Great Filters, Part 6: Civilization Filters

In my previous posts on the topic of Great Filters, I've looked at all the hard science limits on life and intelligence on other worlds. Those limits left us with a ballpark figure of 5,000 intelligent species in the Milky Way.

Now we're shifting to the "softer" sciences, with a focus on history and archaeology. The three questions before us are: how common is civilization, how common is modern technological civilization, and how long do civilizations last? As with our discussions of life and intelligence, the only data point we have is the history of Earth. We know — more or less — how humans went from plains apes to space explorers, and from that I will try to extrapolate how likely this path is for other species.

But first, a definition. What do we mean by "civilization" anyway?

As before, I'm going to stick to functional definitions. A civilization is any social organization which permits specialization of labor and the accumulation of surplus wealth.

The details aren't important. How that specialization happens and how the wealth is accumulated don't matter. We've seen that barbaric tyrannies are perfectly capable of carrying out large cutting-edge technology projects. Consequently we can assume that alien minds with alien social structures can still have something that fits my definition.

Second digression: a warning. In the history biz there's a term which sometimes gets used, called "Whig History." The name comes from the historian Herbert Butterfield, who was critiquing the approach of some 19th Century British historians, who viewed the past as an inevitable progression from the Creation directly to the British Whig Party. Needless to say, this view of history is not confined to Whigs. The grand champions of Whig History are the Marxists, who shoehorn all human societies in all time periods into a schema based on social classes in western Germany circa 1840. More generally, it's any historical approach which assumes history has a goal, whether that's Our Glorious Selves or whatever grand Utopia the historian is trying to wish into existence.

I don't buy it, and I'm not going to sell it. History is a random walk, but we look back and try to make it a vector. The only verifiable historical trends are increasing population, increasing wealth, and increasing knowledge.

What this means is that I'm not going to assume any historical progressions other than wealth and knowledge are either necessary or inevitable. This is especially important because it means we don't have to speculate on the details of history on an alien world.   

Enough digressions.

As with the history of life and intelligence, it's surprising — and ought to be humbling — how long humans managed to exist without civilization. Tool use dates back a couple of million years. Control of fire is a bit less well-defined, but it was at least 700 thousand years ago. But the earliest signs of civilization, at Gobekli Tepe, show up some time around 10,000 B.C. That means humans (or hominids) who were likely as intelligent as the person writing this blog post spent a good two-thirds of a million years as wandering hunter-gatherers.

That suggests that not only is civilization not inevitable, it's not really necessary, either. It gets worse when one considers that some proportion of intelligent species might be physically incapable of doing agriculture and living a sedentary existence. A species of obligate carnivores would likely depelete the local food supply and starve to death if they ever tried to settle down permanently. I'm inclined to guess that civilization is rare. Say one in ten intelligent species ever develops a complex sedentary society with surplus wealth and preservation of knowledge.

That knocks us down to 500 worlds with the potential to reach the stars, or at least send out messages. But I've got another filter up my sleeve.

Any student of history — or anyone who has played a historical 4X game like Civilization — becomes aware of how little technological change occurred over large swaths of recorded history. From before recorded history to around A.D. 1500, wars were waged and won by men with spears and bows. The galleys that fought the Battle of Lepanto in 1571 would have been quite familiar to a Phoenician mariner of 700 B.C.

From 10,000 B.C. to A.D. 1500, the biggest technological innovations were the development of bronze tools and weapons, and then their replacement by iron. That's about it. Aside from relatively minor cultural differences, a peasant anywhere in the world in A.D. 1500 lived in almost exactly the same manner as his ancestor in 1500 B.C.

But around 1500 something changed. It wasn't a single change, either. It was the start of a process of continuous change which has gone on and on, accelerating each decade, right up to this moment. Bits of it have different names: the Scientific Revolution, the Industrial Revolution, the Financial Revolution, the Urban Revolution, and so on. But all of them have been part of an immense and permanent change in how people live.

Broadly speaking, humans right now are a species which lives in cities and large towns, performing a variety of specialized occupations in a money economy. A tiny minority still get by doing subsistence farming or hunting. That's not how it was five hundred years ago. Back then humans were a species which lived in small villages, and farmed. A tiny minority performed specialized tasks for money.

For humans, that revolution, or group of interconnected revolutions, has been as big a shift as the first civilizations themselves. I expect the same may be true on other worlds. Again, the time scale is suggestive: basic agrarian civilization lasted about ten millennia. And during those millennia whole civilizations rose and fell; empires grew and collapsed, without changing how most people lived. But "modern" civilization is less than a thousand years old and has transformed the world. We have no way of knowing how long it will endure (see below). Still, it seems plausible that many worlds could reach a local optimum of stable farming cultures and remain there indefinitely. Whatever happened at the start of the Modern era wasn't inevitable. Let's apply that ten-to-one ratio and say that of those 500 worlds with civilization, only 50 have managed to start the technology rollercoaster ride.

That has thinned out the Galaxy pretty effectively. Fifty civilizations scattered through a hundred billion stars. We'd be separated by more than 20 thousand light years from our closest neighbor.

Surely that settles it? They're just too far apart to detect! No Fermi Paradox necessary . . . right?

Well, not quite. There's the question of age. How long does a technological civilization last? (Here I define "technological civilization" as one capable of sending out interstellar radio signals.)

Our own civilization is less than a thousand years old. It is fashionable to posit that we're on the brink of disaster, and that whatever we're all worried about this year will knock us back to the Stone Age or exterminate us. So the lifetime of a civilization could be just a couple of centuries.

I'm not convinced. In future posts I'll go into more detail, but for now let's just say that I don't believe a large fraction of technological civilizations collapse or go extinct after just a few centuries. In particular, any civilization with self-sustaining colonies in space or on other planets would be effectively invulnerable to any event short of a supernova. I honestly can't think of any end for an advanced civilization. Their lifespans should be unlimited.

And that brings the Fermi Paradox back on stage. Because planets, species, and civilizations presumably don't evolve at the same rates all over the Galaxy. There could have been lifebearing worlds a billion years before the Earth formed. There could have been intelligent beings hundreds of millions of years ago. There could be civilizations which have existed for tens of millions of years.

I've posted about projects going on right now, on Earth, with the goal of sending probes to other stars. That's something achievable with ten-years-out technology and eccentric-billionaire funding. Launching probes at a tenth of the speed of light is doable.

Which means that any civilization older than a couple of million years could have launched probes all over the Galaxy by now. They could be blasting out radio signals, or doing stellar engineering works visible across Galactic distances. Where are they?

There are many answers yet to consider: reasons why interplanetary colonization might be hard, reasons why interstellar probes might not be widespread, and so on. Next time I'll discuss things which might mess with advanced civilizations.

Comments

[MadRocketSci]

This doesn't really address your post directly. I saw that article that floated recently about another SETI survey of some small fraction of the sky turning up empty again.

It seems popular to assume that if our radio waves could have traveled 40LY in principle, that we're visible out to 40 LY, and that we could see aliens broadcasting like we do from such a distance. I think interstellar communication is more difficult, and necessarily more directional than most people assume.

I was doing some physics doodling to get some numbers back into my head: I recall we managed to laboriously drag a 100 bit-per-second signal out of the noise floor from the Gallileo probe at Jupiter. It was an S-band transmission, 20W, which amounts to 2,6E-24 W/m2 on average here at Earth. We had to coordinate several dishes with detectors cooled to 11K to obtain that signal, and we could only do it because we already knew Gallileo was there and where to point the dishes!

A 1GW omni-directional transmitter does roughly 100x worse at 5 LY, the approximate distance of our nearest-neighbor star. Aliens right next door couldn't pick up a massive omnidirectional transmission, nor could we. Practical interstellar communication *must* be directional.

With a 1km dish in the X-band (8-12GHz) we can get gains of something like 2E9, by creating a beam that has a half-angle of 4.57E-5 radians. A 1GW sender could then be heard at "gallileo levels" at 25000 ly, or 1/4 distance across the galaxy. A 1kW sender can be heard at 25 LY at those extreme-limit levels, but the beam would only be about twice as wide as the orbit of pluto at that distance. For economical transmission powers, ranges are in the local-stellar neighborhood and necessarily pointed at specific stars. No one else could intercept that beam unless along a direct line of sight.

Your optics don't have to be as extravagant in optical-laser wavelength ranges. An equivalent optic to that 1km dish for a 535nm laser is a 1.5cm lens: easily doable. You'd have to have a power of >800W to outshine the sun at a 1nm bandwidth, also doable with pulse lasers.

Anyway, I suppose what I'm saying is that it isn't too surprising that a radio survey is coming up empty: Doesn't indicate anything even if the galaxy is full of civilizations, other than that we're not on the "point to" list. (The massive time disparity between "has radios and lasers" and "is moderately intelligent" that you discuss here seems more important.)

[MadRocketSci]

Forgot to say: The Gallileo probe high gain antenna failed to deploy. Earth was trying to find the 20W omnidirectional low-gain antenna to talk to. It's amazing that they could do it at all.

Those big antenna dishes on planetary probes are not there for cosmetic reasons!

[Cambias]

Wow, you're following this thread closely! I hadn't even posted the link to Facebook yet.

Don't worry, I've planned a later post on whether or not we can actually detect alien civilizations, and how little we've actually looked for them.