© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

19.2.5  Galactic Megastructures

What kinds of projects would really tax the abilities of a Type III civilization? Galactic cooperation among literally billions of worlds will permit construction on a scale never before dreamt of by mankind. From our limited terrestrial perspective, there is no way that we can easily imagine the technical skills of a race which commands nearly twenty five orders of magnitude more energy than we. Nor can we imagine the legitimate rationale, if any exists, for building such massive architectures. Fortunately, we can make a few educated guesses what might be possible based solely on the limitations of finite energy and mass resources. No matter how big and how powerful, every culture has its limits.

A good example of a galactic megastructure which humans have no idea how to build is the Megaring (Figure 19.5). Megaring is a mammoth ringworld stretching twenty light-years in diameter. The flat habitable surface is a million kilometers wide, with 60,000-km-high atmospheric retaining walls at the edges. The surface gravity of 1 milligee is slightly augmented by the 0.03 milligee acceleration caused by the rotation of the artifact: Megaring rotates at 10%c.

The air pressure at the base of the Megaring floor is a normal 1 atm, and falls to 0.5 atm only at a height of 6000 kilometers. A man weighing 70 kg on Earth would weigh 70 grams on Megaring. If he could jump 1 meter on Earth, he can leap at least a kilometer on Megaring -- and it will take him 7½ minutes to come back down. Still more fascinating, the gravity is low enough and the air thick enough that mankind could fulfill one of its oldest and dearest dreams: A human being could take to the air from a standing start and fly as a bird, simply by flapping his arms like wings. And he could soar literally thousands of kilometers high.

Figure 19.5 The Megaring

Perhaps not surprisingly, most of the mass of the Megaring is air. About 330 solar masses are used in the structural frame, which is then filled with 3130 solar masses of atmosphere. The ring itself constitutes a mere 3% of the total system weight. Most of the mass -- some 126,000 solar masses worth -- consists of non-disassembled stars. More than a hundred thousand suns circle the Megaring in helical orbits at a distance of 1 AU. (More accurately, the ring orbits the stars and so must be extremely flexible -- but let's not quibble about dynamics.) Held in place by graser assemblies or powerful magnetic fields, this circulating stellar bucket brigade provides Megaring inhabitants with warmth and illumination.

The total energy cost of this project is somewhere in the neighborhood of 10⁴⁵-10⁴⁶ joules, really a modest price for a galactic culture to pay for such an exotic habitat with a livable surface area of more than a trillion Earths. Of course, we don‘t know how to build a Megaring. But the mass and energy bills are well under budget. If Want truly overcomes Big and Costly, and if the expenditure of resources in building this artifact can be ethically justified (Chapter 25), then somewhere in this universe someone may have built such an artifact.

Leaving aside all considerations of ethical energy use, and considering only mass-energy consumption and ignoring structural and construction details, still grander projects are possible in principle, but they will sorely strain the resources of a single Type III civilization. The Big Megaring (Figure 19.6) is a ringworld a thousand light-years in radius, spinning majestically at 10%c along the circumference. Due to the extremely low floor gravity (measured in tens of microgees), the breathable atmosphere doesn‘t drop to half-pressure for many tens of millions of kilometers. As a result, the basic shape of the habitat changes. Rather than a thin ribbon, the Big Megaring more closely resembles the outer section of a very thin canister of movie film -- a kind of hollowed-out Alderson disk, 600 million kilometers wide and a million kilometers deep with kilometer-thick walls.

Figure 19.6 The Big Megaring

Again, the mass of the air predominates. The Big Megaring structure takes only 34 million solar masses, but 3 billion solar masses of air are needed to provide 1 atm base pressure. The estimated assembly energy is 10⁵¹ joules, not counting the quasar which has been moved to the center for heat and light. While the construction of the Big Megaring seems a vast undertaking indeed, a galactic culture will have both the mass and the energy to do the job. There may be quite sound reasons for designing a gigantic space habitat which affords a living area equivalent to 130,000 trillion Earths or more than 240 million Dyson Spheres.

An even bigger artifact is the Megadisk, a scaled up version of the Alderson Disk. The radius of the central hole is 1 light-year, and the distance to the outer edge is 10 light-years. The design is that of a “floppy disk“ configuration, as the base structure is only 100 kilometers thick. The total weight is 11 galactic masses (1M_G = 3 x 10⁴¹ kg), of which 10 are in the Megadisk frame and 1 is air. This provides a 1 atm pressure on both sides of the disk. The energy to assemble this beast is about 10⁵⁴ joules, so a consortium of Type III civilizations could probably handle the assignment. (There may be political problems in obtaining the 11 M_G. Attempting to exercise eminent domain over eleven galaxies cannot be an easy job.) The Megadisk would provide a living area equivalent to 10²⁰ Earths or nearly a thousand Big Megarings.

The Megasphere is probably the ultimate in biosphere engineering. It is a Dyson Sphere 200 light-years in diameter, with a surface area of more than 130,000 square light-years. The surface gravity is measured in milligees, and the air pressure falls to half about 6500 kilometers from the surface. The base structure is 10 kilometers thick, and both sides are equally habitable.

The Megasphere is far beyond the resources of Type III civilizations. About 1300 galactic masses are needed as raw materials, and 10⁵⁶ joules are required for initial assembly. Over a billion year span of time, another 10⁵⁶ joules must be expended simply to maintain the Megasphere at a comfortable 25 ºC. The bill for upkeep is staggering. Only an emergent Type IV universal civilization would dare such an incredible feat.*

* In theory, larger structures are possible. For instance, a 2000-light-year-diameter Big Megasphere could be constructed with 4000 galactic masses and a thickness of 1 kilometer. However, if the air pressure at the surface is more than 0.01 atm (about like Mars), the Big Megasphere is smaller than its Schwartzchild radius and thus becomes a black hole. This should not adversely affect its habitability. However, all journeys to it will be one-way unless tachyonic propulsion is available.

Last updated on 6 December 2008