• 

A spacefaring species could easily settle the entire Milky Way given billions of years. Yet the fact is that there is no obvious one in our solar system right now. The supposed inconsistency between these statements is the Fermi Paradox, named for the Nobel Prize-winning physicist who supposedly first formulated it. In a trenchant formulation of the Fermi Paradox, American astrophysicist Michael H. Hart called the lack of extraterrestrial beings or artifacts on Earth today “Fact A.” He showed that most objections to his conclusion—that a spacefaring civilization could have crossed the galaxy by now—stem from either a lack of appreciation for the timescales involved (it takes a small extrapolation from present human technology to get interstellar ships, and even slow ships can star-hop across our galaxy in less time than the galaxy’s age) or else the dubious assumption that all members of all extraterrestrial species will avoid colonizing behaviors forever (an example of what I’ve called the monocultural fallacy).

William Newman and Carl Sagan later wrote a major rebuttal to Hart’s work, in which they argued that the timescales to populate the entire galaxy could be quite long. In particular, they noted that the colonization fronts Hart described through the Milky Way might move much more slowly than the speed of the colonization ships if their population growth rates were so low that they only needed to spread to nearby stars very rarely. They also argued that being a long-lived civilization is inconsistent with being a rapidly-expanding one, so any species bent on settling the galaxy would not last long enough to succeed. In other words, they reasoned that the galaxy could be filled with both short-lived rapidly expanding civilizations that don’t get very far and long-lived slowly expanding civilizations that haven’t gotten very far—either way, it’s not surprising that we have not been visited.

Being a long-lived civilization is inconsistent with being a rapidly-expanding one.

In a 2014 paper on the topic, my colleagues and I rebutted many of these claims. In particular, we argued that one should not conflate the population growth in a single settlement with that of all settlements. There is no reason to suppose that population growth, resource depletion, or overcrowding drives the creation of new settlements, or that a small, sustainable settlement would never launch a new settlement ship. One can easily imagine a rapidly expanding network of small sustainable settlements (indeed, the first human migrations across the globe likely looked a lot like this).

Another factor affects Newman and Sagan’s numbers on timescales and colonization-front speeds. Most of the prior work on this topic exploits percolation models, in which ships move about on a static two-dimensional substrate of stars. In these models, a star launching settlement ships can quickly settle all of the nearby stars, limiting the number of stars it can settle. But real stars move in three dimensions, meaning that they can carry their orbiting settlements throughout the galaxy, and that a settlement will always have fresh new stars to settle if it waits long enough.

Jonathan Carroll-Nellenback, at the University of Rochester with Adam Frank, not long ago finished work, with Caleb Scharf and me, on analytic and numerical models for how a realistic settlement front would behave in a real gas of stars, one characteristic of the galactic disk at our distance from the galactic center. The big advances here are a few:

1. Carroll-Nellenback validated an analytic formalism for settlement expansion fronts with numerical models for a realistic gas of stars.
2. He accounted for finite settlement lifetimes, the idea that only a small fraction of stars will be settle-able, and explored the limits of very slow and infrequent settlement ships.
3. He also explored a range of settlement behaviors to see how galactic settlement fronts depend on them.

The idea that not all stars are settle-able is important to keep in mind. Adam Frank calls this the Aurora effect, after the Kim Stanley Robinson novel in which a system is “habitable, but not settle-able.”

The results are pretty neat. When we let the settlements behave independently, Hart’s argument looks pretty good, even when the settlement fronts are slow. Even if all the ships have a very limited range (only able to reach, say, the nearest stars to Earth) and even if they are no faster than our own interstellar ships today (like Voyager 2), we find they can still settle the entire Milky Way in less than its lifetime, supporting Hart’s version of the Fermi Paradox.

Carroll-Nellenback also explores regimes where they have been here, but we just don’t notice because it was so long ago. Frank and Gavin Schmidt explored this possibility in their Silurian Hypothesis paper, and I did something similar in my paper on “prior indigenious technological species” in the solar system. The idea is that “Fact A” only applies to technology that has visited very recently or visited and then stayed permanently. Any technology on Earth or the solar system that is not actively maintained will eventually be destroyed and/or buried, so we can really only explore Earth’s history back in time for something like millions of years, and not very well at that.

The question, in other words, isn’t, “Has the solar system ever been visited?” It’s, “Has it been settled recently?”. Carroll-Nellenback shows that there is actually a pretty big region of parameter space where the solar system is amidst many settled systems but just hasn’t been visited in the last 1 million years.

Of course, there are still lots of other reasons why we might not have been permanently settled by a galactic network of settlements—as we note in the paper:

Hart’s conclusions are also subject to the assumption that the Solar System would be considered settleable by any of the exo-civilizations it has come within range of. The most extravagant contradiction of this assumption is the Zoo Hypothesis (Ball 1973), but we need not invoke such “solipsist” positions (Sagan & Newman 1983) to point out the flaw in Hart’s reasoning here. One can imagine many reasons why the Solar System might not be settleable … including the Aurora effect … or the possibility that they avoid settling the environment near the Earth exactly because it is inhabited with life.

In particular, the assumption that the Earth’s life-sustaining resources make it a particularly good target for extraterrestrial settlement projects could be a naive projection onto exo-civilizations of a particular set of human attitudes that conflate expansion and exploration with conquest of (or at least indifference towards) native populations (Wright & Oman-Reagan 2018). One might just as plausibly posit that any extremely long-lived civilization would appreciate the importance of leaving native life and its near-space environment undisturbed.

Our results are a mixed bag for SETI optimists: Hart’s argument that settlement fronts should cross the whole galaxy—which is at the heart of the Fermi Paradox—is robust, especially because of the movements of stars themselves, which should “mix” the galaxy pretty well, preventing simply connected “empires” of settlements from forming. If Hart is correct that this means we are alone in the galaxy, then this is actually very optimistic for extra-galactic SETI, because it means other galaxies with even a single spacefaring species should rapidly become endemic with them. Indeed, our analysis did not even consider the existence of halo stars, which do not rotate with the galactic disk, or the fact that stars closer to the center take less time to go around—both will make settlement timescales even faster than we calculate.

On the other hand, there are a lot of assumptions in Hart’s arguments that might not hold. In particular, his assertion that if the sun has ever been in range of a settled system then it would have been settled and the settlers would still be here. Perhaps Earth life for some reason keeps the settlements at bay, either because “they” want to keep Earth life pristine or it’s just too resilient and pernicious for an alien settlement to survive. Perhaps Earth is Aurora?

• 

     Reid Malenfant

     You know me. And you know I'm a space cadet.
     You know I've campaigned for, among other things, private mining expeditions to the asteroids. In fact, in the past I've tried to get you to pay for such things. I've bored you with that often enough already, right?
     So tonight I want to look a little farther out. Tonight I want to tell you why I care so much about this issue that I devoted my life toil.
     The world isn't big enough any more. You don't need me to stand here and tell you that. We could all choke to death, be extinct in a hundred years.
     Or we could be on our way to populating the Galaxy.
     Yes, the Galaxy. Want me to tell you how?
     Turns out it's all a question of economics.
     Let's say we set out to the stars. We might use ion rockets, solar sails, gravity assists. It doesn't matter.
     We'll probably start as we have in the Solar System, with automated probes. Humans may follow. One percent of the helium-3 fusion fuel available from the planet Uranus, for example, would be enough to send a giant interstellar ark, each ark containing a billion people, to every star in the Galaxy. But it may be cheaper for the probes to manufacture humans in situ, using cell synthesis and artificial womb technology.
     The first wave will be slow, no faster than we can afford. It doesn't matter. Not in the long term.
     When the probe reaches a new system, it phones home, and starts to build.
     Here is the heart of the strategy. A target system, we assume, is uninhabited. We can therefore anticipate massive exploitation of the system's resources, without restraint, by the probe. Such resources are useless for any other purpose, and are therefore economically free to us.
     I thought you'd enjoy that line. There's nothing an entrepreneur likes more than the sound of the word free.
     More probes will be built and launched from each of the first wave of target stars. The probes will reach new targets; and again, more probes will be spawned, and fired onward. The volume covered by the probes will grow rapidly, like the expansion of gas into a vacuum.
     Our ships will spread along the spiral arm, along lanes rich with stars, farming the Galaxy for humankind.
     Once started, the process will be self-directing, self-financing. It would take, the double-domes think, ten to a hundred million years for the colonization of the Galaxy to be completed in this manner. But we must invest merely in the cost of the initial generation of probes.
     Thus the cost of colonizing the Galaxy will be less, in real terms, than that of our Apollo program of fifty years ago.
     This vision isn't mine alone. It isn't original. The rocket pioneer Robert Goddard wrote an essay in 1918—ninety-two years ago—called The Ultimate Migration, in which he imagined space arks built from asteroid materials carrying our far-future descendants away from the death of the sun. The engineering detail has changed; the essence of the vision hasn't.
     We can do this. If we succeed, we will live forever.
     The alternative is extinction.
     And, people, when we're gone, we're gone.
     As far as we can see we're alone, in an indifferent universe. We see no sign of intelligence anywhere away from Earth. We may be the first. Perhaps we're the last. It took so long for the Solar System to evolve intelligence it seems unlikely there will be others, ever.
     If we fail, then the failure is for all time. If we die, mind and consciousness and soul die with us: hope and dreams and love, everything that makes us human. There will be nobody even to mourn us.
     To be the first is an awesome responsibility. It's a responsibility we must grasp.
     I am offering you a practical route to an infinite future for humankind, a future of unlimited potential. Someday, you know it, I'll come back to you again for money: seedcorn money, that's all, so we can take a first step—self-financing even in the medium term—beyond the bounds of Earth. But I want you to see why I'll be doing that. Why I must.
     We can do this. We will do this. We're on our own. It's up to us.
     This is just the beginning. Join me.

(ed note: thanks to Ian Mallett for bringing this quote to my attention)

• 

I came from Deneb," she said. “Do you know it?”

“No.”

“Sixteen hundred light years from Earth—a system settled some four centuries after the start of the Third Expansion. It is quite different from the solar system. It is—organized. By the time the first ships reached Deneb, the mechanics of exploitation had become efficient. From preliminary exploration to working ship yards and daughter colonies in less than a century…Deneb’s resources—its planets and asteroids and comets, even the star itself—have been mined to fund fresh colonizing waves, the greater Expansion—and, of course, to support the war with the Ghosts.”

She swept her hand over the sky. “Think of it, tar. The Third Expansion: between here and Sol, across six thousand light years—nothing but mankind, the fruit of a thousand years of world-building. And all of it linked by economics. Older systems like Deneb, their resources spent—even the solar system itself—are supported by a flow of goods and materials inward from the growing periphery of the Expansion. There are trade lanes spanning thousands of light years, lanes that never leave human territory, plied by vast schooners kilometers wide. But now the Ghosts are in our way. And that’s what we’re fighting for!”

---

Pael said, watching me, “You see, child, as long as the explorers and the mining fleets and the colony ships are pushing outward, as long as the Third Expansion is growing, our economy works. The riches can continue to flow inward, into the mined-out systems, feeding a vast horde of humanity who have become more populous than the stars themselves. But as soon as that growth falters—”

Jeru was silent.

I understood some of this. The Third Expansion had reached all the way to the inner edge of our spiral arm of the galaxy. Now the first colony ships were attempting to make their way across the void to the next arm.

Our arm, the Orion Arm, is really just a shingle, a short arc. But the Sagittarius Arm is one of the galaxy’s dominant features. For example, it contains a huge region of star-birth, one of the largest in the galaxy, immense clouds of gas and dust capable of producing millions of stars each. It was a prize indeed.

But that is where the Silver Ghosts live.

When it appeared that our inexorable expansion was threatening not just their own mysterious projects but their home system, the Ghosts began, for the first time, to resist us.

They had formed a blockade, called by human strategies the Orion Line: a thick sheet of fortress stars, right across the inner edge of the Orion Arm, places the Navy and the colony ships couldn’t follow. It was a devastatingly effective ploy.

This was a war of colonization, of world-building. For a thousand years we had been spreading steadily from star to star, using the resources of one system to explore, terraform and populate the worlds of the next. With too deep a break in that chain of exploitation, the enterprise broke down.

And so the Ghosts had been able to hold up human expansion for fifty years.

Pael said, “We are already choking. There have already been wars, young Case: human fighting human, as the inner systems starve. All the Ghosts have to do is wait for us to destroy ourselves, and free them to continue their own rather more worthy projects.”

Jeru floated down before him. “Academician, listen to me. Growing up at Deneb, I saw the great schooners in the sky, bringing the interstellar riches that kept my people alive. I was intelligent enough to see the logic of history—that we must maintain the Expansion, because there is no choice. And that is why I joined the armed forces, and later the Commission for Historical Truth. For I understood the dreadful truth which the Commission cradles. And that is why we must labor every day to maintain the unity and purpose of mankind. For if we falter we die; as simple as that.”

“Commissary, your creed of mankind’s evolutionary destiny condemns our own kind to become a swarm of children, granted a few moments of loving and breeding and dying, before being cast into futile war.” Pael glanced at me.

“But,” Jeru said, “it is a creed that has bound us together for a thousand years. It is a creed that binds uncounted trillions of human beings across thousands of light years. It is a creed that binds a humanity so diverse it appears to be undergoing speciation…Are you strong enough to defy such a creed now? Come, Academician. None of us chooses to be born in the middle of a war. We must all do our best for each other, for other human beings; what else is there?”

• 

(ed note: obviously this assumes that faster-than-light starships are impossible)

4.1. Light Cage

McInnes shows that an expanding sphere of colonization cannot expand fast enough to avoid complete societal collapse due to exponential population growth and increasing population density (McInnes, 2002). Furthermore, after such a crash, the society may be so impoverished of resources that it is forever blocked from making a second attempt. If this theory is universally true of all civilizations, then it offers significant insight into the Fermi Paradox. McInnes extends the intuitive notion of a constant expansion rate with a steadily increasing population density to a linear expansion rate which maintains constant population density. However, the expansion velocity must have some maximum speed limit. To generate an upper-bound for this theory, McInnes considers a maximum expansion velocity of 1.0c. Obviously, in a real scenario, the expansion rate would be considerably slower, but that only exacerbates McInnes’ fundamental point which is that once the maximum expansion velocity is reached, population density once again begins to rise. Eventually, total societal collapse is inevitable. The only way to avoid catastrophe is to shift from exponential growth to logistic growth.

Given a specified growth rate and an initial sphere of some specified size, one can calculate the size of the expanded sphere when the increasing expansion rate reaches the 1.0c limit. McInnes calls this sphere the light cage. Assuming 1% annual growth and a starting sphere the size of Earth, he determines that the light cage is at a 300ly (light-years) radius and that the time to reach the light cage is 8000 years. Out of curiosity, we ran the numbers for a more realistic maximum expansion velocity. If we assume a maximum flight speed of 0.1c, a common estimate for fusion-powered starships such as Daedalus (Group, 1978; Matloff, 2005), which yields voyages to nearby stars on the order of decades, and if we assume a regeneration period of a few decades — and therefore comparable to the duration of the voyages themselves — then we estimate a more realistic maximum expansion velocity to be ~.05c. When applied to McInnes’ equations, this velocity reveals a far more realistic cage of a mere 15ly, and an associated saturation time of 7000 years. Interestingly, while a considerably slower expansion rate drastically reduces the cage, it only slightly decreases the time until the cage is hit. This follows naturally from exponential growth. To make matters worse, .05c may be entirely too fast if the stasis period between voyages is longer than the few decades of the previous estimate. If we briefly consider a maximum expansion velocity of .01c, we derive a cage of only 3ly!

Considering that there are only 52 stars within 15ly of Earth (and none within 3ly!), and that only some fairly small subset will support habitation, this does not provide us with very many systems to colonize before we implode in a population density catastrophe. More crucially, 15ly probably lies within the single-voyage horizon, which implies that early waves of colonization may fill the cage all at once as opposed to diffusing radially. In other words, according to light cage theory, interstellar travel hardly provides any population relief at all, right from the beginning.

McInnes is careful to admit that his model is continuous and symmetrical. Admittedly, galactic colonization is loosely like an expanding sphere, but more precisely it is like traversal of a rooted tree (a graph), where the root is the homeworld, vertices are solar systems, and labeled edges connect stars whose distances are below a maximum traversal threshold with the labels indicating interstellar distances (and associated travel times). In addition to the inherently discrete nature of this graph, the natural distribution of stars will impose notable asymmetries on the edge length and per-node degree (branching factor). McInnes admits to the simplicity of his model, but points out that such details should not affect the model’s outcome. We agree, but in practicality, a tree of no more than 52 nodes (probably far fewer), all of which have direct edges to the homeworld, no longer resembles a continuous sphere in even the weakest sense. A Monte Carlo simulation might help illuminate any interesting properties of the discrete model.

Assuming that the continuous model is apropos to the discussion, then we envisage two ways by which the model may not predict actual events. The first is admitted by McInnes, that populations may succeed in converting to logistic growth and therefore avoid the prescribed collapse. Given the calculation of a 15ly cage and the observation that it would saturate in a single emigration wave due to direct homeworld access, any hope of survival would depend on adopting logistic growth prior to interstellar colonization; any later would be too late to avoid disaster. However, Earth appears to be following just such a course, leveling off its population growth in the 21st century, long before undertaking interstellar travel (Haqq-Misra and Baum, 2009). Not only does this bode well for humanity, it suggests that other ETI civilizations can do the same...but such a conclusion weakens McInnes’ theory that the Fermi Paradox is resolved due to a finite expansion before societal collapse. If ETI are once again enabled to continue steady outward expansion, then the original questions underlying the paradox resurface.

The second challenge we would raise in response to McInnes’ model will be explained in section 4.3 when we introduce ITB theory. Briefly, we are unconvinced that interstellar distances don’t impose an insurmountable paucity of interaction events between solar systems such that contagions of societal collapse fail to adequately infect neighbors. Our final thoughts on McInnes’ model reflect on his postanalysis. He proposes the example of a civilization which permits exponential growth at its frontier but enforces logistic growth wherever resource limits are reached, i.e., deeper within the sphere. He shows how such a civilization could achieve phenomenal rates of unbounded expansion, filling the galaxy in a few millions years. While McInnes’ concedes this scenario, he nevertheless questions its validity by citing Landis’ percolation model, namely to doubt that the civilization would expand indefinitely. Considering that in section 3 we demonstrate some weaknesses of the percolation model, we likewise conclude that the proposed scenario stands relatively unchallenged. Should it turn out to be reasonable, then the Fermi Paradox remains, well, paradoxical.

---

Group, P.D.S., 1978. Project Daedalus : the final report on the BIS starship study. Journal of the British Interplanetary Society.

Haqq-Misra, J., Baum, S.D., 2009. The sustainability solution to the fermi paradox. Journal of the British Interplanetary Society 62, 47–51.

Matloff, G.L., 2005. Deep Space Probes: To the Outer Solar System and Beyond. Praxis Publishing Ltd

McInnes, C., 2002. The light cage limit to interstellar expansion. Journal of the British Interplanetary Society 55, 279–284

• 

(ed note: this assumes that faster-than-light starships are impossible)

     “Something’s wrong,” Ben whispered.
     “There always is.”
     “I’m serious.” He let his fingers trace out a line across the black sky. “What do you see?”
     With the Sun eclipsed by the shadow of the FGB module, she gazed out at the subtle light. There was that bright planet, andthe dim red disc of rubble surrounding the Chaera black hole, from here just visible as more than a point source of light.
     “There’s a glow around the star itself, covering the orbit of that single planet,” Ben said. “Can you see?” It was a diffuse shine, Madeleine saw, cloudy, ragged-edged. Ben continued. “That’s an oddity in itself. But—”
     Then she got it. “Oh. No zodiacal light.”
     The zodiacal light, in the Solar System, was a faint glow along the plane of the ecliptic. Sometimes it was visible from Earth. It was sunlight, scattered by dust that orbited the Sun in the plane of the planets. Most of the dust was in or near the asteroid belt, created by asteroid collisions. And in the modern Solar System, of course, the zodiacal light was enhanced by the glow of Gaijin colonies.
     “So if there’s no zodiacal light—”
     “There are no asteroids here,” Ben said.
     “Nemoto. What happened to the asteroids?”
     “You already know, I think,” virtual Nemoto hissed.
     Ben nodded. “They were mined out. Probably long ago. This place is old, Madeleine.”
     “It’s like a fragment of a GMC—a giant molecular cloud,” Ben said. “Mostly hydrogen, some dust. It’s thick—comparatively. A hundred thousand molecules per cubic centimeter... The Sun was born out of such a cloud, Madeleine.”
     “But the heat of the Sun dispersed the remnants of our cloud... didn’t it? So why hasn’t the same thing happened here?”
     “Or,” virtual Nemoto said sourly, “maybe the question should be: How come the gas cloud got put back around this star? ”
     They came at the planet with the Sun behind them, so it showed a nearly full disc. It glared, brilliant white, just a solid mass of cloud from pole to pole, blinding and featureless. And it was surrounded by a pearly glow of interstellar hydrogen, like an immense, misshapen outer atmosphere.
     They could see nothing of the surface. Their instruments revealed a world that was indeed like Venus: an atmosphere of carbon dioxide, kilometers thick, scarcely any water.
     There was, of course, no life of any kind.
     Ben was troubled. “There’s no reason for a Venus to form this far from the Sun. This world should be temperate. An Earth.”
     “But,” Nemoto hissed, “think what this world has that Earth doesn’t share.”
     “The gas cloud,” Madeleine said.
     Ben nodded. “All that interstellar hydrogen. Madeleine, we’re so far from the Sun now, and the gas is so thick, that the hydrogen is neutral—not ionized by sunlight.”
     “And so—”
     “And so the planet down there has no defense against the gas; its magnetic field could only keep it out if it was charged. Hydrogen has been raining down from the sky, into the upper air.”
     “Once there, it will mix with any oxygen present,” Nemoto said. “Hydrogen plus oxygen gives—”
     “Water,” Madeleine said.
     “Lots of it,” Ben told her. “It must have rained like hell, for a million years. The atmosphere was drained of oxygen, and filled up with water vapor. A greenhouse effect took off—”
     “All that from a wisp of gas?”
     “That wisp of gas was a planet killer,” Nemoto whispered.
     “But why would anyone kill a planet?”
     “It is the logic of growth,” Nemoto said. “This has all the characteristics of an old system, Meacher. Caught behind a wave of colonization—all its usable resources dug out and exploited...”
     Madeleine frowned. “I don’t believe it. It would take a hell of a long time to eat up a star system.”
     “How long do you think?”
     “I don’t know. Millions of years, perhaps.”
     Nemoto grunted. “Listen to me. The growth rate of the human population on Earth, historically, was two percent a year. Doesn’t sound like much, does it? But it’s compound interest, remember. At that rate your population doubles every thirty-five years, an increase by tenfold every century or so. Of course after the twentieth century our growth rates collapsed; we ran out of resources.”
     “Ah,” Ben said. “What if we’d kept on growing?”
     “How many people could Earth hold?” Nemoto whispered. “Ten, twenty billion? Meacher, the whole of the inner Solar System out to Mars could supply only enough water for maybe fifty billion people. It might have taken us a century to reach those numbers. Of course there is much more water in the asteroids and the outer system than in Earth’s oceans, perhaps enough to support ten thousand trillion human beings.”
     “A huge number.”
     “But not infinite—and only six tenfold jumps away from ten billion.”
     “Just six or seven centuries,” Ben said.
     “And then what?” Nemoto whispered. “Suppose we start colonizing, like the Gaijin. Earth is suddenly the center of a growing sphere of colonization whose volume must keep increasing at two percent a year, to keep up with the population growth. And that means that the leading edge, the colonizing wave, has to sweep on faster and faster, eating up worlds and stars and moving on to the next, because of the pressure from behind...”
     Ben was doing sums in his head. “That leading edge would have to be moving at light speed within a few centuries, no more.”
     “Imagine how it would be,” Nemoto said grimly, “to inhabit a world in the path of such a wave. The exploitation would be rapid, ruthless, merciless, burning up worlds and stars like the front of a forest fire, leaving only ruins and lifelessness. And then, as resources are exhausted throughout the light-speed cage, the crash comes, inevitably. Remember Venus. Remember Polynesia.”
     “Polynesia?”
     “The nearest analog in our own history to interstellar colonization,” Ben said. “The Polynesians spread out among their Pacific islands for over a thousand years, across three thousand kilometers. But by about A.D. 1000 their colonization wave front had reached as far as it could go, and they had inhabited every scrap of land. Isolated, each island surrounded by others already full of people, they had nowhere to go.
     “On Easter Island they destroyed the native ecosystem in a few generations, let the soil erode away, cut down the forests. In the end they didn’t even have enough wood to build more canoes. Then they went to war over whatever was left. By the time the Europeans arrived the Polynesians had just about wiped themselves out.”
     “Think about it, Meacher,” Nemoto said. “The light-speed cage. Imagine this system fully populated, a long way behind the local colonization wave front, and surrounded by systems just as heavily populated—and armed—as they were. And they were running out of resources. There surely were a lot more space dwellers than planet dwellers, but they’d already used up the asteroids and the comets. So the space dwellers turned on the planet. The inhabitants were choked, drowned, baked.”
     “I don’t believe it,” Madeleine said. “Any intelligent society would figure out the dangers long before breeding itself to extinction.”
     “The Polynesians didn’t,” Ben said dryly.

---

(ed note: The Chaera are the aliens who formerly lived on the murdered Venus-like planet. They currently live in miserable tiny space colonies orbiting a dangerous black hole artifact.)

     “But there remain mysteries,” Ben said. “The Chaera look too primitive to have constructed that artifact. After all, it manipulates a black hole’s gravity well. Perhaps their ancestors built this thing. Or some previous wave of colonists, who passed through this system.”
     “You aren’t thinking it through,” virtual Nemoto whispered. “The Chaera have eyes filled with salty water. They must have evolved on a world with oceans. They can’t have evolved here.”
     “Then,” Madeleine snapped, “why are they here?”
     “Because they had no place else to go,” Nemoto said. “They fled here—even modified themselves, perhaps. They huddled around an artifact left by an earlier wave of colonization. They knew that nobody would follow them to such a dangerous, unstable slum area as this.”
     “They are refugees.”
     “Yes. As, perhaps, we will become in the future.”
     “Refugees from what?”
     “From the resource wars,” Nemoto said. “From the hydrogen suffocation of their world. Like Polynesia.”
     The core artifact trembled.
     And Nemoto kept talking, talking. “This universe of ours is a place of limits, of cruel equations. The Galaxy must be full of light-speed cages like this, at most a few hundred light-years wide, traps for their exponentially growing populations. And then, after the ripped-up worlds have lain fallow, after recovery through the slow processes of geology and biology, it all begins again, a cycle of slash and burn, slash and burn... This is our future, Meacher: our future and our past. It is after all a peculiar kind of equilibrium: the contact, the ruinous exploitation, the crash, the multiple extinctions—over and over. And it is happening again, to us. The Gaijin are already eating their way through our asteroid belt. Now do you see what I’m fighting against?”
     Madeleine remembered the burster, the slaughter of the star lichen fourteen times a second. She remembered Venus and Australia, the evidence of ancient wars even in the Solar System—the relics of a previous, long-burned-out colonization bubble.
     Must it be like this?

(ed note: As it turns out, our solar system is not primordial. Venus used to be a Terra type planet, before a colonization wave swept through and murdered the planet. Millions of years later a second wave from a subsequent civilization swept through. This one killed most life on Terra with an asteroid bombardment. That is the reason Terra has continental plates and plate tectonics.)

---

     Life, Cassiopeia (the Gaijin alien) said, was emergent everywhere. Planets were the crucible. Life curdled, took hold, evolved, in every nook and cranny it could find in the great nursery that was the Galaxy.
     Characteristically life took hundreds of millions of years to accrue the complexity it needed to start manipulating its environment on a major scale. On Earth, life had stuck at the single-celled stage for billions of years, most of its history. Still, on world after world, complexity emerged, mind dawned, civilizations arose.
     Most of these cultures were self-limiting.
     Some were sedentary. Some—for instance, aquatic creatures, like the Flips—lacked access to metals and fire. Some just destroyed themselves, one way or another, through wars, or accidents, or obscure philosophical crises, or just plain incompetence. The last, Madeleine suspected, might have been mankind’s ultimate fate, left to its own devices.
     Maybe one in a thousand cultures made it through such bottlenecks.
     That fortunate few developed self-sustaining colonies off their home worlds, and—forever immune to the eggs-in-one-basket accidents that could afflict a race bound to a single world—they started spreading. Or else they made machines, robots that could change worlds and rebuild themselves, and sent them off into space, and they started spreading.
     Either way, from one in a thousand habitable worlds, a wave of colonization started to expand.
     There were many different strategies. Sometimes generations of colonists diffused slowly from star to star, like a pollutant spreading into a dense liquid. Sometimes the spread was much faster, like a gas into a vacuum. Sometimes there was a kind of percolation, a lacy, fractal structure of exploitation leaving great unspoiled voids within.
     It was a brutal business. Lesser species—even just a little behind in the race to evolve complexity and power—would simply be overrun, their worlds and stars consumed. And if a colonizing bubble from another species was encountered, there were often ferocious wars.
     “It’s hard to believe that every damn species in the Galaxy behaves so badly,” Madeleine said sourly.
     Malenfant grinned. “Why? This is how we are. And remember, the ones who expand across the stars are self-selecting. They grow, they consume, they aren’t too good at restraining themselves, because that’s the way they are. The ones who aren’t ruthless predatory expansionists stay at home, or get eaten.”
     Anyhow, the details of the expansion didn’t seem to matter. In every case, after some generations of colonization, conflicts built up. Resource depletion within the settled bubble led to pressure on the colonies at the fringe. Or else the colonizers, their technological edge sharpened by the world-building frontier, would turn inward on their rich, sedentary cousins. Either way the cutting-edge colonizers were forced outward, farther and faster.
     Before long, the frontier of colonization was spreading out at near light speed, and the increasingly depleted region within, its inhabitants having nowhere to go, was riven by wars and economic crisis.
     So it would go on, over millennia, perhaps megayears.
     And then came the collapse.
     It happened over and over. None of the bubbles ever grew very large—no more than a few hundred light-years wide—before simply withering away, like a colony of bacteria frying under a sterilizing lamp. And one by one the stars would come out once more, shining cleanly out, as the red and green of technology and life dispersed.
     “The Polynesian syndrome,” Madeleine said gloomily.
     “But,” Malenfant growled, “it shouldn’t always be like this. Sooner or later one of those races has got to win the local wars, beat out its own internal demons, and conquer the Galaxy. But we know that not one has made it, across the billions of years of the Galaxy’s existence. And that is the Fermi paradox.”
     YES, Cassiopeia said. BUT THE GALAXY IS NOT ALWAYS SO HOSPITABLE A PLACE.
     Now a new image was overlaid on the swiveling Galaxy: a spark that flared, a bloom of lurid blue light that originated close to the crowded core. It illuminated the nearby stars for perhaps an eighth of the galactic disc around it. And then, as the Galaxy slowly turned, there was another spark—and another, then another, and another still. Most of these events originated near the Galaxy core: something to do with the crowding of the stars, then. A few sparks, more rare, came from farther out—the disc, or even the dim halo of orbiting stars that surrounded the Galaxy proper.
     Each of these sparks caused devastation among any colonization bubbles nearby: a cessation of expansion, a restoring of starlight.
     Death, on an interstellar scale.
      
     Her attention came to rest, at last, on a pair of stars—small, fierce, angry. These stars were close, separated by no more than a few tens of their diameters. The two stars looped around each other on wild elliptical paths, taking just seconds to complete a revolution—like courting swallows, Madeleine thought—but the orbits changed rapidly, decaying as she watched, evolving into shallower ellipses, neat circles.
     A few wisps of gas circled the two stars. Each star seemed to glow blue, but the gas around them was reddish. Farther out she saw a lacy veil of color, filmy gas that billowed against the crowded background star clouds.
     “Neutron stars,” Malenfant said. “A neutron star binary, in fact. That blue glow is synchrotron radiation, Madeleine. Electrons dragged at enormous speeds by the stars’ powerful magnetic fields...”
     The Gaijin said, PERHAPS FIFTY PERCENT OF ALL THE STARS IN THE GALAXY ARE LOCKED IN BINARY SYSTEMS—SYSTEMS CONTAINING TWO STARS, OR PERHAPS MORE. AND SOME OF THESE STARS ARE GIANTS, DOOMED TO A RAPID EVOLUTION.
     Malenfant grunted. “Supernovae.”
     MOST SUCH EXPLOSIONS SEPARATE THE RESULTANT REMNANT STARS. ONE IN A HUNDRED PAIRS REMAIN BOUND, EVEN AFTER A SUPERNOVA EXPLOSION. THE PAIRED NEUTRON STARS CIRCLE EACH OTHER RAPIDLY. THEY SHED ENERGY BY GRAVITATIONAL RADIATION—RIPPLES IN SPACETIME.
     The two stars were growing closer now, their energy ebbing away. The spinning became more rapid, the stars moving too fast for her to see. When the stars were no more than their own diameter apart, disruption began. Great gouts of shining material were torn from the surface of each star and thrown out into an immense glowing disc that obscured her view.
     At last the stars touched. They imploded in a flash of light.
     A shock wave pulsed through the debris disc, churning and scattering the material, a ferocious fount of energy. But the disc collapsed back on the impact site almost immediately, within seconds, save for a few wisps that dispersed slowly, cooling.
     “Has to form a black hole,” Malenfant muttered. “Two neutron stars... too massive to form anything less. This is a gamma-ray burster. We’ve been observing them all over the sky since the 1960s. We sent up spacecraft to monitor illegal nuclear weapons tests beyond the atmosphere. Instead, we saw these.”
     THERE IS INDEED A BURST OF GAMMA RAYS—VERY HIGH-ENERGY PHOTONS. THEN COMES A PULSE OF HIGH-ENERGY PARTICLES, COSMIC RAYS, HURLED OUT OF THE DISC OF COLLAPSING MATTER, FOLLOWING THE GAMMA RAYS AT A LITTLE LESS THAN LIGHT SPEED.
     THESE EVENTS ARE HIGHLY DESTRUCTIVE.
     A NEARBY PLANET WOULD RECEIVE—IN A FEW SECONDS, MOSTLY IN THE FORM OF GAMMA RAYS—SOME ONE-TENTH ITS ANNUAL ENERGY INPUT FROM ITS SUN. BUT THE GAMMA-RAY SHOWER IS ONLY THE PRECURSOR TO THE COSMIC RAY CASCADES, WHICH CAN LAST MONTHS. BATTERING INTO AN ATMOSPHERE, THE RAYS CREATE A SHOWER OF MUONS—HIGH-ENERGY SUBATOMIC PARTICLES. THE MUONS HAVE A GREAT DEAL OF PENETRATING POWER. EVEN HUNDREDS OF METERS OF WATER OR ROCK WOULD NOT BE A SUFFICIENT SHIELD AGAINST THEM.
     “I saw what these things can do, Madeleine,” Malenfant said. “It would be like a nearby supernova going off. The ozone layer would be screwed by the gamma rays. Protein structures would break down. Acid rain. Disruption of the biosphere—”
     A COLLAPSE IS OFTEN SUFFICIENT TO STERILIZE A REGION PERHAPS A THOUSAND LIGHT-YEARS WIDE. IN OUR OWN GALAXY, WE EXPECT ONE SUCH EVENT EVERY FEW TENS OF THOUSANDS OF YEARS—MOST OF THEM IN THE CROWDED GALAXY CORE.
     Madeleine watched as the Galaxy image was restored, and bursts erupted from the crowded core, over and over.
     Malenfant glared at the dangerous sky. “Cassiopeia, are you telling me that these collapses are the big secret—the cause of the reboot, the galactic extinction?”
     Madeleine shook her head. “How is that possible, if each of them is limited to a thousand light-years? The Galaxy is a hundred times as wide as that. It would be no fun to have one of these things go off in your backyard. But—”
     BUT, Cassiopeia said, SOME OF THESE EVENTS ARE... EXCEPTIONAL.
     They were shown a cascade, image after image, burst after burst.
     Some of the collapses involved particularly massive objects. Some of them were rare collisions involving three, four, even five objects simultaneously. Some of the bursts were damaging because of their orientation, with most of their founting, ferocious energy being delivered, by a chance of fate and collision dynamics, into the disc of the Galaxy, where the stars were crowded. And so on.
     Some of these events were very damaging indeed.
     FROM THE WORST OF THE EVENTS THE EXTINCTION PULSE PROCEEDS AT LIGHT SPEED, SPILLING OVER THE GALAXY AND ALL ITS INHABITANTS, ALL THE WAY TO THE RIM AND EVEN THE HALO CLUSTERS. NO SHIELDING IS POSSIBLE. NO COMPLEX ORGANISM, NO ORGANIZED DATA STORE, CAN SURVIVE. BIOSPHERES OF ALL KINDS ARE DESTROYED...
     So it finishes, Madeleine thought: the evolution and the colonizing and the wars and the groping toward understanding. All of it halted, obliterated in a flash, an accident of cosmological billiards. It was all a matter of chance, of bad luck. But there were enough neutron-star collisions that every few hundred million years there was an event powerful enough, or well-directed enough, to wipe the whole of the Galaxy clean.
     It had happened over and over. And it will happen again, she saw. Again and again, a drumbeat of extinction. That is what the Gaijin have learned.
     “And for us,” Malenfant growled, “it’s back to the f**king pond, every damn time... So much for Fermi’s paradox. Nemoto was right. This is the equilibrium state for life and mind: a Galaxy full of new, young species struggling out from their home worlds, consumed by fear and hatred, burning their way across the nearby stars, stamping over the rubble of their forgotten predecessors.”
     But her mind was racing. “There must be ways to stop this. All we have to do is evade one collapse—and gain the time to put aside the wars and the trashing, and get a little smarter, and learn how to run the Galaxy properly. We don’t have to put up with this sh*t.”
     Malenfant smiled. “Nemoto always did call you a meddler.”
     BUT YOU ARE RIGHT, the Gaijin said. SOME OF US ARE TRYING...

(ed note: a coalition of advanced civilizations has located the next sterilize-the-entire-freaking-galaxy gamma-ray burster and are trying to prevent or at least delay the onset of its burst)

• 

      "WE WERE EXCITED,"said the captain of the Federation vessel USS Carrizal during the postmission debriefing following its return from the Trianguli stars, a little more than a hundred years ago. "It was the first hit we'd had in nine months of scouring that sector. A hominid culture, obviously highly developed, a large population, it was everything we had hoped for. Better yet, the same people were on two worlds … an Earth-Moon configuration. Mike Maliani, our astrogator, suggested Romus and Remus as nicknames for the planets until we found out their real names from the people who lived there. After the twin brothers in an ancient Italian myth." On the debriefing tape, Captain Dini smiles rather ruefully. "I was never a specialist in the classics: I wish I were. Mike's misspelling of 'Romulus' is going to haunt me to the grave. But at that point I thought he knew what he was talking about."

     A pause. "Anyway, we were really excited. You know how few spacefaring species there are: the standing orders are to closely examine any we find. But we weren't so excited that we went in without the proper protocols. We gave them everything we had: the classic first-contact series—atomic ratios, binary counting, pictures. You name it. There was never any answer, even though we're sure they knew we were there. They had an outer cordon of defense satellites that noticed us, and after the messages from the satellites were received on the planets, the message traffic on the bigger planet increased by about a thousand percent. But there was nothing we could do with it by way of translation—after that first message there was silence, and everything that came later was encoded—some kind of closed-satchel code, very sophisticated, and no way to break it in anything short of a decade, without a supercomp or the code key."

     There is a long silence on the tape as Captain Dini shakes his head and looks puzzled. "We never came any closer to their planet than two orbits out," he says, "right beyond the fifth planet in the system. We never came near them. We just observed, and took readings, and went away quietly. I'll never understand what happened."

     What happened was the First Romulan War, as the Federation later called it. What it looked like, from the Federation side, was a long, bloody conflict started without provocation by the Romulans. From the other side it wore a different aspect.

     The appearance of Carrizal caused such a panic as the Rihannsu (Romulans) had never known since they became Rihannsu. In terms of a Rihanha's lifetime, it was thirty generations and more since the Settlement, and the actual records of the appearance of aliens on Vulcan, all those many years ago, were not so much lost as largely ignored. People knew through the history they were taught in the academies what had happened on Vulcan in the old days … and the history had bent and changed, what with telling and retelling, and neglecting to go back to the original source material. Not that that would have helped much. The source material itself had been altered in the Journey, but very few—scholars and historians—knew this, or cared. What the Rihannsu knew about this incursion into their space was that it closely matched the pattern followed by the Etoshans so long ago: quiet observation coupled with or followed by proffers of peaceful contact. They were not going to be had that way again.

(ed note: The Etoshans were the dreaded Pirates of Orion. When the Rihannsu still lived on planet Vulcan, the Etoshans made the exact same first contact overtures as did the the Carrizal. The Etoshans used the contact to learn all they could about the people of Vulcan. Then they kidnapped some Vulcan leaders and held them for ransom. The Vulcans refused to pay, and the Vulcan-Etoshan insterstellar war started.)

     Ch'Rihan and ch'Havran had, over sixteen hundred years, become superbly industrialized. The Rihannsu have always had a way with machines: and this, coupled with their great concern for taking care of the worlds they found after such Journey and suffering, produced two planets that were technologically most advanced at manufacture, without looking that way. Few factories were visible from the atmosphere, let alone from space. Aesthetics required that they be either pleasant to look at, or completely concealed. Many factories were underground. Release of waste products into the ambient environment, even waste so seemingly innocuous as steam or hot water, was forbidden by Praetorial indict, and a capital crime. A starship passing through, even one looking carefully, as Carrizal did, would see two pastoral-looking worlds, unspoiled, quiet. One would hardly suspect the frenetic manufacture that was to start after Carrizal's departure.

     There was frantic action elsewhere as well. In the Praetorate and the Senate some heads rolled, and the survivors scrambled to start working on the defense of the planet—or to otherwise take advantage of the situation. The defense satellites had not been approached closely enough by the invading ship to trigger their weaponry. Cannily, it had stayed out of range. There was no way to tell if the Two Worlds could be defended against the ship that had appeared there, no telling what kind of weaponry it had. But from their experience in air combat (almost every nation of each planet had its own air force, which they used liberally for both friendly and unfriendly skirmishes), the Rihannsu military specialists knew that even a heavily armed ship should not be able to do much against overwhelming numbers.

     They got busy, digging frantically through ancient computer memories and printouts and film and metal media for the forgotten space technology they needed. Had the Ships been spared, even one of them—had their data been preserved in one place rather than scattered all over two worlds—the Federation's boundaries might be much different now. But even what remained was useful, and the Rihannsu were frightened. It is unwise to frighten a Rihanha. Within a year after Carrizal's visit, ch'Rihan's numerous nations had built, among them, some three thousand spacecraft armed with particle-beam weapons and the beginnings of defensive shields. Ch'Havran had built four thousand. They were crude little craft, and their cylindrical shapes recalled those of the Ships, though there was no need for them to spin for gravity: artificial gravity had been mastered a century or so earlier.

     It was three more years before the next ship came. The unlucky Balboa came in broadcasting messages of peace and friendship, and was blown to bits by the massed particle beams of a squadron of fifty. After that the Rihannsu grew a little bolder, and went hunting: a task force caught Stone Mountain, to which Balboa had sent a distress call, and captured her by carefully using high-powered lasers to explosively decompress the crew compartments. They towed Stone Mountain home, took her apart, and shortly thereafter added warp drive to their little cruisers.

     The Federation considers the War to have begun with the destruction of Balboa. In the twenty-five years of warfare that followed, no less than forty-six Federation task forces of ever-increasing size and firepower went into Rihannsu space to deal with the aggression against them, and even with vastly superior firepower, most of them suffered heavy damage if not annihilation. "I can't understand it," says one fleet Admiral in a debriefing. "Their ships are junk. We should be able to shoot them down like clay pigeons." But the huge numbers of the Rihannsu craft made them impossible to profitably engage; even "smart" photon torpedoes could target only one vessel at a time. When there were twelve more climbing up your tail, the situation became impossible. Starfleet kept trying with bigger and better weapons—until two things happened at once: there was a change of administration, and the Vulcans joined the Federation.

     The only indication of what the Rihannsu looked like had come from a very few burned and decompressed bodies picked up in space. When Vulcan was discovered, and after negotiation entered the Federation, their High Council was pointedly asked whether it knew anything about these people. The Vulcans, all logic—and selective truth—told the Federation that they were not sure who these people were. There had indeed been some attempts to colonize other worlds, they admitted, but those ships had been out of touch with Vulcan for some seventeen hundred years. The first Vulcan Ambassador, a grimly handsome gentleman who had just been posted to Earth, made this statement to the Admirals of Starfleet in such a way that they immediately found it politic to drop the subject. But through the ambassador, the Vulcan High Council gave the Federation a piece of good advice. "Make peace with them," Sarek told the Admirals, "and close the door. Stop fighting. You will probably never beat them. But you can stop your ships being destroyed."

(ed note: the Romulans were descendants of the hot-headed dissidents who left Vulcan 1,700 years ago in sublight starships, unwilling to adopt the new Vulcan way of removing emotions. The Vulcans knew this, but was not about to tell the Federation High Council that these warriors had anything to do with Vulcans. )

• 

     The advice went down hard, and Starfleet tried to do it their own way for several years more. But finally, as Vulcan's increasing displeasure became plainer, Fleet acquiesced. The War ended with the Treaty of Alpha Trianguli, probably the first treaty in Federation history to have been negotiated entirely by data upload. No representatives of the two sides ever met. The Rihannsu had no interest in letting their enemies find out any more about them than could be revealed by autopsy. They might be back someday.

     The treaty established what came to be called the Romulan Neutral Zone, an egg-shaped area of space about ninety lightyears long and forty wide, with 128 Trianguli at its center. The Zone itself was the "shell" of the egg, a buffer area all the way around, one lightyear thick, marked and guarded by defense/ monitoring satellites of both sides. Everything inside the Zone was considered "the Romulan Star Empire," even though there was as yet no such thing. The Federation was not exactly hurt by this treaty: as far as they were concerned, there were no strategically promising planets in the area. Perhaps they were not looking hard enough. Later some Federation officials would kick themselves when finding out about Rhei'llhne, a planet just barely inside the Neutral Zone in Rihannsu space, and almost richer in dilithium than Direidi.

     So the war ended, and as far as the Federation was concerned, for fifty years nothing came out of the Zone, not a signal, not a ship. Perhaps, some thought, the people in there had gotten sick of fighting. Wiser heads, or those who thought they knew what stock the "Romulans" had sprung from, suspected otherwise.

     The Rihannsu had stopped fighting indeed, but as for being tired of it, this was unlikely. There was a matter of honor, mnhei'sahe, still to be resolved. So much of the Two Worlds' economies were poured into starship weapons research that they still have not recovered entirely from the austerity it caused the contributing nations. They rebuilt the defense satellite system to hundreds of times its former strength, and trained some of the best star pilots ever seen in any species anywhere.

     They also decided not to make the mistake their forefathers had made with the Etoshans. The Rihannsu scientists spent literally years translating the complete contents of the reference computers of the Federation ships they had so far managed to capture. They realized from what they found that they were one small pair of planets caught between two Empires (Federation and Klingon), and that to survive, they were going to have to have an Empire themselves.

     So began the "expansionist" period of Rihannsu history, in which they tackled planetary colonization with the same ferocious desperation they had used to build a fleet out of nothing. They needed better ships to do this, of course. They wound up reconstructing numerous large people-carriers along the Ship model, though, of course, with warp drive these craft did not need generation capability. Twenty planets were settled in eighteen years, and population-increase technology was used of the sort that had made ch'Rihan and ch'Havran themselves so rapidly viable. Not all the settlements were successful, nor are they now: Hellguard was one glaring example.

• 

• 

Kuramesu Drift: A modestly-sized modular drift-habitat located in the Omane (First Expanses) System, at the Solar-Diageri (Omane IV) trailing libration point.

Kuramesu Drift is an independent drift, unaffiliated with any of the polities or law providers of Omane Actual, the freesoil world with which it shares a system. Rather, Kuramesu Drift is chartered to the Microstatic Commission, providing a data haven and negotiation space for the Worlds’ many micronations and small freeholds to play politics out from under the eyes of their much larger cousins. Omane, one link outside the Empire’s border, protected from intimidation by other polities by its position in an isolated loop route only accessible by passing through an Imperial border world – Ionai (First Expanses) – and yet only 13 links from the Conclave Drift by optimal routing, is essentially perfect for these purposes.

Naturally, Kuramesu Drift has a very high density of spies per capita. In fact, gentle reader, you may find it easiest to assume that everyone not an actual delegate or you, yourself, is a spy for someone.

The drift is, however, well worth visiting for reasons other than espionage. The lifestyles of even minor notables ensure that Kuramesu Drift is blessed with excellent shopping districts, banking facilities, and cultural events, including a spintronic symphony orchestra, tholin baths, and microgravity ballet, and the Commission offsets the running costs of the Drift by renting out their facilities to a variety of conferences (especially those seeing an advantage in a location near, but not within, the Empire) and conventions when they are not otherwise in use.

Meanwhile, the Agent’s Rest offers one of the finest polyspecific selections of liquors and other hedonics to be found in the central Worlds. Just don’t ask for a double – everyone’s heard that one already.

– Leyness’s Worlds: Guide to the Ecumene

Ever since I became an environmentalist, the potential destruction wrought by aggressively expanding civilizations has been haunting my thoughts. Not just here and now, where it’s easy to see, but in the future.

In October 2006, I wrote this in my online diary:

A long time ago on this diary, I mentioned my friend Bruce Smith’s nightmare scenario. In the quest for ever faster growth, corporations evolve toward ever faster exploitation of natural resources. The Earth is not enough. So, ultimately, they send out self-replicating von Neumann probes that eat up solar systems as they go, turning the planets into more probes. Different brands of probes will compete among each other, evolving toward ever faster expansion. Eventually, the winners will form a wave expanding outwards at nearly the speed of light—demolishing everything behind them, leaving only wreckage.

The scary part is that even if we don’t let this happen, some other civilization might.

The last point is the key one. Even if something is unlikely, in a sufficiently large universe it will happen, as long as it’s possible. And then it will perpetuate itself, as long as it’s evolutionarily fit. Our universe seems pretty darn big. So, even if a given strategy is hard to find, if it’s a winning strategy it will get played somewhere.

So, even in this nightmare scenario of "spheres of von Neumann probes expanding at near lightspeed", we don’t need to worry about a bleak future for the universe as a whole—any more than we need to worry that viruses will completely kill off all higher life forms. Some fraction of civilizations will probably develop defenses in time to repel the onslaught of these expanding spheres.

It’s not something I stay awake worrying about, but it’s a depressingly plausible possibility. As you can see, I was trying to reassure myself that everything would be okay, or at least acceptable, in the long run.

Even earlier, S. Jay Olson and I wrote a paper together on the limitations in accurately measuring distances caused by quantum gravity. If you try to measure a distance too accurately, you’ll need to concentrate so much energy in such a small space that you’ll create a black hole!

That was in 2002. Later I lost touch with him. But now I’m happy to discover that he’s doing interesting work on quantum gravity and quantum information processing! He is now at Boise State University in Idaho, his home state.

But here’s the cool part: he’s also studying aggressively expanding civilizations.

Expanding bubbles

What will happen if some civilizations start aggressively expanding through the Universe at a reasonable fraction of the speed of light? We don’t have to assume most of them do. Indeed, there can’t be too many, or they’d already be here! More precisely, the density of such civilizations must be low at the present time. The number of them could be infinite, since space is apparently infinite. But none have reached us. We may eventually become such a civilization, but we’re not one yet.

Each such civilization will form a growing ‘bubble’: an expanding sphere of influence. And occasionally, these bubbles will collide!

Here are some pictures from a simulation he did:

As he notes, the math of these bubbles has already been studied by researchers interested in inflationary cosmology, like Alan Guth. These folks have considered the possibility that in the very early Universe, most of space was filled with a ‘false vacuum’: a state of matter that resembles the actual vacuum, but has higher energy density.

A false vacuum could turn into the true vacuum, liberating energy in the form of particle-antiparticle pairs. However, it might not do this instantly! It might be ‘metastable’, like ball number 1 in this picture:

It might need a nudge to ‘roll over the hill’ (metaphorically) and down into the lower-energy state corresponding to the true vacuum, shown as ball number 3. Or, thanks to quantum mechanics, it might ‘tunnel’ through this hill.

The balls and the hill are just an analogy. What I mean is that the false vacuum might need to go through a stage of having even higher energy density before it could turn into the true vacuum. Random fluctuations, either quantum-mechanical or thermal, could make this happen. Such a random fluctuation could happen in one location, forming a ‘bubble’ of true vacuum that—under certain conditions—would rapidly expand.

It’s actually not very different from bubbles of steam forming in superheated water!

But here’s the really interesting Jay Olson noted in his first paper on this subject. Research on bubbles in the inflationary cosmology could actually be relevant to aggressively expanding civilizations!

Why? Just as a bubble of expanding true vacuum has different pressure than the false vacuum surrounding it, the same might be true for an aggressively expanding civilization. If they are serious about expanding rapidly, they may convert a lot of matter into radiation to power their expansion. And while energy is conserved in this process, the pressure of radiation in space is a lot bigger than the pressure of matter, which is almost zero.

General relativity says that energy density slows the expansion of the Universe. But also—and this is probably less well-known among nonphysicists—it says that pressure has a similar effect. Also, as the Universe expands, the energy density and pressure of radiation drops at a different rate than the energy density of matter.

So, the expansion of the Universe itself, on a very large scale, could be affected by aggressively expanding civilizations!

The fun part is that Jay Olson actually studies this in a quantitative way, making some guesses about the numbers involved. Of course there’s a huge amount of uncertainty in all matters concerning aggressively expanding high-tech civilizations, so he actually considers a wide range of possible numbers. But if we assume a civilization turns a large fraction of matter into radiation, the effects could be significant!

The effect of the extra pressure due to radiation would be to temporarily slow the expansion of the Universe. But the expansion would not be stopped. The radiation will gradually thin out. So eventually, dark energy—which has negative pressure, and does not thin out as the Universe expands—will win. Then the Universe will expand exponentially, as it is already beginning to do now.

(Here I am ignoring speculative theories where dark energy has properties that change dramatically over time.)

Jay Olson’s work

Here are his papers on this subject. The abstracts sketch his results, but you have to look at the papers to see how nice they are. He’s thought quite carefully about these things.

• S. Jay Olson, Homogeneous cosmology with aggressively expanding civilizations, Classical and Quantum Gravity 32 (2015) 215025.

Abstract. In the context of a homogeneous universe, we note that the appearance of aggressively expanding advanced life is geometrically similar to the process of nucleation and bubble growth in a first-order cosmological phase transition. We exploit this similarity to describe the dynamics of life saturating the universe on a cosmic scale, adapting the phase transition model to incorporate probability distributions of expansion and resource consumption strategies. Through a series of numerical solutions spanning several orders of magnitude in the input assumption parameters, the resulting cosmological model is used to address basic questions related to the intergalactic spreading of life, dealing with issues such as timescales, observability, competition between strategies, and first-mover advantage. Finally, we examine physical effects on the universe itself, such as reheating and the backreaction on the evolution of the scale factor, if such life is able to control and convert a significant fraction of the available pressureless matter into radiation. We conclude that the existence of life, if certain advanced technologies are practical, could have a significant influence on the future large-scale evolution of the universe.

• S. Jay Olson, Estimates for the number of visible galaxy-spanning civilizations and the cosmological expansion of life.

Abstract. If advanced civilizations appear in the universe with a desire to expand, the entire universe can become saturated with life on a short timescale, even if such expanders appear but rarely. Our presence in an untouched Milky Way thus constrains the appearance rate of galaxy-spanning Kardashev type III (K3) civilizations, if it is assumed that some fraction of K3 civilizations will continue their expansion at intergalactic distances. We use this constraint to estimate the appearance rate of K3 civilizations for 81 cosmological scenarios by specifying the extent to which humanity could be a statistical outlier. We find that in nearly all plausible scenarios, the distance to the nearest visible K3 is cosmological. In searches where the observable range is limited, we also find that the most likely detections tend to be expanding civilizations who have entered the observable range from farther away. An observation of K3 clusters is thus more likely than isolated K3 galaxies.

• S. Jay Olson, On the visible size and geometry of aggressively expanding civilizations at cosmological distances.

Abstract. If a subset of advanced civilizations in the universe choose to rapidly expand into unoccupied space, these civilizations would have the opportunity to grow to a cosmological scale over the course of billions of years. If such life also makes observable changes to the galaxies they inhabit, then it is possible that vast domains of life-saturated galaxies could be visible from the Earth. Here, we describe the shape and angular size of these domains as viewed from the Earth, and calculate median visible sizes for a variety of scenarios. We also calculate the total fraction of the sky that should be covered by at least one domain. In each of the 27 scenarios we examine, the median angular size of the nearest domain is within an order of magnitude of a percent of the whole celestial sphere. Observing such a domain would likely require an analysis of galaxies on the order of a giga-lightyear from the Earth.

Here are the main assumptions in his first paper:

1. At early times (relative to the appearance of life), the universe is described by the standard cosmology – a benchmark Friedmann-Robertson-Walker (FRW) solution.

2. The limits of technology will allow for self-reproducing spacecraft, sustained relativistic travel over cosmological distances, and an efficient process to convert baryonic matter into radiation.

3. Control of resources in the universe will tend to be dominated by civilizations that adopt a strategy of aggressive expansion (defined as a frontier which expands at a large fraction of the speed of the individual spacecraft involved), rather than those expanding diffusively due to the conventional pressures of population dynamics.

4. The appearance of aggressively expanding life in the universe is a spatially random event and occurs at some specified, model-dependent rate.

5. Aggressive expanders will tend to expand in all directions unless constrained by the presence of other civilizations, will attempt to gain control of as much matter as is locally available for their use, and once established in a region of space, will consume mass as an energy source (converting it to radiation) at some specified, model-dependent rate.

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(ed note: Web And Starship by Greg Costikyan is a table-top wargame of conflict between interstellar empires. He does have some other interesting things to say about multi-empire interactions.)

      There are two things I tried to do in Web And Starship. One is fairly evident: to design a viable three-player game. Three-player games are inherently unstable: they tend to devolve into two-player alliances against a third. It is possible to design a game with this element built in — one strong versus two weaker players — but that makes the three-player game merely a variant on two-player ones. The goal is to produce a game which can support shifting alliances and preserve some element of multi-player diplomacy. The only truly successful game I know of which does this is Conquistador, which succeeds by pitting the players more against the game system than against one another. In Web And Starship, I decided to tackle the problem head on.

     The economic game/war game dichotomy is an important element of my strategy; the idea is that wars will be interspersed with breathing periods, during which time players may back off from alliance commitments and rebuild. Another attempt to break the two-player alliance lock is the nature of the player’s positions; Terra is midway between the other two players and, initially, practically defenseless. Either player can, with some ingenuity, knock the Terrans out in the first few turns of the game — unless, that is, the other player comes to Terra’s aid, a virtual given. Too, the channelized nature of the board means conflict among all three players is likely; Terra and the Gwynhyfarr are in conflict virtually from the beginning, the Gwynhyfarr and Pereen will clash in the 70 Ophiuchi/41 Arae area, and the Terrans and Pereen in the 61 Cygni/Sigma Draconis area. Every player has a reason to fight every other — so, one hopes, a breakdown into a permanent two-on-one alliance will rarely occur.

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     Another goal was to depict two incompatible systems of warfare. An analogy to Renaissance naval warfare may be useful here; warfare in the Mediterranean was dominated by the galley. that in the Atlantic by the sailing ship. Gunnery was considerably more primitive than in later eras. primarily because gunports had not been invented, but also because bronze cannon were hideously expensive and iron cannon had a nasty tendency to explode. Consequently, sailing ships. with their relatively small manpower component, were unable to do much damage to galleys. Galleys were crammed with men — who could do little faced with a sheer wall of wood. A sailing vessel could not reasonably hope to attack a galley, nor vice versa.

     The situation in Web And Starship is similar; the Gwynhyfarr are masters of space, able to strike where they will, but without the ability to transport large numbers of troops quickly. FTL navies are all very fine, but in the final analysis, it’s the poor bloody infantry who gets the job done, as always. The Pereen can mass troops at a moment’s notice — but have very limited ability to send them anyplace. The result is a series of strategic and tactical problems that require some ingenuity to solve.

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     Finally, Web And Starship was the next in series of games which I privately call the “Earth's Future" series. The goal of the series is not, as with a fiction series, to put the same characters through different adventures, but instead to examine a spectrum of possible futures, given interstellar travel.

     The first game of the series was Trailblazer, published by Metagaming and hence now out of print; it was a simulation of free-market micro-economics — and assumed that nothing much was out there, that humanity would have the stars to itself.

     The assumption of Web And Starship is that we’ll be faced with powerful competitors, but, despite our weakness, will be able to develop fast enough to deal with them.

     I plan several future games, but the next two will be Terra Uber Alles, which, as one might guess, will pit a small but vigorous Earth against a nasty but decadent alien empire;

     and The Treaty of Io, which will be a diplomatic game in which a pitifully weak Earth tries to survive by playing several alien races off against one another.