### Browsed byMonth: September 2018

Modeling Galactic History (Wrap-up)

## Modeling Galactic History (Wrap-up)

This post is a little math-heavy, and yet I’ve left out a few explicit steps here and there. Consider this a wrap-up of the posts I made a few weeks back, laying out the assumptions I’ve made with respect to the prevalence of interstellar cultures in the galaxy.

Assumption #1: In the solar neighborhood, there exists about one stellar system for every 300 cubic light-years of space. Stellar systems average about 1.2 stars each.

Assumption #2: There exists one habitable world for every 16 stellar systems, or about one habitable world for every 4800 cubic light-years of space.

Commentary: As I remarked before, these are based on HIPPARCOS data for the solar neighborhood, with some educated guesswork as to how many very dim red dwarfs are not properly accounted for by HIPPARCOS. The Architect of Worlds draft design sequence was used to generate planetary systems for a large sample of the solar neighborhood, which yielded a rough estimate for the prevalence of habitable worlds.

Assumption #3: On any given habitable world, a sentient tool-using civilization will appear about once every 500 million years.

Commentary: Based on the experience of Earth as our one data point (one sentient tool-using civilization – that we know of – since the Earth became habitable back in the Devonian era).

Assumption #4: Left to themselves, sentient tool-using civilizations have a 75% chance of an average lifespan of about 12,400 years, remaining in a pre-industrial status, followed by extinction. They have a 25% chance of an average lifespan of about 12,800 years, attaining industrial status late in that period, followed by extinction. The probability of a sentient tool-using civilization attaining interstellar capability on its own is vanishingly small.

Commentary: This is one of the foundational assumptions of the Human Destiny setting, and a partial solution to the Fermi Paradox. Effectively, I’m arguing for a set of “Great Filters.” Most civilizations never reach an industrial era before falling to some natural disaster. The few that do almost invariably destroy themselves.

Result: Assuming no interstellar-capable civilizations appear, then at any given time there should be about one tool-using civilization for every 40,000 habitable worlds.

Assumption #4: The first culture to attain interstellar capability filled up the galaxy in a short period of time (<< 250,000 years) and gave rise to an era of pan-galactic civilization which lasted several hundred million years. This period is usually called the “era of the Precursors” today.

Assumption #5: The Precursor meta-civilization collapsed in an era of pan-galactic conflict. The winning (or surviving) Precursor faction gave rise to a meta-civilization called the Synarchy, which has carefully limited the growth of all subsequent interstellar-capable cultures. The Synarchy “cultivates” the galaxy by recruiting new interstellar-capable cultures as its proxies.

Assumption #6: Once a civilization has attained interstellar capability under the Synarchy or one of its proxies, its average lifespan after that point is about 250,000 years.

Assumption #7: The Synarchy normally permits one of its proxies, together with its client cultures, to occupy no more than about 25,000 habitable worlds. This allocation is divided between the proxy and its clients, with the expectation that the proxy itself may occupy no more than 5,000 habitable worlds, and each client culture may occupy no more than 1,000 habitable worlds. Within those constraints, a given proxy culture has broad discretion as to how to manage the volume of space under its supervision.

Commentary: More foundational assumptions. The specific numbers have been set to lead to a model which permits many small interstellar societies to exist at the same time in the galaxy. More to the point, aside from the very first “Precursor” civilization, no culture has ever had the opportunity to overrun the galaxy. The Synarchy values variety and acts to prevent such runaway growth.

Results: The policies maintained by any given Synarchy proxy can be characterized by several variables.

• M, the number of habitable worlds considered to be within the proxy’s supervised volume. In a typical case, many or even most of these worlds will be maintained in a “fallow” condition, unoccupied and waiting for the appearance of a naturally evolved tool-using civilization.
• N, the number of client cultures normally maintained by the proxy at any given time (can be any value but is generally <= 20).
• KP, the number of habitable worlds occupied by the proxy (must be < 5,000).
• KS, the average number of habitable worlds occupied by client cultures (must be < 1,000).

We observe that if RE is the rate of extinction of client cultures, then:

$R_E=N/250,000$

To maintain a steady state, the proxy must have a rate of uplift RU equal to RE. The rate of uplift is dependent upon M and the proxy’s level of selectivity when choosing candidate cultures to uplift. Some typical values are:

• If the proxy uplifts no candidate cultures, then $R_U=0$, $N=0$, and M can be any value < 5,000.
• If the proxy uplifts all candidate cultures which reach the industrial era, then $R_U=M/2,000,000,000$ and $M\approx8,000\times N$.
• If the proxy uplifts all candidate cultures, even at a pre-industrial level, then
• $R_U=M/500,000,000$ and $M\approx2,000\times N$.
• If the proxy seeks out pre-sentient species for uplift, then RU is undetermined but possibly as large as $R_U=M/50,000,000$, which would imply $M\geq200\times N$.

Assumption #8: The volumes of space occupied by a proxy culture or by one of its clients will tend to obey the following guidelines.

• The supervised volume of a proxy culture is likely to be a compact volume of space, such as a sphere with radius roughly equal to $\sqrt[3]{1,150\times K}$. A habitable world which drifts away from that volume will no longer be monitored, and any civilization which arises there will be on its own.
• The volume occupied by a given culture will not be compact, since stellar drift and the requirement to leave certain habitable worlds “fallow” will create gaps and voids. If K is the number of habitable worlds currently occupied by a culture, and A is its age as an interstellar-capable civilization, then as a rough estimate its occupied space will be bounded by a sphere of radius S, where:

$S>\sqrt[3]{1,150\times K}+\frac{A}{6,000}$

• The first term in that estimate is related to the size of the compact volume that might contain the appropriate number of habitable worlds, while the second term is related to the scattering of stellar systems over time due to their different space velocities.
• This last estimate may be reduced if the culture in question practices migration, moving populations back toward the core as their stellar systems drift too far away. Most proxy cultures will engage in this kind of behavior. Otherwise, some of their occupied systems would tend to drift out of the supervised volume over time.

Assumption #9: For at least the last 600 million years, and until quite recently, Sol and Earth have not fallen within the volume of supervision of any Synarchy proxy culture.

Results: Within the past 600 million years, the 25,000 nearest habitable worlds to Sol would have given rise to 30,000 tool-using civilizations. This suggests that without intervention and uplift, the probability for any one tool-using civilization to attain interstellar capability is no greater than about 1 in 60,000.

Combining this with the galactic-history model developed earlier, we find that:

• The Precursor civilization most likely appeared 9.8 billion years ago, and the era of great galactic conflicts was about 9.4 billion years ago.
• Today, depending on the typical lifespan of a proxy culture (probably longer than the quarter-million years of a client civilization), there are likely to be several thousand such proxies in the galaxy at any given time.

Another result is that it would be very unusual for more than one proxy culture to appear independently in the same galactic neighborhood at the same time. However, it may be possible for client civilizations to “graduate” to independent proxy status rather than “ascension” or voluntary extinction. Proxy cultures might be thought of as reproducing by “budding” during periods of stability. Also, when the proxy eventually passes on, it may reproduce by “spawning” some of its last few client civilizations into independent proxy status.

Khedai Hegemony Reference Map Complete

## Khedai Hegemony Reference Map Complete

Well, this map stretched my technique a little further than before, but after quite a bit of research and development, and a couple dozen hours of painstaking Photoshop work, it’s done. I now have a reference map for the galactic neighborhood of the Khedai Hegemony, covering a decent chunk of the Orion Spur in the process.

Here’s a thumbnail for the new map:

As before, this is a pretty hefty file, so you might do well to download it and view it locally. Alternatively, here’s a link to the pertinent page in my DeviantArt gallery.

This was world-building with a purpose! Not only did this exercise give me a new reference map for our galactic neighborhood, on a larger scale than I had ever done before, but it drove me to build a definitive model for interstellar cultures that I can continue to use later. I also came up with at least a high-concept description for over a dozen alien civilizations that I can now use in my stories.

In fact, once I get my notes collated and write down all the conceptual material that’s floating around in my head, I may have the basis for a fairly complete world-book for GURPS or some similar tabletop game. Between this map and the previous one, I have capsule descriptions for dozens of star systems, and by the time I’ve fleshed out all sixteen or so cultures I’ll have plenty of alien character templates. I may need to confer with the folks at Steve Jackson Games and see what the current limits are for publishing anything using GURPS language. It’s been a while since I was on their editorial staff, so their policies may have changed. At the very least, I ought to be able to post all of the pertinent material to the web for free.

Short term goal, though, is to buckle down and produce a publishable version of the novella In the House of War. With this map, I now have a much better idea where everything is, and what sort of aliens Aminata is likely to encounter during her first ventures out into the galaxy.

Real progress. Feels good.

Status Report (22 September 2018)

## Status Report (22 September 2018)

Very good progress, over the last few days, on a draft map of Khedai Hegemony space. Rather than post the unfinished map here, I’ll give you a link to the item in my Scraps folder on Deviant Art. I’d encourage anyone who’s interested in this project to have a look.

Probably a few more days to work on the map itself, and then I may also be able to put together a gazetteer for Hegemony space. I’m beginning to think this may work as a high-space-opera GURPS setting, among other things. Most importantly, of course, the exercise of finally mapping all of this out is giving me lots of ideas for Human Destiny stories . . .

Last of the Nine

## Last of the Nine

Sometimes, an idea for a story gets into my head, and wedges itself in there so tightly that I know there’s nothing to be done but to write the thing. Given how many hours I’ve spent over the past week playing Middle-earth: Shadow of War for the first time, it was probably inevitable that I would get the urge to write some fan-fiction for it. At least this time, it wasn’t a whole novel’s worth (or a whole series of novels’ worth).

The story is available on FanFiction.Net: Last of the Nine. It tells the story of a brief meeting between Talion, the protagonist of the Shadow games, and a disguised Ranger of the North. The idea was rather irresistible, given that if we accept the departures from Tolkien lore that the game has already committed, something like this story almost has to have happened just offstage . . .

Okay, now that the imp has been exorcised, and I’m almost finished with my play-through of the game as well, maybe I can get back to my galactic modeling and preparation for the next Human Destiny stories.

Status Report (14 September 2018)

## Status Report (14 September 2018)

Just a quick note, since it’s been a few days since I’ve posted anything here. Been rather distracted by finally picking up the video game Shadow of War, which is iffy from the standpoint of a Tolkien scholar but quite entertaining from a gameplay perspective.

I was about ready to wrap up my modeling of galactic history and drill down to the structure of the Khedai Hegemony (the interstellar polity that conquers and rules Earth in my Human Destiny setting). Then I had a sudden realization that caused me to re-think a lot of the chain of reasoning. To wit: stars move.

Okay, yes, that isn’t a great revelation. We all know that stars have proper motion in the sky; over long periods of time the configuration of stars around Sol (for example) will change dramatically. What I realized is that the time-scale on which this is significant is well within the periods of time I was working with for the Human Destiny setting. Interstellar civilizations can’t be treated as nice, compact, spherical volumes of space – not if they last long enough that their colony worlds are going to scatter across dozens or even hundreds of light-years.

So I’ve made a few tweaks to the chain of logic, and in the process have improved it somewhat. I can now model different interstellar civilizations based on the strategy they select as to which new cultures they choose to “uplift” into the galactic community. I also now have a solid chain of reasoning that indicates why any given interstellar culture might have neighbors, to serve as enemies or at least competitors. I believe I’m now in a position to publish my revised model here, and work on a larger-scale map of the entire Hegemony that I can use as reference when writing stories. Look for that over the next few days, so long as I can tear myself away from mowing through hordes of Sauron’s orcs.

Modeling Galactic History (Part II)

## Modeling Galactic History (Part II)

So far, I’ve gotten through some of the chain of logic that sets up the structure of galactic civilization in the Human Destiny setting. Today I’m going to work through a few more steps.

Assumption #7: On the average, one Synarchy proxy can manage a volume containing about 100 subordinate cultures.

Commentary: This is a remarkable span of control. No empire in human history has managed to survive for long with a 100-to-1 disparity between the subordinate populations and the metropole. I’ll assume that the Synarchy chooses its proxies carefully, supports them effectively, and permits them a little more expansionism than the client species. Also, the Synarchy’s normal methods probably involve guiding client civilizations into a quietist lifestyle, thus discouraging rebellion.

I’ll assume that no proxy ever grows much larger than this, no matter how long it remains stable.

If the typical proxy can manage 100 subordinate civilizations, that implies that it will govern a volume containing about 40,000 habitable worlds. In the solar neighborhood, that implies a volume of about 192 million cubic light-years, or a sphere about 360 light-years in radius.

At this point, I should be able to place an upper bound on the rate at which interstellar-capable civilizations appear in the galaxy.

Consider that for all the 4.6-billion-year history of Earth, there has been a population of the 10,000 habitable worlds “closest” to Sol. Here, “closest” is in the sense that if an interstellar civilization appeared on any of those worlds, it would have been recruited by the Synarchy as a proxy, and Earth would soon have been terraformed and colonized by one of the proxy’s client civilizations.

Now, across 10,000 habitable worlds and during 4.6 billion years, we expect 92,000 tool-using civilizations to have arisen (number of habitable worlds, multiplied by the time, divided by 500 million years). If we want the expected number of interstellar civilizations to be less than one-half, then only one in about 184,000 tool-using civilizations will attain interstellar capability on their own. Let’s round that up to 200,000. That’s a very strict Great Filter (or, more likely, a very strict set of several Slightly Lesser Filters).

Assumption #8: Faster-than-light travel has three modes, which tend to limit the reach of any one interstellar culture.

Commentary: Based on the lore I’ve already established in completed stories, the primary interstellar mechanism is a relatively slow Alcubierre-like warp drive. Under the GURPS definitions, this functions as a “hyperdrive” (see p. 37-38 of GURPS Space). Ships can enter FTL anywhere, so long as they’re a safe distance away from any large mass (say, a few AU away from a star or solar mass). While in FTL, a ship is entirely isolated from the rest of the universe – all it can do is wait until it emerges at the pre-planned point. Emergence from FTL can also be done anywhere in open space, although navigation to a point of emergence tends to be rather inexact. The machinery for the FTL drive is carried on board the starship itself, and it’s the only means available for a ship to travel FTL independently. The warp drive provides variable interstellar speed, but most ships can manage up to about 90c, or about one light-year in four days. It’s available at GURPS TL10 to all interstellar cultures.

The second method of FTL travel is by wormhole bridge. Wormhole bridges are built between pairs of stellar systems, usually placed in orbit around a gas giant planet or some other convenient gravitational anchor. They are very expensive, but once built they permit almost instantaneous travel between their endpoints. Synarchy proxies build wormhole bridges between major worlds in their space, to facilitate trade and military movement. The technology for wormhole construction is available at GURPS TL11 to Synarchy proxies.

The Synarchy is believed to have a third FTL method, its operating principles mysterious, which appears capable of transiting the entire galaxy at will. This method is inferred only by those who have witnessed the Synarchy itself in direct action. Fleets appear, carry out their missions, and then vanish, never to be seen again. This method would seem to be available at GURPS TL12 to the Synarchy alone.

This combination of FTL methods has implications for the physical layout of space controlled by a Synarchy proxy culture. There’s probably a dense inner core, where the member civilizations are packed as tightly as they can go, connected by a network of wormhole bridges. On the edges of this core, there may be a middle region that’s more loosely packed, where the proxy doesn’t bother to uplift all the candidate civilizations that appear. Beyond the last outposts of the wormhole network, there’s probably a frontier zone, where the proxy keeps an eye on things but is very unlikely to uplift any civilizations that appear.

Working through some back-of-the-envelope calculations:

• If 80% of a proxy’s member cultures are within the packed inner zone, that should be about 80 subordinate civilizations, packed into a compact volume containing about 32,000 habitable worlds. In the solar neighborhood, that implies a sphere about 335 light-years in radius.
• Suppose the remaining 20% of the member cultures are in the middle zone, and that zone is about one year’s travel by slow FTL deep (90 light-years). That shell has inner radius 335 light-years and outer radius 425 light-years, for a total volume of about 164 million cubic light-years. That implies about 34,170 habitable worlds, of which only about 2,000 are occupied, or about 6% as opposed to the 25% or so in the inner zone.

### Some Corrections

After writing the section I published here on 1 September, I became more and more uneasy with one of my assumptions – the wild-guess estimate that the first interstellar-capable civilization would appear about 6 billion years after the formation of the first stars. Instead of continuing to press forward with that guess, I went back and did some modeling of the history of star formation in the Milky Way galaxy.

With a little digging, I located a recent paper which yielded a reasonably clear profile of the galaxy’s star-formation rate throughout its history:

• Starting at the beginning, and running for about a billion years, the galaxy formed stars at a little higher than the present-day rate. At this point, the galaxy had little shape – stars forming in this era were in the galactic halo.
• Starting about 12.5 billion years before present, the galaxy began to form the “thick disk” of stars, forming stars at about three times the rate we see today. This burst of star formation seems to have lasted about 2.7 billion years.
• Once the thick disk had formed, about 9.8 billion years ago, star formation fell off to about its original rate, slightly higher than today. This period lasted about 1.3 billion years.
• From about 8.5 billion years before present, to about 7 billion years before present, star formation in the Milky Way almost stopped. Very few stars appear to have formed in this period, which marks a clear deficit in the age distribution of stars to the present day. This period seems to bracket the era during which the “thin disk” was forming. This probably isn’t a coincidence. The compression of the interstellar medium into the thin disk would have heated it, slowing down star formation.
• Star formation from about 7 billion years ago to the present day seems to have been happening at a reasonably constant rate.

Okay, so given this profile, and a few wild-guess assumptions about the rate of stellar deaths and the rate of enrichment of the galactic medium with metals, I built a rough model of the number of stars, the number of habitable worlds, and the number of civilizations that might have existed in the galaxy throughout its history. I ended up with the following charts:

Looking at these data and applying the natural rate of appearance of interstellar cultures derived above, I found that the first interstellar culture in the Milky Way – the Precursors discussed above – probably appeared about 9.6 billion years ago. Their home stellar system was probably a halo star, remarkably rich in metals for its time, and their home planet likely reached the stage of complex ecologies much more quickly than the norm. Mildly surprising . . . but here’s the thing: the galaxy is big, and even rare cases are likely to occur somewhere.

As we’ve seen, the Precursors would have filled the galaxy in the blink of a cosmic eye. By about 9.2 billion years ago, the Precursors would have had to deal with a dozen or so local civilizations that managed to reach interstellar capability on their own. I’ll pin the era of galactic conflicts to about this time, and the foundation of the Synarchy within 50 million years or so after that.

At about this time, my spreadsheet tells me that new interstellar civilizations were appearing in the galaxy about once every 50 million years. That would have given the Synarchy plenty of time to place the whole galaxy under monitoring, so that it could begin recruiting any new interstellar cultures as its proxies.

While the galaxy’s thin disk and spiral arms formed, the Synarchy would have remained in control, “cultivating” the galaxy and preventing any new episodes of chaos such as had occurred under the Precursors. By the time Sol formed in some obscure corner of the galaxy, Earth would have been well-protected from being overrun many times over by undisciplined interstellar cultures.

With the Fermi Paradox secure, I think I’m ready to build an outline of galactic history, and to sketch out the shape of the Synarchy proxy that conquers Earth in the Human Destiny setting. All that will be for next time.

Modeling Galactic History (Part I)

## Modeling Galactic History (Part I)

Over the next two or three posts, I’m going to be going through my reasoning for some of the background assumptions of the Human Destiny setting. This is probably going to come across as being a little stream-of-consciousness. I’m trying to work my way through a logic chain without necessarily knowing where it will need to go before I’m done.

Incidentally, although I’m not going to make any specific references to GURPS in these next few posts, I’ll continue to tag them that way – this kind of thought process is an extension of a few items in the last GURPS Space edition. I’m thinking specifically of the section titled Mapping the Galaxy, on pages 67-72. GURPS referees might find this process of some interest as a worked example.

So let’s get started.

Assumption #1: In the solar neighborhood, there exists about one stellar system for every 300 cubic light-years of space.

Commentary: The usual figure given for the stellar density in the solar neighborhood is about 0.004 stars per cubic light-year. Applying this to the 10-parsec-radius neighborhood of my map, we would predict the presence of about 580 stars. Clearly, the HIPPARCOS data set I work from is missing a lot of stars, since it only has 327 stars in that space (about 56% of the predicted number). As a cross-check, a 5-parsec radius should have about 72.5 stars, but the HIPPARCOS data set shows 64 in that space (about 88% of the predicted number). As expected, we’re missing more and more stars as we get further away from Sol.

On the other hand, we can expect that most “missing” stars are among the smallest and least luminous red dwarfs, increasingly difficult to observe with any significant distance. Even in the HIPPARCOS data set, there’s plenty of evidence that our data on individual red dwarfs becomes very poor well within the 10-parsec radius. Many stars have no other catalog designation, their spectral class isn’t at all certain, and so on. While proceeding through this analysis, I’ll bear in mind that very few of the “missing” stars are likely to have habitable planets.

Close to Sol, stellar systems seem to have an average of 1.2 stars each. This gives us an average density of about one stellar system for every 300 cubic light-years. That indicates about 480 stellar systems in the 10-parsec solar neighborhood.

Assumption #2: Within 10 parsecs of Sol, there exist about 30 habitable worlds.

Commentary: Applying the Architect of Worlds design sequence, I ended up with 28 habitable worlds in that space. Twenty of these (about 70%) appeared in stellar systems that included at least one K-class or more luminous star. None of them had a primary star of less than about 0.15 solar masses (about an M4 or M5). That suggests that habitable worlds circling the very dimmest red dwarfs – the ones by far most likely to be “missing” from the HIPPARCOS data set – are going to be very rare. I therefore assume that there might be one or two habitable worlds that I’ve overlooked, but no more than that.

Combining this assumption with the implications of Assumption #1, we get about one habitable world for every 4800 cubic light-years, or about one habitable world for every 16 stellar systems. We can apply this as a rough estimate for most regions of the galaxy.

Assumption #3: Habitable worlds that currently carry native tool-using civilizations (defined as capable of basic cultivation agriculture at a minimum) are very rare, about one in 40,000 habitable worlds.

This assumption breaks into two sub-assumptions:

• Assumption #3A: A world will give rise to a tool-using civilization about once in every 500 million years of its habitable lifespan.
• Assumption #3B: Almost all tool-using civilizations have a finite lifespan averaging about 12,500 years, after which they succumb to natural or sentient-made disaster, without ever developing interstellar capability.

Commentary: The computation is straightforward – divide the average lifespan of a typical civilization into the rate of their occurrence.

Naturally, both parameters are taken from the history of Earth. It should be noted that we have a very limited capability to identify other tool-using civilizations that may have occurred on Earth in the distant path. Still, we’ve never seen any evidence of prior non-human civilizations here, since the post-Cambrian appearance of complex land-based ecologies roughly 500 million years ago.

Meanwhile, humans have engaged in basic agriculture for about 11,500 years at this point. A more dramatic way of stating #3B is that, left to our own devices, we humans will drive ourselves into barbarism and extinction within another millennium or so. That gives us a total lifespan of about 12,500 years, which I’ll take as an average.

Combining this assumption with the others, we determine that at any given time in the natural steady state, tool-using civilizations appear about once in every 192 million cubic light-years, or once for every 640,000 stellar systems. That suggests the average distance between neighboring civilizations (using one of the formulae on page 72 of GURPS Space) is well over 600 light-years!

The natural state of the galaxy is many thousands of primitive cultures in existence at any given time, separated from each other by gulfs of hundreds of light-years, unable ever to see the slightest sign of each other’s presence.

Now, that assumes that no civilization ever attains interstellar capability. What happens if a few of them do?

Assumption #4: Given the possibility of interstellar (FTL) travel, as soon as one interstellar-capable civilization appears, it will no longer be subject to quick extinction and will fill the galaxy in a trivial amount of time.

Commentary: It seems reasonable to assume that an FTL civilization will no longer be subject to all the forces which drive a planet-bound culture to extinction. Only the very largest-scale natural disasters (enormous gamma-ray bursters, galactic core explosions, and so on) could destroy an FTL-capable culture. Such a culture might conceivably destroy itself through internecine warfare, but it seems reasonable to assume any culture likely to do such a thing would have done it before attaining FTL.

Meanwhile, an FTL-capable culture whose numbers expanded at even a modest rate would fill up the galaxy in a short time. Assume an annual rate of expansion as low as 0.01% – very slow, given FTL – then the Milky Way is filled up in as little as 250,000 years.

The question arises, then: given this event occurred, when?

The oldest stars in the galaxy formed about 13.5 billion years ago, but the environment for high-technology civilizations in such an early galaxy was probably very poor. Assume a minimum of four billion years for the first life-bearing planets to give rise to complex land-based ecologies. Then assume a further delay of about two billion years, for early civilizations to overcome the disadvantages of a metal-poor environment, and for more high-technology civilizations to appear at any given time. Then the first FTL-capable cultures may have appeared about 7.5 billion years in the past.

As it happens, this is after the formation of the galactic disk and spiral arms, and after a lengthy period of relatively slow star formation. If we assume one or more FTL-capable cultures appeared about then, they would have had a newly formed spiral galaxy to expand into, and a new flood of young stars to explore and colonize. These Precursor cultures might have hurried the process along, engaging in large-scale terraforming projects to create more habitable worlds.

(By an odd coincidence, the current Architect of Worlds design sequence yields that any potentially habitable world that is at least 7.6 billion years old, if its primary is not a class-IV subgiant, is guaranteed to have a complex biosphere. I didn’t plan that, but it fits! We can imagine that lots of small, cool stars that have been around since before the Precursor era were seeded with their favored ecologies back then.)

Any new FTL-capable cultures that arose during this period would have found the galaxy already full and busy. The Precursors may have been benevolent toward newcomers, or they may have been cruel and aggressive. In either case, newcomers would have had little chance to repeat the Precursors’ success. They would have been forced to survive in the margins of the elder galactic cultures.

We probably can set aside any concept of a unified Galactic Empire. Even with FTL, the natural unit of government is going to be no larger than the single star system. Such a system, if densely populated and developed, is likely to be economically self-sufficient and almost impossible to conquer.

The Precursor era was likely one of many millions of local civilizations, all in constant contact with one another, all of them rising and falling over time. Many single-system cultures may well have collapsed back into barbarism from time to time. Even whole regions of the galaxy might have fallen victim to some disaster or another. Alistair Reynolds’s concept of the “churn” (from his novel, House of Suns) seems likely to be appropriate here. Even so, the galactic association of cultures would have endured, possibly for a very long time.

Now, clearly this isn’t the situation we see now. The galaxy appears to be a wilderness. Something brought this Precursor era to an end, and something is preventing the galaxy from returning to that state today.

Assumption #5: The Precursor era was the only point in galactic history at which nearly every habitable world was occupied by high-technology civilization. Since then, the expansion of new FTL-capable cultures has been strictly limited.

Commentary: I choose to assume that in this setting, the galactic Precursor culture eventually fell victim to a massive conflict, driven by disagreements over several major issues. Among others:

• Many local civilizations came under the domination of powerful AI, becoming machine cultures. These tended to replace their biological antecedents, through benevolent “mandatory pampering,” through non-violent competition, or through violent extermination. Naturally, civilizations which remained largely biological often regarded this development with alarm.
• Some local civilizations found ways to “ascend” to new styles of life, often esoteric and incomprehensible to those who remained. Often this was associated with a shift to machine-culture status, as the “ascending” biological sentients abandoned their machine servants and guardians. Such “ascension” meant effectively dropping out of the galactic churn, often vanishing entirely to leave behind apparently empty worlds. Some cultural movements asserted that such “ascension” was the natural outcome and implied purpose of any sentient community. Other cultures rejected any such idea with horror.
• Some local civilizations became concerned that the galactic community suffered from a lack of variety. They argued that ever since a single original civilization had given rise to the galactic community, all newcomers had been crippled, forced into an unnatural accommodation with that one dominant society. Over time, this became regarded as a fundamental question of justice.

Over millions of years, disputes over these issues gave rise to an epic series of wars. While a star-system community was a difficult thing to conquer, a sustained effort could sometimes do the trick. Of course, such a community was much easier to destroy. A barrage of relativistic kinetic-kill missiles, directed at every inhabited planet and space habitat in the target system, was one of the less destructive methods applied. Over time, the Precursor community collapsed across most of the galaxy, and high-technology culture was nearly eradicated.

Near the end of the conflict, an alliance of local cultures formed to defend what remained of the community, and to impose a specific solution on the galaxy:

• Certain forms of “ascension” were accepted as the ultimate end of any sentient culture. One of the galactic community’s goals was to facilitate safe methods for such evolution, and to protect elder cultures as they proceeded toward it.
• As elder cultures “ascended,” this would naturally make room for new biological cultures to arise, thus providing the galactic community with much-needed variety it needed. A second goal for the community was to protect such newcomers, helping them to survive the transition to FTL-capable status, and integrating them into galactic society. This specifically required leaving large volumes of space “fallow,” preventing any one culture from expanding too quickly or too far at the expense of others.
• Powerful AI, and the machine cultures they tended to create, had a clear role in the galactic community, but they could not be permitted to harm or crowd out organically evolved cultures. Strict limits were placed on the use of AI by non-ascended civilizations. The creation of self-replicating AI was specifically forbidden.

This post-war settlement remains in effect, down to the present. Somewhere in the galaxy, very far from the solar neighborhood, a very powerful network of beings still works tirelessly to manage the galaxy, as if it were a vast garden. This network is called the Synarchy.

The Synarchy manages the galaxy by:

• Intervening at certain points in the history of developing civilizations, helping them to avoid self-destruction and move toward readiness for participation in the interstellar community. This intervention is usually subtle but may involve overt conquest if necessary.
• Enforcing certain foundational laws designed to prevent any one culture from overrunning the galaxy. Notably, no one civilization may claim or occupy more than a small fraction of the galaxy, and no civilization may build independent or self-replicating AI. Civilizations which break these laws may be brought into line by force.
• Preserving knowledge and making it available to all participants in good standing in the galactic community, through the promulgation of a galactic Library. The evolutionary pathways that end with “ascension” are specifically revealed to all interstellar cultures at a certain level of maturity.

Much of the Synarchy’s work is done through proxies. These “mature interstellar empires” have generally been in existence for at least a few million years, have a good record of adherence to the Synarchy’s law, and have exhibited the ability to coexist smoothly with younger civilizations. The Synarchy deputizes such cultures to manage their areas of the galaxy, generally concealing its own existence from less mature civilizations.

So, what does an area of the galaxy overseen by one of the Synarchy’s proxies look like?

Assumption #6: In an area of space currently governed by a Synarchy proxy civilization, habitable worlds that currently serve as the home-worlds of native tool-using civilizations are much more common, about one in 400 habitable worlds.

This assumption derives from Assumption #3A, and from the following sub-assumptions:

• Assumption #6A: About one in four tool-using civilizations survives long enough to develop a high-technology culture that will require intervention.
• Assumption #6B: After intervention and emergence into the galactic community, civilizations have a finite lifespan averaging about 5 million years, after which they either voluntarily die out, or they “ascend” to the Synarchy and beyond.

Commentary: These assumptions imply that the average lifespan of a tool-using civilization is about 1.25 million years. Dividing this into the rate of occurrence of new civilizations (about once in 500 million years per habitable planet) gives us about one civilization per 400 habitable planets.

It should be noted that a Synarchy proxy could apply a different strategy, giving rise to a much higher density of high-tech civilizations. For example, a proxy could locate and intervene in the development of even pre-industrial cultures. Or it could even seek out promising pre-sentient species for “uplift” and civilization. I’ll assume that the Synarchy discourages such intense interventionism, possibly because it would lead the intervening civilization to force its clients into too restrictive a cultural mold. This would lead to a loss in the variety that the Synarchy values.

Without assuming anything (yet) about the shape or configuration of any volume of space governed by a Synarchy proxy, let’s examine how that space might be populated. If a given proxy governs space that includes N habitable worlds, then:

• On the average, a new tool-using civilization will appear in that space every 500 million divided by N years.
• On the average, a new high-technology civilization will appear, ready for intervention, every 2 billion divided by N years. This is also the rate at which established civilizations within the proxy’s sphere of influence will vanish into voluntary extinction or “ascension,” maintaining a steady state.
• The current population of that space at any given time will be about N divided by 400 FTL-capable civilizations.

Suppose each FTL-capable civilization is allocated about 100 habitable worlds to colonize and occupy throughout its lifespan. If the space containing these worlds is compact, that implies a volume of about 480,000 cubic light-years, or a sphere with radius of about 48.5 light-years.

About 75% of the habitable worlds in a proxy’s volume will be left “fallow” at any given time. This should allow plenty of space for likely candidate species that might give rise to high-technology civilizations over the next million years or so. Potential colony worlds can be allocated to minimize the probability that a new civilization will appear on a world that’s already occupied.

The question arises: just how much space will a given Synarchy proxy be able to govern? Suppose a proxy can last much longer than the average of 5 million years for a full FTL-capable culture? Does it continue to grow, accepting responsibility for more and more space? Will the collective of all the Synarchy’s proxies fill the galaxy, or will there be “empty” space?

These questions are important, since we’re modeling a setting that needs to be consistent with what we’ve seen so far of the real universe. Results which indicate that Sol and Earth should have been visited and colonized many times in the past will mean that something has gone wrong.

I’ll examine some of these questions in the next post.