermonuclear war several times, rendering the surface of the planet a radioactive wasteland, fit only for the likes of insects and invertebrates. Every technology we build gets used for good and bad, and the more powerful those technologies become, the more at risk we put our very existence. Maybe this is some intractable truth for for every civilization in the universe, and not one of them has managed to control the technological genie once it’s out of the bottle.

OK. Whew. That’s it. Take a moment to walk off the existential dread there if you need to.

For purposes of argument, the literature about the we-are-alone hypotheses presume that there is one challenge that is greater than all the rest, and calls this the Great Filter. We’ll use the term as well. If the great filter is behind us, well, lucky us, but that will prove intergalactically lonely and a little depressing. This is it. We’re stuck with us. But hey, buck up little camper, at least we made it. We’re here. We got the chance to do the right thing.

If the great filter is ahead of us, in the form of a natural disaster or self-destruction, it goes from lonely to terrifying. Can we survive? What exactly is the nature of the threat? What do we need to do to prepare ourselves? Will our humanity survive The Thing We Must Do? Where do we put our resources in preparation?

Which brings us, as so many things do, to sci-fi, which I’ll talk about in the next post.