Back in 1969 Nicoli Karadashev came up with the Kardashev scale to define the scale of future civilizations. This was based on energy use.
Kardashev I
A civilization that uses all of the energy that falls that is available on its planet. This is in effect all the solar energy falling on the planet for Earth approximately 1.74×1017 watts. Currently we are using about 2×1013 watts. This makes us a Kardashev 0.7 civilization according to Carl Sagan in 1973.
Kardashev II
A civilization that uses all the energy of their parent star in their solar system, 4×1026 watts. This civilization has colonized their other planets. More than likely they will have a started to dismantle their planets to create space habitats. The typical view in some science fiction is building a Dyson Sphere around their sun completely to use all its energy. However even with advanced technology this seems unlikely. The only way to make it work would be to have some type of artificial gravity plating and even and this may be impossible for even hyper-advanced civilizations.
Stephen Baxter in The Time Ships had the Morlock civilization living in a Dyson Sphere that rotated around the sun generating gravity from centrifugal force. It made for 1 g at the equator and then falling off depending on your latitude inside the sphere.
A more realistic structure is a Dyson Swarm. In this case you build up a series of habitats around the sun infrastructure of solar panels beaming power. Various habitats maybe specialized for farming, population or industry.
Kardashev III
A civilization that uses all the power of its galaxy, 4×1037 watts. This is effectively a galaxy that has been colonized with every star enclosed by a Dyson Sphere or swarm. This is truly a galactic empire. Although some functions we would have to consider here is if the speed of light is a factor. If faster than light communication or travel is possible you could have a galactic government. If it is not, then maybe just a whole cluster of K II societies with confederations of nearby stars. Eventually you will have a galaxy of many races due to divergent evolution.
So an interesting question might be is if the speed of light is a limitation is a K III civilization even possible.
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