What some science fiction writers have been calling a Dyson sphere is something else: a hollow spherical shell, like a ping pong ball with a star in the middle. Mathematically at least, it is possible to build such a shell without leaving the solar system for materials. The planet Jupiter has a mass of 2 x lO^30 grams, which is most of the mass of the solar system excluding the sun. Given massive transmutation of elements, we can convert Jupiter into a spherical shell 93 million miles in radius and maybe ten to twenty feet thick. If we don't have transmutation, we can still do it, with a thinner shell. There are at least ten Earth masses of building material in the solar system, once we throw away the useless gasses.
The surface area inside a Dyson sphere is about a billion times that of the Earth. Very few galactic civilizations in science fiction have included as many as a billion worlds. Here you'd have that much territory within walking distance, assuming you were immortal.
Naturally we would have to set up a biosphere on the inner surface. We'd also need gravity generators. The gravitational attraction inside a Uniform spherical shell is zero. The net pull would come from the sun, and everything would gradually drift upward into it.
So. We spot gravity generators all over the shell, to hold down the air and the people and the buildings. "Down" is outward, toward the stars.
We can control the temperature of any locality by varying the heat-retaining properties of the shell. In fact, we may want to enlarge the shell, to give us more room or to make the permanent noonday sun look smaller. All we need do is make the shell a better insulator: foam the material, for instance. If it holds heat too well, we may want to add radiator fins to the outside.
Note that life is not necessarily pleasant in a Dyson sphere. We can't see the stars. It is always noon. We can't dig mines or basements. And if one of the gravity generators ever went out, the resulting disaster would make the end of the Earth look trivial by comparison.
But if we need a Dyson sphere, and if it can be built, we'll probably build it.
Now, Dyson's assumptions (expanding population, expanding need for power) may hold for any industrial society, human or not. If an astronomer were looking for inhabited stellar systems, he would be missing the point if he watched only the visible stars. The galaxy's most advanced civilizations may be spherical shells about the size of the Earth's orbit, radiating as much power as a Sol-type sun, but at about 1O angstroms wavelength-in the deep infrared...
...assuming that the galaxy's most advanced civilizations are protoplasmic. But beings whose chemistry is based on molten copper, say, would want a hotter environment. They might have evolved faster, in temperatures where chemistry and biochemistry would move far faster. There might be a lot more of them than of us. And their red-hot Dyson spheres would look deceptively like red giant or supergiant stars. One wonders.
In The Wanderer, novelist Fritz Leiber suggested that most of the visible stars have already been surrounded by shells of worlds. We are watching old light, he suggested, light that was on its way to Earth before the industrial expansion of galactic civilization really hit its stride. Already we see some of the result: the opaque dust clouds astronomers find in the direction of the galactic core are not dust clouds, but walls of Dyson spheres blocking the stars within.
I myself have dreamed up an intermediate step between Dyson spheres and planets. Build a ring 93 million miles in radius—one Earth orbit—which would make it 600 million miles long. If we have the mass of Jupiter to work with, and if we make it a million miles wide, we get a thickness of about a thousand meters. The Ringworld would thus be much sturdier than a Dyson sphere.
There are other advantages. We can spin it for gravity. A rotation on Its axis of 770 miles/second would give the Ringworld one gravity outward. We wouldn't even have to roof itover. Put walls a thousand miles high at each rim, aimed inward at the sun, and very little of the air will leak over the edges.
Set up an inner ring of shadow squares—light orbiting structures to block out part of the sunlight—and we can have day-and-night cycles in whatever period we like. And we can see the stars, unlike the inhabitants of a Dyson sphere.
The thing is roomy enough; three million times the area of the Earth. It will be some time before anyone-complains of the crowding.
As with most of these structures, our landscape is optional, a challenge to engineer and artist alike. A look at the outer surface of a Ringworld or Dyson sphere would be most instructive. Seas would show as bulges, mountains as dents. River beds and river deltas would be sculpted in; there would be no room for erosion on something as thin as a Ringworld or a Dyson sphere. Seas would be flat-bottomed—-as we use only the top of a sea anyway—and small, with convoluted shorelines. Lots of beachfront. Mountains would exist only for scenery and recreation.
A large meteor would be a disaster on such a structure. A hole in the floor of the Ringworld, if not plugged, would eventually let all the air out, and the pressure differential would cause storms the size of a world, making repairs difficult.
The Ringworld concept is flexible. Consider:
(1) More than one Ringworld can circle a sun. Imagine many Ringworlds, noncoplanar, of slightly differing radii—or of widely differing radii, inhabited by very different intelligent races.
(2) We'd get seasons by bobbing the sun up and down. Actually the Ring would do the bobbing; the sun would stay put. (One Ring to a sun for this trick.)
(3) To build a Ringworld when all the planets in the system are colonized to the hilt (and, baby, we don't need a Ringworld until it's gotten that bad!) pro tem structures are needed. A structure the size of a world and the shape of a pie plate, with a huge rocket thruster underneath and a biosphere in the dish, might serve to house a planet's population while the planet in question is being disassemb1ed. It circles the sun at 770 miles/second, firing outward to maintain its orbit. The depopulated planet becomes two more pie plates, and we wire them in an equilateral triangle and turn off the thrusters, evacuate more planets and start building the Ringworld.
Dyson Spheres 2
I pointed out earlier that gravity generators look unlikely. We may never be able to build them at all. Do we really need to assume gravity generators on a Dyson sphere? There are at least two other solutions.
We can spin the Dyson sphere. It still picks up all the energy of the sun as planned; but the atmosphere collects around the equator, and the rest is in vacuum. We would do better to reshape the structure like a canister of movie film; it gives us greater structural strength. And we wind up with a closed Ringworld.
Or, we can live with the fact that we can't have gravity. According to the suggestion of Dan Aiderson, Ph.D., we can built two concentric spherical shells, the inner shell transparent, the outer transparent or opaque, at our whim. The biosphere is between the two shells.
It would be fun. We can build anything we like within the free fall environment. Buildings would be fragile as a butterfly. Left to themselves they would drift up against the inner shell, but a heavy thread would be enough to tether them against the sun's puny gravity. The only question is, can humanity stand long periods of free fall?
Hold it A Minute
Have you reached the point of vertigo? These structures are hard to hold in your bead. They're so flipping big It might help if I tell you that, though we can't begin to build any of these things, practically anyone can handle them mathematically. Any college freshman can prove that the gravitational attraction inside a spherical shell is zero. The stresses are easy to compute (and generally too strong for anything we make). The mathematics of a Ringworld are those of a suspension bridge with no-endpoints.