A few comments on the ‘Culture’….The origins of the interstellar civilization, as stated by
the author of the novels, occurred when an unspecified number of autonomous star ships
and artificial habitats began to federate with a (small?) number of Artificial Intelligences
and a much smaller number of planetary based polities. In the period the novels are set,
many Culture citizens (this mainly refers to the (post-) human population but not exclusively)
live aboard so-called ‘Rocks’, habitable, space-faring asteroids. This number presumably
makes up a significant proportion (10% of all humans, drones etc) of the Culture’s population
that live at any one time aboard their interstellar craft. It is safe to assume that these people
are the descendants of the Culture’s founders. Most of the (human) population, however, live
on artificial worldlets called ‘Orbitals’.

There is much ambiguity in the novels over what constitutes a habitat and what constitutes
a starship, simply because even Orbitals can move under their own power to some extent.
To our minds, Orbitals would be spaceships, albeit not ‘true’ starships capable of traversing
interstellar distances in a human lifetime. In the novels, what constitutes a Starship is that
it is controlled by a Mind (or similar level AI), can carry crew/passengers of at least one and
can easily exceed the speed of light by at least two-hundred times. In fact, for a Starship,
200c is considered pretty slow-this is the average/maximum cruising speed of the more
advanced Modules the Culture sometimes uses. Rocks usually have an AI-core if not a true
Mind, while it would appear that velocities of 250c are their limit. For this purpose, Rocks
are considered alongside regular (i.e. non-Orbital, median population 5 or so million) Habitats.

The Culture could probably produce about a million Starships. This would work out as one for
every eighteen million active Human citizens (i.e. not including those in Storage, existing in
VR or group minds, or the many drones and other devices considered Sentient), so actually
owning one’s own Starship would be a luxury akin to one owning a planet! If we have an upper
limit of a thousand people per Starship, this allows a billion Humans to actually live aboard
Starships, which is still a small fraction of the Culture’s population, even smaller than the
planetary population.

Out of a population of eighteen trillion:

This gives a (estimated)-
1. Planetary population of 300 billion
2. ditto aboard archaic and non-Culture starships-500 billion
3. Aboard Rocks (and Habitats)-150 billion
4. Aboard various Systems Vehicles -1.2 trillion
5. Orbital population-16 trillion (calculated as 15,850 billion)

On the face of it, it is difficult to see where the large numbers of people would come from to
populate the thousands of Orbitals in existence, but certain hints of the Culture’s beginnings
would explain this. Some of the genetic alterations ‘standard’ Culture Humans are born with
(being able to biologically swap gender, postpone or terminate pregnancies, the ability to
regenerate limbs and other abilities) would be of more use in situations among less developed
peoples as the technology to achieve these alterations genetically would coincide with other
technologies that enable individuals to perform similar feats anyway. It would appear that,
at some early point, the ancestors/founders of [many people in] the Culture were living
alongside less developed, possibly aggressive, humans for some period and/ or among
advanced aliens. Most of the Culture’s founding starships had probably been mothballed
by the period the novels are set and the original habitats were probably more crowded
(or, more importantly, had a policy of zero-population growth which was later relaxed)
at the point before/during the Culture’s brief period of terraforming (c.4K BCE?) which
happened before the first Orbitals were constructed.

Rings

Presumably a ‘Ring’ encircles an entire star. These include Morthanveld Nestworlds*
(“Matter”), and Rings are mentioned as being [capable of being] built by the Culture.
It could be expected that the Culture would have hundreds of Rings, each with a
population in the trillions, but it would seem that their main purposes are for manufacturing
and maintaining Starships and other products as opposed to being lived-in environments.
A Ring would most likely be built by an advanced society based in the same solar system
and would take advantage of both solar power and cosmic radiation for harvesting energy/
anti-matter. Considering the Culture, on a planetary level, has a small number of home worlds
(approx a dozen), there are unlikely to be more than about twenty, as Culture citizens/
machines could presumably have occupied Rings which were build and then abandoned
by other (long-since Sublimed) civilizations, each with a population of probably no more
than a billion or so, considering it would be easier to build an Orbital than to provide
workable/simulated gravity throughout something the size of a Ring (which would have
a diameter of at least 1AU).

“Spheres” are also mentioned, which could be Dyson Spheres (an encircled Star) but are
more likely to be habitable shells built around a Gas Giant or a Brown Dwarf (failed star)
at a distance which would provide a gravity of around or exactly 1g (suitable for Humans)
at their surface. Again, while these would be more habitable than a Ring (which, if a whole
structure, would have to rotate or provide artificial gravity in selected areas) but would have
their own power source in exploiting the strong magnetic fields involved. The problem with
Spheres (and planets) is that a ship leaving one would have to be capable of achieving the
Escape Velocity for that body. While the Culture is more than capable of building such
craft, this is very energy-intensive for inter-body travel and especially the transport of
consumer goods around a habitat. Air-spheres are also mentioned as being lived-in from
time-to-time by various “Involved”-level civilizations but not fully Culture-occupied.



*These are basically large conglomerations of small-to-medium sized habitats encircling a star,
so in this case for any Culture Ring based on this format its population would be considered
among those living in habitats