Certain genres of science fiction are in love with busy, talky universes, full of interesting alien species with which the heroes can interact. The beloved Lensman series of E. E. “Doc” Smith is probably the ur-example, with its galactic “Civilization” made up of hundreds of cooperating species. Today we can see more superb examples in literature (David Brin’s Uplift universe), in cinema and television (Babylon 5, Stargate, Star Trek, and Star Wars), in tabletop gaming (Traveller), and in video games (Halo and Mass Effect).

It’s an appealing set of tropes. Aliens provide us with the opportunity to build an interesting Other, against which our characters (or our readers!) can measure themselves. Here I’d like to look at some of the implications of a setting like that.

When we’re considering what kind of tropes to use in a science-fictional setting, we need to be aware of an observation most commonly called the Fermi Paradox. It’s named after Enrico Fermi (image above), who made the observation while visiting the Los Alamos national laboratory in 1950. The observation – which isn’t really a paradox, as such – goes something like this:

• The galaxy is very large (hundreds of billions of stars) and very old (billions of years).
• Stars with planets appear to be very common.
• It seems reasonable to assume that many planets exist that provide conditions suitable for life.
• Given enough time, there seems to be a significant probability that any planet supporting life will eventually give rise to an intelligent species capable of tool use and a high-technology civilization.
• If a high-technology civilization becomes capable of interstellar travel, even using very slow methods, it should be able to colonize the entire galaxy within a few million years. If easier or faster interstellar travel turns out to be possible, that process could take considerably less time.
• Therefore, we should see evidence of previous visits to and colonization of our own solar system. Possibly a lot of such evidence.
• We don’t. Where is everybody?

An astute reader will notice all manner of hand-waving in that argument. When Enrico Fermi walked through it back in 1950, we didn’t know very much about the galaxy around us. Most of the probabilities and quantities implicit in the argument were unclear. Today we have more evidence for a few items. We know that most stars probably have planets, for example, because we’ve detected thousands of them in recent years. Still, at a lot of points we’re arguing from a sample size of one, and that’s always dangerous.

It’s entirely possible that Fermi’s observation isn’t a paradox at all. Perhaps life is much rarer than we assume. Or perhaps complex life is vanishingly rare; the universe may be crammed full of bacteria, with the appearance of big tool-using animals like us as an aberration. Or perhaps high-technology cultures almost never figure out the trick of interstellar travel, either because they don’t survive long enough, or because interstellar travel is even harder than we think. We need more data.

On the other hand, when we want to design a space-operatic setting, we’re required to implicitly assign values to several of those quantities. Hence it behooves us to assign values that make sense together, and don’t run us straight into Fermi’s Paradox at warp speed.

I’d like to suggest the following rule of thumb:

If a given science-fiction setting has multiple interstellar civilizations, and the typical civilization undergoes territorial expansion at a rate of 1% per year, then no civilization should be expected to survive longer than 1,000 years. For every factor of ten by which the growth rate is reduced, the allowable lifespan for interstellar civilizations will increase by a factor of ten.

The reasoning here is straightforward.

Starting with a single fully occupied star system, a civilization which grows at 1% per year doubles its territory in not quite 70 years. Now it fully occupies two star systems (or, more likely, it has that one home system and small colonies in several other star systems). In another 70 years, it occupies the equivalent of four systems. In another 70 years, it occupies the equivalent of eight. The power of compound-interest expansion: in about 2,500 years that civilization has occupied one hundred billion star systems, and at that point the Milky Way is full to bursting.

If there are multiple interstellar cultures around, and that kind of growth is typical for them, then we have a problem. In the past billion years, Earth should have been overrun many times over. The Fermi Paradox is in full force, unless something comes along to eat civilizations for dinner long before they reach that point. That could be a recurring natural disaster, or an intelligent super-cultural force that cuts young civilizations short. Or maybe civilizations tend to stop their territorial expansion, turn to other concerns, and then die out. It’s your setting, your choice.

For this rule of thumb, I stipulate that the lifespan limit is only 1,000 years. This is a nice round number, and it permits us to assume the presence of many interstellar civilizations at any given time, all of them following the same dynamics of growth and decay.

If we want star-faring cultures to live longer, then we must adjust the other parameter in the model: their typical rate of growth. Given how the math works, if we divide the growth rate by ten, the allowable lifespan in turn grows almost exactly by a factor of ten. Hence if we want our interstellar civilizations to last on the order of ten thousand years, we need to assume a growth rate of 0.1% per year. A hundred thousand years, 0.01% per year.

Notice what this says about interstellar cultures, assuming we aren’t living in a “Rare Earth” universe in which there just aren’t any intelligent beings other than ourselves. The Fermi Paradox seems to suggest that longevity requires very slow growth. The growth rates required to permit the existence of million-year-old civilizations are so low that they’re just about indistinguishable from a steady state.

Perhaps this shouldn’t surprise us. After all, on our little planet and for most of human history, our own population growth rates were very low. Only the Industrial Revolution, and subsequent improvements in agricultural technology, sanitation, and medicine, permitted us to undergo a period of rapid expansion. Human population growth peaked at a little over 2% in the early 1960s, is currently back down to a little over 1%, and may not be sustainable at even that pace for very long. The whole galaxy is a much bigger field of endeavor . . . but given even a little time, compound interest has a way of overwhelming such differences in scale.

Now, notice one other implication: none of this should be a surprise to any culture that manages to figure out the trick of interstellar travel. By the time such a culture has been out among the stars for a while, it should have a good estimate for every parameter in the relevant mathematics. Which means that if our characters live in a fast-growing interstellar civilization, or they know of other such cultures, they should be very worried.

Why? Well, let’s look at a specific science-fictional universe that I’ve been playing with for some time: the one in the popular Mass Effect series of video games.

In Mass Effect, humanity emerges into the galaxy in the mid-22nd century, to find several other interstellar civilizations already well-established. The oldest of these civilizations reached the stars about three thousand years ago. We learn of dozens or even hundreds of colony worlds settled in that time, some of them with populations in the billions. That suggests a typical territorial growth rate that’s modest but still significant. Say, about 0.2% or so per year. We also learn that there have been plenty of former interstellar cultures, all of them now extinct.

During the first Mass Effect game, the protagonist discovers the existence of the Reapers, a force of godlike sentient machines that periodically sweep the Milky Way, exterminating every advanced civilization they find. The story details the effort to delay the return of the Reapers, and then to defeat them and win the survival of galactic civilization once they do return.

It’s all very well-done space opera, with plenty of attention given to a plausible setting and plot. But there’s one detail that should set off alarms for us: the protagonist has a great deal of difficulty persuading anyone in authority that the Reapers even exist, until it’s far too late.

From a narrative perspective, of course, this is fine. If the hero is forced to act on her own, because those in authority don’t take a threat seriously, that’s a perfectly useful narrative trope. Yet after we’ve given the Fermi Paradox some thought, we should ask ourselves how the rulers of any galactic civilization could remain fully ignorant of the implications.

Notice what our rule of thumb suggests for this universe. Interstellar cultures grow at about 0.2% per year; that tells us that the maximum lifespan for any civilization is somewhere around five thousand years. Most of that time has already passed. Just about everyone who’s paying attention should probably be looking around with a great deal of apprehension right now.

We know that interstellar travel is easy, and that civilizations can grow with significant speed. We know that there have been other interstellar cultures before our own. Why wasn’t the galaxy already full when we arrived? Why are all those other cultures extinct?

What made them extinct? What might be waiting to make us extinct before we manage to fill up the galaxy with our own colonies? Shouldn’t we be trying to find out?

If you’re a potential author, maybe your fictional galaxy won’t have anything in it like the Reapers. But there should be something to keep the galaxy from becoming over-crowded, many times over during its long history. You need to take a moment and consider what that might be.

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Author’s Note: This is one of the few posts I’ve preserved across several iterations of my blog, mostly because I think it presents a corollary to the Fermi Paradox that hasn’t often been stated explicitly.

It may amuse my readers to know that I’ve also laid out a similar argument in some of my fiction – in this case, a chapter of my Mass Effect novel The Illium Years. Here’s a link to the chapter in question. It should be readable even if you aren’t familiar with Mass Effect beyond what you could pick up from this article.

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“Fermi’s Nightmare” © 2020 by Jon F. Zeigler. All rights reserved.