Just The Caffeine Talking

Great Filters, Part 9: Odds and Ends

Well, I think I've come to the end of my series on the Great Filters and the Fermi Paradox. There are a few bits which didn't quite fit into any of the earlier posts. So here they are, in more or less random order.

Have We Looked?

First, there's the issue of what Michael Hart called "Fact A" — we don't observe any extraterrestrial civilizations. But the actual fact is that we haven't detected them, not that they don't exist.

The shocking truth is that we haven't actually looked very hard. For the first fifty years of SETI, searches for extraterrestrial signals were small, spare-time affairs, involving a few hours of instrument time now and then, aimed at a handful of stars. More recently, donations by whimsical tech billionaires and a thin trickle of government funds have finally made it possible to set up dedicated radiotelescopes scanning the skies.

But even the mighty Breakthrough Listen project has only managed to search about 700 stars for signs of radio transmissions comparable to modern radar systems. That's the biggest SETI survey ever attempted, representing a decade of work. It represents one quarter of one-billionth of the stars in the Milky Way. Another billion years or so and they'll be done!

We don't know if there are any other civilizations in the Milky Way. We can't even really be sure there aren't any alien probes parked at Earth's Lagrange points or on the surface of a near-Earth asteroid, calmly watching us.

The actual, known facts are really as follows:

1: We are reasonably certain there are no giant omnidirectional beacons using the entire energy output of a star to power transmissions. At least not on this side of the Galaxy.

2: We are sure there are no civilizations within a few hundred light years beaming messages directly at us with an antenna the size of the Arecibo radiotelescope.

3: We know Earth was not colonized by an alien civilization eons ago (Or was it? See below!), nor dismantled by Von Neumann machines.

Other than that, we don't know!

Someone's Got To Be First

Someone's got to be the first species to reach a level of technology capable of interstellar communication. Maybe it's us. I've even toyed with a mathematical argument in favor of that. If you're born randomly in the Galaxy, which species are you most likely to be born into? Obviously, the odds favor the species which lasts the longest and exists in the greatest numbers. So the fact that I am a human supports the idea that humanity is the Elder Race of the Milky Way.

Of course the same argument can be used to demonstrate that we're on the verge of going extinct, since the odds favor being born at the time of humanity's greatest population. I suspect the entire argument is bogus, but I haven't been able to prove it yet.

Anyway, if my earlier estimates about the number of civilizations in the Milky Way are in the right ballpark, then there's about a 1 in 50 chance we're the oldest technological species. Those are long odds . . . but they're not impossible ones. Unless and until we do detect another civilization, this seems like the best assumption.

A Wizard Did It

Or God, or the programmer of the simulated reality we're all living inside, or whatever. The assumption, again, is that the Galaxy (if not the whole Universe) was either created specifically for us, or the simulation was run over and over and we're the first ones to appear this time (see above). Amusingly, one of the arguments in favor of the simulation hypothesis is exactly the same as my mathematical proof that we're the first.

Once you add a supernatural component, two things happen. First, any analysis based on the observed Universe becomes pointless, because the only elements that matter are the choices of whatever intelligence is directing and arranging things. Study theology instead of astronomy.

Second, all the questions just get moved up a level. Where did that creator or simulator come from? What are the rules of the "real" universe in which the simulation is running? Again, study theology.

We're Them

Or, perhaps we've been ignoring a key possibility. Maybe life didn't originate on Earth or in the Solar System. Maybe we owe our existence to some seeding expedition a couple of billion years ago. Rather than disproving the existence of ancient starfarers, maybe life on Earth is evidence they stopped off here.

That doesn't really negate any of the other Great Filters I've discussed, but it would change the numbers around. We could expect to find life everywhere, which makes intelligence and technology even less likely.

Of course, as Arthur C. Clarke suggested in 2001, perhaps our intelligence is also the result of alien meddling. Again, if we are made rather than the result of evolution, see the section on supernatural creators above.

They're Everywhere

One topic which keeps cropping up in discussions of the Fermi Paradox is that even if the aliens aren't interested in transmitting messages in our direction, we should still be able to detect any large-scale activities of theirs. Dyson spheres, stellar mining, laser-launched interstellar missions, and so forth. But (so far) we don't. See above for not looking much.

Unless, maybe we have but don't know it yet. One of my personal theories is that some astronomical phenomenon we've attributed to natural processes is really the work of intelligence elsewhere in the Galaxy. If any astronomers can think of something which is uncommon but shows up in other galaxies and has only tentatively been explained, maybe that should be re-examined as a potential sign of extraterrestrial intelligence.

All right, I'm done now. What now? Now we wait and see.

Great Filters, Part 8: Space Filters

Wow, I've been doing this for two months and am not done yet. Now I know how Proust must have felt. Assuming Proust was writing blog posts about the Fermi Paradox, of course. So far I've looked at all the Great Filters which lie in our past — barriers to the evolution of life, barriers to intelligence and technology, and potential civilization-ending disasters. They certainly do reduce the number of possible advanced civilizations in the Milky Way, but only to a point.

Right now we're stuck at an estimate of about fifty civilizations out there among the stars, of which perhaps two-thirds have the wealth and ability to construct radiotelescopes or interstellar probes. Call it thirty. That's an awkward number because there's a very strong chance that many of them would be thousands if not millions of years older and more advanced than we are, which should make their activities easy to spot even across interstellar distances. But as yet we see nothing.

Today I'd like to consider three possible Great Filters which might explain why those civilizations — assuming they exist — are not doing anything we can detect. So these are Space Filters.

The first is the simplest, and most likely. Space travel, particularly interstellar travel, is just really hard. We may be able to send microchip-sized probes to nearby stars, but no colonists, no flag-planters. And given that it's likely to be tens of lightyears to the nearest world with even simple microbial life, maybe the effort is too great.

We do know that dangers wait in interstellar space: in the past few years we've detected two comets originating outside the Solar System. Now since presumably they didn't just start showing up right after humans invented ways to detect them, this suggests that interstellar wanderers are fairly common, and that space between the stars may be studded with rocks and shoals.

This would certainly put a damper on fast interstellar travel. Hitting a full-sized comet nucleus at any large fraction of the speed of light wouldn't just damage the starship, it would literally vaporize it. Even small rocks, pebbles, and dust grains — which, statistically, are undoubtedly far more common than visible objects like Oumuamua — would be catastrophic obstacles for star voyagers.

So go slowly, then. One thing which Oumuamua did demonstrate is that it's entirely possible to travel between the stars, as long as you don't mind spending tens or hundreds of thousands of years doing it. But is it really possible for any civilization to create a self-contained ship which can keep running that long? Or for any civilization to want to send out an expedition which won't report back for a thousand centuries?

Those are strong objections. Even the notion of firing off a bunch of Von Neumann self-replicating probes gets a little unattractive if your civilization might not last long enough to get the data they send back.

Now, it's possible that individuals in a sufficiently rich civilization might be able to launch their own interstellar missions. Right now a few tech billionaires on Earth are funding interstellar research essentially out of personal whims. The inhabitants of a Dyson Sphere with the full energy output of their home star to play with could do it as the equivalent of a high school science club project. But it's very hard to imagine any sustained, large-scale, systematic interstellar project under those circumstances.

I am going to call this one another Great Filter. Let's assume that half of those thirty advanced civilizations out there find the prospect of interstellar exploration too difficult, and so never do it. That cuts the number down to fifteen. Still awkward.

The second Space Filter is what some have called the "Dark Forest" problem. The idea goes like this: if you make yourself known across interstellar distances, some other civilization might decide to conquer or annihilate you. So everyone just keeps silent. The analogy is of a group of armed men stumbling around in a dark forest at night. None of them dares turn on a flashlight or call out, because anyone who draws attention could become a target.

I've written another long-winded series of blog posts about why I don't buy the Dark Forest idea at all. In addition to the reasons cited, there's another factor which I've only become aware of since writing those original pieces. Quite simply, our existence disproves it.

An advanced civilization can construct immense space-based telescopes, both radio and optical. Our own observatories are already looking for "biosignatures" and "technosignatures" on exoplanets within several hundred light-years. A civilization with kilometer-wide telescopes could do the same across a significant fraction of the Galaxy. This would be an especially important project for a paranoid xenophobic civilization planning to shoot first — the equivalent of night-vision goggles for that dark forest.

Our planet has had biosignatures in its atmosphere for billions of years. It has had detectable technosignatures for a few centuries, depending on how clever any potential observers are. The point is, we can already be seen across hundreds of light-years. Maybe thousands. Yet here we are. No interstellar killer probes yet.

Also, the detection and industrial infrastructure needed to monitor the whole Galaxy and zap any emerging civilizations should itself be detectable. So I'm calling the Dark Forest filter a dud. I just don't think it's likely.   

And now the final filter, the one that I actually do worry about. In my discussion of civilization filters a couple of weeks ago I mentioned the danger that some civilizations might reach some kind of local optimum and simply stop advancing. No Scientific/Industrial/Economic/etc. Revolutions, just endless generations of Iron Age peasants, ruled by an endless series of alternating warlords and bureaucrats, until the Sun swells in the sky and the oceans dry up. Not a bad life, really, but one which leaves no mark on the Galaxy.

We got through that filter . . . or did we? It's possible there are several such dead ends in history, including one or more in our future. We may be on the cusp of one right now.

It's alarmingly easy to imagine a future for humanity in which we never leave Earth at all. Unmanned probes explore the Solar System a few square kilometers at a time, but that's all. No space colonies, no terraforming Mars, no Dyson Sphere Kardashev II civilization for us. Just endless "sustainable" generations of Information Age consumers, ruled by an endless series of alternating demagogues and bureaucrats, until the Sun swells in the sky and the oceans dry up (because nothing is really sustainable over time).

And across the Galaxy there may be dozens of civilizations caught in just the same trap, stuck on a single planet and unable to bootstrap themselves up to becoming interplanetary. Maybe . . . all of them?

That's a Great Filter I can see coming, and one we may be able to avoid. It means taking action — investing, donating, and letter-writing. If the Universe isn't worth fighting for, what is?

A Prediction

If you read this article — "The Bias that Divides Us," by Keith Stanovich — you will nod sagely about the cognitive bias it describes. And then you will spend ten or fifteen minutes reassuring yourself that you aren't affected by it.

Great Filters, Part 7: Disaster Filters!

We've brought ourselves up to the present day in the evolution of the Earth, life, and human civilization. My estimates for the various filters we've passed through indicate that there ought to be around fifty other civilizations in the Milky Way.

The principle of mediocrity suggests that a good half of those fifty should be older than our own by at least an order of magnitude, if not many. Which raises the Fermi Paradox question again: Where Are They? Why don't we see any sign of ancient supertech civilizations enveloping stars with Dyson Spheres, building artificial black holes, dismantling stars, and doing other cool stuff like that? Why don't we detect their interstellar laser messages? Why don't we see their starship engines? Why do we exist at all when they might have colonized Earth and replaced our vertebrate ancestors with alien organisms?

This raises the worrying possibility that there are still some Great (or at least Great-ish) Filters in our future — filters which evidently could and did thwart the growth of beings as rich and clever as ourselves. Filters which may have ended civilizations bigger and wiser than our own. Scary stuff.

One's thoughts immediately stray to big disasters which might somehow wipe out a technological civilization. That's what I'm going to discuss today: disaster filters.

The trouble with disasters is that they're actually kind of wimpy, at least when you're talking about intelligent tool-using beings with large social structures who live all over the planet.

Consider everyone's favorite civilization-ender: nuclear war. Long, long ago, back in the glorious 1980s, I sat down to create a postapocalyptic roleplaying setting based on real-life North America. I knew how many bombs the Russians had, and various nuclear-disarmament groups published helpful lists of potential targets in the United States. So just black out a twenty-mile radius around each of those targets and presto! There's your Thundarr the Barbarian world.

Except for two problems.

First, the areas which weren't blacked out still included a heck of a lot of towns and small cities, plenty of farmland, forests, oil fields, coal mines, and other resources. Plenty of people, too. They could communicate, organize, and rebuild. Sure, there would likely be bandit gangs and some regional warlords, at least at first, but it didn't seem very likely that civilization would vanish — or even regress much. My postapocalypse would look more like the 1930s than the 1980s, but it wouldn't have illiterate savages with MTV hairstyles gazing in superstitious awe at the ruins of Disney World.

The second problem was when I remembered that the world exists outside the borders of the United States. Even if the whole Warsaw Pact plus Red China unloaded everything they could throw at the US, NATO, France, Australia, and Japan . . . what about Brazil? What about Mexico? What about Thailand or India or Ethiopia or Nigeria or Zaire?

The notion of "nuclear war blows up the world" seems to incorporate a lot of not-so-hidden racist assumptions about the inhabitants of the warmer parts of the planet. All those countries I listed above had burgeoning industries even in the 1980s.

Even the much-feared "nuclear winter" which would leverage the damage of a nuclear war into a planetary ice age wouldn't be enough to wipe out civilization. How do I know that? Simple: we've done ice ages already. Our ancestors rode out ten thousand years of glaciation equipped with tools made of wood and stone, with only wood fires and animal furs to fight the cold. They not only survived, they expanded across the globe during that period. An ice age made humanity.

Much the same objections apply to giant asteroid impact as a civilization-ender. Right now we're just on the edge of being able to detect and deflect an object headed for Earth. But even if we fail, and an object like the Chicxulub impactor — the asteroid that killed the dinosaurs — hits Earth, it probably wouldn't wipe out humanity.

Sure, billions would die. Coastal areas would likely be ravaged by tsunamis. Another ice age might follow. But . . . Zaire would be in fine shape. Paraguay would be unscathed. The American Great Plains and Ukraine and western China and Zambia and Bolivia and Chile would come through intact, all able to cooperate and rebuild.

Humans have been through planetary disasters. They would definitely harm our civilization and change it, but I don't think they would destroy it.

What about plagues? That was the original question which started this epic. Well, the worst plague on record, the Black Death, may have killed as much as one-fourth of the humans alive at the time. Civilization barely noticed. Some borders changed, there were economic shifts, but no knowledge was lost. One would not call the world of A.D. 1400 "less civilized" than the world of A.D. 1200, before the plague arrived.

In the modern day, we are right now living through a global crisis resulting from a disease about a thousandth as deadly as the Black Death. It's a crisis for us because we've gotten so good at fighting and eradicating diseases that any fatal virus is now unusual and alarming. Yet even the current crisis is temporary: we're arguing about what to do until a vaccine is developed. Not "if" but "when."

Again, we've survived much worse diseases while relying on medical technology that was either utterly useless or actively harmful. Using scientific medicine, the question is simply how long it takes to stop an epidemic. Like ice ages, we've done plagues.

Okay, but those are natural viruses. What about a man-made menace? What if some paranoid regime or mad genius builds a virus that is easily transmissible, incurable, and utterly lethal? And what if it gets released in every airport in the world simultaneously?

Even a hypothetical superbug would leave survivors. And humanity has been through some severe population bottlenecks before — it has been suggested that after the Toba supervolcano eruption 75 thousand years ago, the human population was reduced to fewer than 10 thousand individuals. Other studies have posited that earlier in human history the population could have been as low as just a couple of thousand. We got through those crises, even without the resources of an entire global civilization sitting idle around us.

So, again, disease might simply slow down human population growth and technological progress for a few centuries, but not make us extinct.

A more high-tech version of the plague filter is the "gray goo apocalypse" — the notion that some super-advanced form of self-replicating nanotechnology would get out of control and consume everything on the Earth.

There are two problems with this notion. The first is that Earth has been covered by voracious self-replicating machines for about four billion years already. Any cell-sized nanobots escaping into the wild would likely fall victim to feral microorganisms. A nanotech plague sophisticated enough to overcome living things would essentially be a life form itself. See plagues, above, for how we could cope with that.

Finally there's the macroscopic literalized metaphor of technology out of control: the Robot Apocalypse. This is the fear that if we can someday create artificial intelligences which are as smart as we are, or smarter, then they would exterminate and replace us.

That's a plausible fear, but it's utterly irrelevant to the Fermi Paradox. It just means that our world would be home to a technological civilization of digital intelligences rather than biological ones. Getting wiped out by robots is rough on your species, but not your civilization.

So I honestly can't think of any disaster which might wipe out humanity or permanently end our technology. Now, admittedly, the Great Filter concept includes the assumption that the terrible filter in our future might well be impossible to imagine until it's too late — since otherwise presumably some civilization might have avoided it.

Next time I'll discuss the one possible future Great Filter which does keep me up at nights.

Hanson Again

No, I don't have some kind of Internet stalker crush on Robin Hanson. I know I've spent six weeks writing 'blog posts about an idea he had a decade ago, but that doesn't prove anything. Posting a link to a recent piece by him about technological progress (or the lack thereof) doesn't prove anything either.

Tomorrow: more Great Filters!

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.