Just The Caffeine Talking

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.

Great Filters, Part 5: Intelligence Filters

I finished up last week's post on Great Filters by looking at the Galaxy according to the numbers I've cooked up. It produced the surprising figure of lifebearing planets every 50 light-years or so. In cosmic terms that's right next door. It means that in the near future we have a pretty decent chance of detecting some "biosignatures" emanating from planets orbiting other stars.

Of course, that conclusion is also kind of alarming. If life really is that common, then the real Great Filter might be looming up ahead of us, unforeseeable and unavoidable.

But there are still some obstacles between life, even complex land-dwelling animal life, and the ability to make yourself heard across interstellar distances. The biggest one is, how common is intelligence?

First, of course, there's the issue that intelligence is maddeningly hard to define. Does it mean communication, abstract reasoning, problem-solving, tool-making, social organization, all of the above, or something else? For the purposes of this essay I'm going to fall back on another functional definition: "Intelligence" is the ability to invent some method of communicating across interstellar distances or conduct activities which are detectable at that range.

I'm aware that this is incredibly reductive, but it's still useful. If the Galaxy turns out to be full of planets inhabited by beings with complex languages, abstract philosophies, and sophisticated art forms — but none of them can even conceive of the idea of building a radiotelescope or an interstellar probe, then we've solved Fermi's Paradox. If human-style intelligence is unique, then that's our Great Filter.

So . . . how common is intelligence? Obviously we have no idea. But looking at the history of life on Earth gives some hints. Complex life has existed on Earth since the Cambrian Era, about half a billion years ago. The hominins (including the genera Australopithecus and Homo), appeared about 4 million years ago. So intelligence — by a very generous definition — has existed for less than 1 percent of the history of life on Earth. Evidently intelligence is not an inevitable evolutionary strategy.

That suggests that intelligence is rare. It certainly requires a huge biological investment in energy-hungry brain tissue. But the payoff turns out to be huge! Why didn't any other species do it? There's at least one theory that there has been a selection pressure in favor of intelligence over the history of life. There's some support for the idea. Vertebrates are (mostly) smarter than invertebrates, and eventually took over. Mammals are (mostly) smarter than non-mammals, and eventually took over.

Well, maybe. Yes, I know about cephalopods. And I know about birds. But without a time machine we can't go back 50 or 100 million years to see if Paleogene-era birds were as smart as contemporary parrots and crows, or if Cretaceous cephalopods were as unnervingly intelligent as contemporary octopuses.

Ultimately it doesn't matter. If cephalopods got as smart as they are now hundreds of millions of years ago, they obviously hit a hard limit and stopped. If birds got smart tens of millions of years ago, they must have hit a wall, too (or a plate-glass window). It's possible that different methods of being intelligent have inherent limits built in, and only hominin intelligence avoided those limits. In that case we can rephrase our question of the frequency of intelligence as the frequency of "intelligence without built-in limits."

Or, if there really is a selective pressure in favor of intelligence, then it seems to have cooked up near-human intellects in a bunch of different genera all right at the same time, and winner takes all. And so in that case we can rephrase the filter as "where selective pressure acts in favor of intelligence at a similar rate."

Either way, I'm going to stick with the 1 percent figure. That winnows down the number of potential intelligent species in the Galaxy to just 10,000. That's still a lot, but it means an average separation of a few thousand light-years between worlds with intelligent beings on them. No more planet-of-the-week Star Trek episodes.

The second big intelligence-related Great Filter is tool use. Now things are going to get very fuzzy, because defining tool use is hard. Is picking up a stick to poke an anthill tool use? Is bending a wire rod into a hook to get food out of a bottle tool use? Does it matter if this behavior only happens in an animal-behavior lab when humans set up the conditions? Does it matter if there is no transmission of "technological" knowlege from generation to generation?

A key element of the tool use question is how important are hands? We've got great hands, combining strength and dexterity, but our hands are the product of a complicated and unlikely evolutionary history. Elephant trunks are stronger but not as dextrous — and they've only got one each. Octopus tentacles are fantastically dextrous, but of course they're mostly stuck in the water. Meanwhile birds have to make do with laborious combinations of beak and feet, and even apes have trouble carrying things because they need their hands to help with locomotion.

I'm going to say that hands aren't an important filter. When I was designing a handless alien species for my short story "The Alien Abduction" I had to come up with some rather implausible anatomy in order to make them truly incapable of manipulating objects. Even a jaw or a pair of toes would be enough.

Our hominid ancestors seem to have started making tools between 2 and 3 million years ago, which suggests that tool use and intelligence went hand in hand (so to speak). However, I'm going to stick a Great Filter in here, another coin-flip 50 percent cut. This is to cover all the potentially intelligent species with really inconvenient anatomy, beings which evolved in environments unsuitable for making tools, beings so physically capable they don't need tools, and those who just never quite got around to banging the rocks together.

It also covers beings who get stuck at a local optimum. They learn to make stone hand-axes and stop there. Our own ancestors did that for nearly 2 million years (although there's a huge sampling bias in that because stone hand-axes survive much better than tools made of wood and hide).

So we're down to just 5,000 species capable of making and using tools in the Milky Way, and presumably comparable figures for other galaxies. That's still an impressive number. Why haven't they said hello?

Next time: Civilization Filters.

A Pirate's Life for Me!

With the approach of Talk Like a Pirate day (Sept. 19) I'm pleased to be able to talk about pirates. In this case, space pirates. Baen Books has just released a new anthology of classic space-pirate yarns, called Cosmic Corsairs. It's edited by Hank Davis and Christopher Ruocchio, and features some excellent piratical science fiction — including a Fritz Leiber story which has never been reprinted since it appeared in 1941. 

I'm particularly excited because the volume includes my short story "To the Barbary Shore," which eventually grew into my second novel Corsair. You can read my own "How I Did It" post about the genesis of that story, and you can listen to the Baen Free Radio Hour podcast about the new anthology.

The book's available everywhere, so go get it!

Great Filters, Part 4: Life Filters

Having considered how uncommon Earthlike worlds are, now we're going to look at potential Great Filters in the history of life itself.

The first is more of a meta-filter, and affects some of the possible stellar and planetary filters already discussed. It's simply this: what is the range of potential forms of life in the Galaxy?

The most restrictive answer is "life is like Earth life." But biologists and SF writers have come up with a vast range of potential kinds of "life" which are very unlike Earth life. (To thwart any pointless tail-chasing about the definition of life, I'll use this one: Life is any naturally-arising system capable of reproduction and evolution.) Here's a very quick rundown of types, from hottest to coldest.

• Plasma beings storing "genetic" information in magnetic fields, living on the surfaces of stars;

• Beings of fluorosilicone living in liquid sulfur oceans;

• Carbon-based life living in ammonia oceans;

• Carbon polymer life living in liquid hydrocarbon oceans;

• Beings of liquid helium.

There's no evidence that any kinds of exotic life like these exists, but there's no compelling physical reason why they couldn't — and in a vast universe of billions of galaxies, are they any less likely than ourselves? This is especially important because many of the Great Filters already described wouldn't catch exotic life. The high-temperature ones probably wouldn't care much about radiation, and the cold ones have much bigger "habitable" zones and more suitable star systems than we do.

Which makes this a big problem. We've been looking at Great Filters which make life like ourselves rare, but when considering exotic forms of life we either have to discover filters which could snare things we can only theorize about . . . or we run bang into the Fermi Paradox again. Where are they? Where are the fast-evolving high-temperature civilizations which could race to intelligence and Kardashev-III technology in half the time it took us to get here? Where are the vast and slow cold-life empires spreading from Kuiper Belt to Kuiper Belt across the Galaxy, ignoring the warm dry worlds circling close to the deadly stars?

We don't see them. To be fair, we're not really looking for them, either. That's because it's really hard to devise an experiment to detect biosignatures for completely hypothetical forms of life, and even harder to get funding for it.

All of which means this discussion is limited to Life As We Know It. Either our kind of life is the only possible kind of life, or there is a whole alternate set of Great Filters which apply to exotic life types and has so far prevented any of them from doing anything detectable across interstellar distances.

My personal opinion is that Earth-type life really is the only game in town. Carbon is a uniquely adaptable element, and I think it's entirely plausible that only carbon can form information-rich molecules like DNA, and only at liquid-water temperatures. The rest of this post will operate under that assumption, but note that it is an assumption.

Digression over.

The simplest Great Filter related to life is simply that life is rare. On Earth, the first living things date back nearly 4 billion years, out of the planet's 4.5 billion year history. That strongly suggests that life formed as soon as conditions were even marginally suitable, but we really don't know. Perhaps Earth just got really lucky. This question won't be settled for a while. Even if probes to Mars uncover traces of ancient life there, it's entirely possible that the two worlds traded primitive organisms and life only had to happen once.

Finding life on Europa or Enceladus would be a bit more suggestive. It's much less likely that organisms could make the voyage from Earth to the outer moons. If life can evolve on bodies as different as Earth and Europa, then presumably it arises anywhere conditions are right.

The early date of life on Earth is a strong clue that life is nearly inevitable. I'm going to make this one another 50-50 coin flip filter. If a world has suitable conditions, life arises half the time. I personally think that's pessimistic, but we'll stick to it for now. That cuts the number of potentially life-bearing worlds to just over 2 million in the Milky Way Galaxy.

The Drake Equation jumps directly from worlds-with-life to intelligence, and that seems to be a common fallacy both in discussions of SETI and in science fiction. Even now one can see films or TV shows depicting planets with trees and grass and furry critters, which characters describe as having "no life forms." But there are still some hurdles between some self-replicating goop in a warm little Archaean pond and Our Glorious Selves.

First, while life may be common, eukaryotic life, with a distinct cell nucleus, may not be. Most species on Earth are prokaryotes — smaller and simpler forms with no nucleus. Eukaryotes dominate Earth's biomass because they mastered the trick of symbiosis with other single-celled organisms. Every one of your cells includes mitochondria, which are apparently prokaryotes incorporated by some distant ancestor, making you capable of oxygen respiration. The cells in every green leaf include chloroplasts, which are basically blue-green algae co-opted by eukaryotes to allow photosynthesis.

How common are eukaryotes (or some analog) in the Galaxy? No way to tell, but here's a significant fact: they came along in the Proterozoic era, about 2 billion years ago. In other words, half the history of life on Earth consisted of nothing but prokaryotes living their single-celled lives. That gives us a useful number: assume another 50 percent chance of eukaryotic life (or an alien equivalent). That knocks us down to 1 million potential civilizations.

But we're not done yet. The next potential hurdle for life on Earth was multicellularity. Eukaryotes are all very well, but it's hard to have specialized tissues like, say, brains, when you've only got one cell. However, history suggests multicellularity is an easy hurdle to get over. It has evolved independently a whole bunch of times in Earth's history. There are even some multicellular prokaryotes. I'm going to declare that this filter isn't really a filter at all. If you've got cells, apparently at some point they're going to start clumping together, and the evolutionary advantages are strong.

Those are the life-related filters. There is a third — the emergence of life onto land, but I covered that one last time in the planetary filters, with the water world filter. I think it's otherwise pretty inevitable that organisms would colonize any environment they can reach.

Let's stop a moment and look at the Galaxy. It's full of stars, and many of them have planets. But there are huge patches where radiation from the galactic core or active star-formation regions has sterilized every world. Squeezed in between those deadly zones are safe areas, with Sun-like stars separated by about 10 light-years. But potentially habitable worlds are more like 50 light-years apart. Most of those have oceans full of single-celled organisms or clumps of simple cells, but nothing more complex. You'd have to go more than a hundred light-years in the right direction to find a planet with complex organisms.

And how many of those worlds have someone on them looking back with curiosity? I'll take that up next time.

The Star Trek Horta image is almost certainly copyrighted by Paramount or Viacom or CBS or whoever owns the rights to it.

Great Filters, Part 3: Planetary Filters

I've discussed galactic and stellar-scale Great Filters. Now it's time to look at them on a planetary level.

The first is the question of how stable planetary systems are. This is a problem which goes back to Isaac Newton. When Sir Isaac compiled his great work the Principia, he was bothered by one implication of his discoveries. There didn't seem to be any general solution to the "Three-Body Problem" — that is, how the interactions of multiple bodies in a planetary system would work. A general solution would show obvious regimes of stability; otherwise you'd just have a constant iterative process of constant perturbations with planets shuffling around and occasionally getting flung out into interstellar space.

The Italo-French mathematician Joseph-Louis Lagrange discovered one set of special conditions for the three-body problem, the famous "Lagrange Points" of stability around a moon or planet orbiting a larger body.

Before the discovery of exoplanets, the question of stable planetary systems was very much up in the air (beyond it, really). With only one sample, our own Solar System, we really had no way of knowing if we lived in a freak example of a stable planetary system, or if such things were commonplace.

The answer seems to lie between those two poles. There are lots of planetary systems, so many that it appears to be the norm. Only stars too young to have formed planets at all may lack them.

But the study of the orbits of those exoplanets has revealed a surprising amount of instability. Apparently planets migrate all over the place in stellar systems, shifting orbits, switching places, and in all likelihood getting kicked out into interstellar space. The stable systems that remain are simply the aftermath of chaos, smoothed down by billions of years.

So how is this a filter?

Well, we still need to figure out what proportion of planetary systems are unstable in the long term. Even if a potentially lifebearing world doesn't get tossed into space, jostling around its home system can't be a good thing for struggling life forms on its surface. We need to know how long planets are likely to remain in the habitable zones of their parent stars, where temperatures are warm enough for liquid water but not hot enough to boil away all the lighter elements.

This isn't an easy number to pick. Astronomers are still figuring out how planets form and how they interact over billions of years. Since we see lots of planetary systems, with plenty of worlds, instability can't be too great a problem. And many of our best methods for detecting exoplanets are biased toward weird systems — it's easier to detect a large world orbiting close to its parent star, and it's precisely those systems which are the product of lots of planetary migration and interaction.

I'm going to make a completely wild-assed guess and assume ten percent of star systems have chaotic interactions among their planets which make it unlikely for any of those worlds to develop life. In last week's post I winnowed the number of star systems which might be home to lifebearing worlds to about 10 billion in the Milky Way. This lops a billion off that figure.

Next comes the big one: how many Earthlike planets are there? By "Earthlike" I mean a rocky planet with liquid water and an atmosphere density somewhere between 1 percent and 200 percent of Earth's, orbiting in the habitable zone (sometimes called the "Goldilocks Zone" of its star system.

At present we know of some 4,000 exoplanets. Of those between 20 and 50 are in their star's habitable zone, depending on whether you take the optimistic or pessimistic view of limits on that zone. For simplicity's sake we'll say 1 percent of planets are in the habitable zone. For the Milky Way, that means 90 million worlds.

Now, some of those bodies are not exactly what you'd call "Earthlike," with masses more comparable to Neptune or even Saturn. It's hard to see how a giant planet like that, even with a rocky core, could avoid a lethal greenhouse effect. Roughly half the worlds on the list of exoplanets in the habitable zone of their parent stars have masses greater than 10 times that of Earth, which I consider beyond the pale. So our 90 million becomes 45 million.

After that comes a grab-bag of filters based on what we know of Earth's history, with absolutely no hard numbers to base our estimates on. These potential planetary filters include:

Giant Impacts: We believe the Moon was formed when Earth collided with a Mars-sized body early in its history. This may have affected Earth's chances of developing life, by blowing off the planet's original atmosphere and preventing Earth from winding up like Venus. That's highly speculative, and it's also unclear how common such impacts might be. A lot of airless bodies with ancient surface features show really big impact basins, so it's quite possible that almost planet gets walloped by something big in its early years. For lack of any hard numbers I'll call that one a coin toss: 50 percent.

Moons: It's been suggested that the Moon makes Earth a home for life by stabilizing the planet's rotation and axial tilt. I'm not sure I buy it — it sounds a lot like correlation (Earth has a big moon, Earth has life) being turned into causation (Earth's big moon is responsible for Earth having life). Since moon formation appears to be tied to impacts, I'm just going to fold this one into the previous filter.

Snowball Earth: When plant life evolved on Earth and began transforming the planet via toxic waste products, one side effect was a drastic drop in temperature as the plants sucked up all of the atmospheric carbon dioxide, reducing the greenhouse effect keeping Earth warm. Earth's surface was covered by ice, with life surviving only in the oceans under the icecap. The "Cryogenian" era may have lasted up to 200 million years, ending only when the Sun's gradual increase in temperature overcame the Earth's cooling.

But what if Earth had been at the outer edge of the habitable zone, or the Sun's output wasn't increasing as rapidly? Snowball Earth could have lasted hundreds of millions of years longer, or been permanent. To me, this one seems like an important filter, one likely to repeat elsewhere. How to estimate the outcome of two independent processes is very hard, so I'm just going to make this one another coin-flip: 50 percent.

Waterworlds: Earth's surface is 75 percent ocean. Raise that sea level by just a couple of miles and the figure would be close to 100 percent. If we assume that more massive worlds retain more light elements, then any world more than, say, two or three times Earth's mass might be entirely covered with water. This wouldn't prevent life from evolving, but might well prevent technological civilizations from arising. While it is possible that an aquatic alien civilization might come up with ways to communicate across interstellar distances before they discover fire or electricity, I'd hardly call it likely. That one's another coin-flip.

Summing up these guesswork filters gives us 1 in 8, which I'm going to adjust to 1 in 10 for simplicity, and to cover any other planetary filters I didn't think of. Applying that to our existing figure we get 4.5 million worlds in the Milky Way which might be home to life and civilization. That's still a lot, but it's certainly less than the 10 billion figure we had at the end of my last post.

It's also cause for some limited optimism. Our world is already a .0045 percent long shot. But with literally millions of possible inhabited worlds, we're certainly not out of the (dark) woods yet. Where is everybody?

Next time: Life Filters.

Great Filters, Part 2: Galactic and Stellar Filters

The first set of potential "Great Filters" I'm going to discuss are those which operate at the level of stars and galaxies — things which might make most galaxies, or most star systems, unsuitable for the development of life.

But first, a digression. In science, especially in cosmology, the number 1 is a very funny number. Scientists don't like unique phenomena. They like to put objects or processes into a class, and study the characteristics of that class. So in the Solar System, planetary scientists study the four rocky worlds, the two gas giants, the two ice giants, and the low-mass dwarf planets. They look at similarities and differences.

Unfortunately, Earth is unique in many ways, the most obvious being that it's covered with life. A single lifebearing planet in the Milky Way — or in the entire observable universe — is just weird. There are lots of stars, lots of planets, lots of galaxies. But only one Earth. If there is some factor which prevents lifebearing worlds, but somehow skips Earth, that's weird, too. On the cosmic scale, 0 is a much more plausible number than 1. There should be no lifebearing worlds, or many.

Digression over. Let's look at some Great Filters.

The first is Active Galaxies, or Active Galactic Nuclei. One thing which radio astronomers began to discover in the 1960s was that some galaxies emit a heck of a lot of energy. Here's a NASA Web page explaining it in more detail. 

For a long time the mechanism wasn't well understood, but astrophysicists have figured out that supermassive black holes at the core of certain galaxies are the likely culprits. As matter falls into a black hole, it gives off a huge amount of energy, nearly the equivalent of converting its mass into energy. Active galactic nuclei have black holes which are taking in a lot of mass, releasing a lot of energy, and — via magnetic or gravitational effects —blasting out that energy in huge beams or jets. Obviously, galaxies which are pumping out enormous amounts of X-ray and gamma radiation are going to be a pretty deadly places. Most kinds of life we can imagine would have a very hard time evolving and surviving in an environment like that.

But active galaxies aren't that common. There may be tens of thousands of active galaxies we can observe, but there are hundreds of billions of galaxies in the sky. So as a filter, active galactic nuclei aren't very important. There's also the issue of time: the closest active galaxy is hundreds of millions of light-years away, which means it's hundreds of millions of years in the past. It's entirely possible for a galaxy to go through an active phase and then settle down, perhaps eventually becoming an environment where life could evolve.

The notion of a lethal galaxy is a chilling one if you think about it: a hundred billion stars, many with planets, and all utterly dead. You could stand on a world (wearing a thick lead suit), look up at the sky, and know that you were utterly alone. Brr.

Of course, even a "safe" galaxy can still have regions which are unsuitable for life. This leads to the next large-scale Great Filter, the Galactic Habitable Zone. The concept of the "Galactic Habitable Zone" is analogous to the region with in a star system where lifebearing worlds may exist. 

Simply put, there are parts of any galaxy you just don't want to be in: the galactic core is bathed in radiation from the central black hole, and is rich in bright, short-lived stars which end their lives in massive supernova explosions. Such explosions bombard nearby systems with blasts of high-energy particles, making them unfriendly places for living things. Gas and dust lanes along the spiral arms also have too many supernovas for comfort. According to some estimates, about half of the galaxy might be outside the Habitable Zone.

A third galaxy-scale filter relates to the formation of necessary elements, which astronomers charmingly simplify as "metalicity" (to an astronomer, the universe consists of hydrogen, helium, and metals). Hydrogen and helium are primordial elements, but everything else is made in the cores of giant stars, then flung out into space when those stars explode. Obviously, regions without a lot of star formation don't have many supernovas and thus are low in metals. Current theories suggest that the outer reaches of the galaxy are too metal-poor to have many planets at all, let along life-bearing worlds like Earth.

So you can't be in a part of the galaxy with too many supernovae, or you'll get cooked. But if you're in a region with too few exploding stars, you won't have the metals to build planets or living things. Combined with the Habitable Zone limits, this suggests that only about a third of the galaxy is both habitable and likely to have planets. Of course that still represents tens of billions of stars.

Once again, there's also a time component. As generations of giant stars form, make heavy elements, and explode, the average metalicity of a galaxy gradually increases. This, plus the notion that active cores are more common in young galaxies, does at least give us a rough upper bound for how old any civilization could be. Our own solar system may be part of the first cohort of star systems with enough heavy elements to form planets and potentially have life.

There is a lot of wiggle room in those estimates, though. While there may be no planets ten billion years old in our galaxy, it's possible some stars got a head start on our sun by a couple of billion years. So one might have lifebearing worlds that are six or seven billion years old, instead of Earth's four to five. They'd be less common, but not impossible.

Now, on the cosmic scale this makes a HUGE difference. We can more or less rule out looking for signs of life or civilization in any galaxies more than a couple of billion years younger than our own. Which means any galaxy more than, say, two billion light-years away is underage and off-limits. At a stroke we've filtered out three-quarters of the Universe.

Our fourth and final stellar-scale Great Filter is the fraction of stars that are likely to have lifebearing planets. A star needs to have a lifespan long enough for planets to form, and its energy output needs to be steady enough for life to evolve. This rules out nearly all the brighter classes of stars, because they just don't last long enough. Most of the visible stars in the sky, especially the low-magnitude ones with names you'd recognize, are too big and short-lived to have planets with life.

However, because they don't last long, those big bright stars represent only a tiny fraction of the galaxy's stellar population. All the type O, B, and A stars combined make up only one percent of stars.

But at the low end of the stellar scale, small dim red dwarf stars make up the majority of all stars, by a veto-proof margin, and they may also be unsuitable for lifebearing worlds.

There are several reasons to reject red dwarf stars. A lot of them are very old, and thus formed before the heavier elements were abundant enough for solid planets. Their dimness means any planets warm enough for liquid water would have to orbit closer than Mercury circles the Sun, and that puts those worlds at risk from flares — something red dwarfs are very prone to. It's also likely that planets orbiting close to a red dwarf would be tidally locked to their parent stars, and it's very hard to see how a habitable environment could exist on such a world.

There are some dodges to get around these problems. A large moon circling a gas giant in close orbit around a red dwarf would avoid tidal locking (to the star, anyway) and would gain the protection of the giant planet's magnetic fields. There might be red dwarf stars which are stable and don't emit deadly flares. A double-planet system could also avoid tidal locking. These are all unlikely, but not impossible.

Red dwarfs make up three-quarters of all stars. The objections already noted may eliminate about ninety percent of them. (This is a bit of a wild-ass guess, but because there are so many unknowns I'm leaving some room in this filter.) Combined with the other filters resulting from the Galactic Habitable Zone and the metalicity issue, we've eliminated a good 90 percent of the stars in the galaxy from consideration.

That still leaves something like 10 billion stars in our galaxy — and equivalent numbers in most other galaxies — which might have lifebearing worlds, but star type and location in the galaxy are definitely a Great Filter, one which we have obviously passed through.

Ten billion is still a big number, and leaves the question open: Where Are They?

Next time: Planetary Filters

 

Great Filters, Part 1

Two weeks ago, during the world's first CYBER WorldCon, I participated in a panel discussion called "COVID-19: A Great Filter?" The panelists were myself, writer and Coronavirus expert Vylar Kaftan, and astrophysicist Valentin Ivanov. Our topic was the concept of the "Great Filter" and whether a plague like the current Coronavirus outbreak could qualify.

         The more I thought about Great Filters, the more I realized there was no way to fit the topic into one 50-minute Zoom panel, or even a single 'blog post. So I'm going to write a series of posts about potential Great Filters, working my way through different types and examining what we know and how plausible they are.

         But first: what the heck is a Great Filter?

         The term comes from an essay by economist and futurist Robin Hanson, which you can read here. To summarize, we know that technological civilization capable of communicating across interstellar distances is possible, because we exist and could do it ourselves. But if that's the case, as Enrico Fermi famously asked, "Where is everybody?"

         Science doesn't like unique phenomena. There should be either multiple technological civilizations, or none. We know it's not impossible because we exist, so . . . where is everybody?

         Just to complicate matters, it's unlikely that most civilizations out there would be about the same age as our own. It took billions of years for Earth to produce hominids, and hundreds of thousands of years for humans to develop radio telescopes and space travel. Even a variation of just one percent would mean other civilizations would be millennia, if not eons older than our own. Even at modest rates of growth they should be detectable across interstellar distances even if they aren't deliberately signaling to us.

         Frank Drake tried to quantify the probabilities of such civilizations arising in his famous Drake Equation, which you can read about here. He broke apart the likelihood of civilizations existing into a number of sub-probabilities. How likely are stars to have planets? How likely is it for a planet to support life? How likely is it for intelligence to arise? And so forth.

         Hanson took that another step. Obviously something keeps the number of technological civilizations very low. How do we know that? Again, because we're here. If civilizations were common, either we'd have heard from them, or seen signs of their existence . . . or we wouldn't exist because a bunch of settlers from the Perseus Arm of the Galaxy colonized Earth right after dropping an asteroid on Yucatan to clear away those pesky dinosaurs 65 million years ago.

         The big question is whether the Great Filter — the real bottleneck keeping the number of civilizations vanishingly small — is behind us or ahead of us. In other words, is the origin of life or the origin of intelligence the Great Filter, in which case we've passed through; or is there something about to happen to us which we can't predict or prevent, and which has hit every other nascent civilization in the Milky Way?

         As I said, this is a subject too big for a single post, so I'm breaking down the potential filters into categories. First come Stellar Filters: things related to the Galaxy or to stars which make life or intelligence unlikely. Next is Planetary Filters, reasons to think Earth might be extremely rare in some way. Then Life Filters, looking at just how likely it is for life to arise and evolve into something which could become intelligent. After that comes Intelligence Filters, also including issues of language and tool use. Then Civilization Filters, which might make large-scale social systems uncommon in the Galaxy.

         That brings us up to the present, and then I'll start looking at Filters we still may have to get through. Disaster Filters are things which might bring down or destroy our civilization. Space Filters are things which might confine us to a single planet or a single star system. Perception Filters are reasons for optimism: maybe we just haven't seen everybody yet. And finally there are Meta Filters, which are essentially ways to reject the entire question.

         And so, having finished that long-winded introduction, I will stop here and leave everyone hanging. Next week: Stellar Filters!

The First CYBER WorldCon!

This year's World Science Fiction Convention was to have taken place in New Zealand, but since that country is entirely sealed off from the outside world, the convention has gone entirely on-line. It's a CYBER WorldCon! See the link here.

Despite obstacles the show will go on! I'm participating in several events, without leaving my house in Massachusetts. Here's my schedule (all times are in US Eastern Daylight Time):

Wednesday, July 29, 5:00 p.m.: Kaffeeklatsch

An hour-long informal hangout with me and nine other fans. Grab a drink and join us! (I probably will be having something stronger than coffee.)

Wednesday, July 29, 7:00 p.m.: Staying Closer to Home: Science Fiction in the Solar System

A panel featuring Daniel Abraham, Jeffrey Carver, myself, G. David Nordley, and Ian McDonald; focusing on the future of science fiction set within the Solar System rather than among the stars, and how our rapidly changing understanding of the Sun's other planets is affecting that.

Thursday, July 30, 5:00 p.m.: COVID-19: A Great Filter?

A panel featuring Valentin Ivanov, Vylar Kaftan, Ion Newcombe, and myself. We'll discuss whether pandemics like the current Coronavirus outbreak might explain why we don't see any sign of technological civilizations elsewhere in the Galaxy.

Thursday, July 30, 6:00 p.m.: Reading

I'll be doing a 20-minute reading of "Calando," a new short story of mine from the original anthology Retellings of the Inland Seas.

Friday, July 31, 6:00 p.m.: The Ascent of Wonder: What Makes Hard SF

Panel starring Kenn Bates, myself, Mark English, Harun Siljak, and Deborah Munro. We'll focus on how important it is to get the science right, what rules can we break, and who does it well?

Friday, July 31, 11:00 p.m.: RPG Design (And What to Do Next)

Join Michael Sands, Morgan Davie, Brandon O'Brien, myself, and Stephen Dedman for a discussion of how to design role-playing games and how to get them published.

Another Cup of Tea

The complete "Teatime Readings" series is now available for viewing on YouTube. You can see them here. Or you can skip right to my reading from The Initiate.

There's a lot of good stuff and some interesting comments and conversation. Check it out.

Inland Seas Reviews

The first reviews of Retellings of the Inland Seas are out, and it looks as though this collection is getting a good reception. 

Nerds of a Feather reviews it here.

Treehouse Writers blog reviews it here.

It's always interesting to see how different reviewers look at the same book. I like to see varying takes. When everyone focuses on the same things I'm always suspicious they're just reading the press release.