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.