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.