Bigger Than Worlds
Analog - March, 1974
(1974)*
Larry Niven

Contents

	The Multigeneration Ship
	Gravity
	Flying Cities
	Inside-Outside
	Macrolife
	Worlds
	Dyson Spheres
	Ringworld
	Dyson Spheres II
	Hold It a Minute
	The Disc
	Cosmic Macaroni
	The Mesgasphere
	Final Solution

	Just because you've spent all your life on a planet doesn't mean that everyone always will. Already there are alternatives to worlds. The Russian space station may have killed its inhabitants, and the American Skylab has had its troubles, but the Apollo craft have a good record. They have never killed a man in space.

	Alas, they all lack a certain something. Gravity. Permanence. We want something to live on, or in, something superior to a world: safer, or more mobile, or roomier. Otherwise, why move?

	It's odd how much there is to say about structures larger than worlds, considering that we cannot yet begin to build any one of them. On the basis of 'size, the Dyson Sphere—a spherical shell around a sun comes about in the middle. But let's Start small and work our way up.

The Multigeneration Ship

	Robert Heinlein's early story, "Universe," has been imitated countless times by most of the writers in the business.

	The idea was this: Present-day physics poses a limit on the speed of an interstellar vehicle. The ships we send to distant stars will be on one-way journeys, at least at first. They will have to carry a complete ecology; they couldn't carry enough food and oxygen in tanks. Because they will take generations to complete their journeys, they must also carry a viable and complete society.

	Clearly we're talking about quite a large ship, with a population in the hundreds at least: high enough to prevent genetic drift. Centrifugal force substitutes for gravity. We're going to be doing a lot of that. We spin the ship on its axis, and put all the things that need full gravity at the outside, along the hull. Plant rooms, exercise rooms, et cetera. Things that don't need gravity, like fuel and guidance instruments, we line along the axis. If our motors thrust through the same axis, we will have to build a lot of the machinery on tracks, because the aft wall will be the floor when the ship is under power,

	The "Universe" ship is basic to a discussion of life in space. We'll be talking about much larger structures, but they are designed to do the same things on a larger scale: to provide a place to live, with as much security and variety and pleasure as Earth itself offers—or more.

Gravity

	Gravity is basic to our life style. It may or may not be necessary to life itself, but we'll want it if we can get it, whatever we build.

	I know of only four methods of generating gravity aboard spacecraft.

	Centrifugal force seems to be most likely. There is a drawback: coriolis effects would force us to relearn how to walk, sit down, pour coffee, throw a baseball. But its effects would decrease with increasing moment arm—that is, with larger structures. On a Ring City, for example, you'd never notice it.

	Our second choice is to use actual mass: plate the floor with a neutronium, for instance, at a density of fifty quadrillion tons per cubic foot; or build the ship around a quantum black hole, invisibly small and around as massive as, say, Phobos. But this will vastly increase our fuel consumption if we expect the vehicle to go anywhere.

	Third choice is to generate gravity waves. This may remain forever beyond our abilities. But it's one of those things that people are going to keep trying to build, forever, because it would be so damn useful. We could put laboratories on the sun, or colonize Jupiter. We could launch ships at a million gravities, and the passengers would feel nothing.

	The fourth method is to accelerate all the way, making turnover at the midpoint and decelerating the rest of the way. This works fine. Over interstellar distances it would take an infinite fuel supply—which we may have, in the Bussard ramjet. A Bussard ramjet would use an electromagnetic field to scoop up the interstellar hydrogen ahead of it—with an intake a thousand miles or more in diameter—compress it, and burn it as fuel for a fusion drive. Now the multigeneration ship would become unnecessary as relativity shortens our trip time: four years to the nearest star, twenty-one years to the galactic hub, twenty-eight to Andromeda galaxy—all at one gravity acceleration.

	The Bussard ramjet looks unlikely. It's another ultimate, like generated gravity. Is the interstellar medium sufficiently ionized for such finicky control? Maybe not. But it's worth a try.

	Meanwhile our first step to other worlds is the "Universe" ship—huge, spun for gravity, its population in hundreds, its travel time in generations.

Flying Cities

	James Blish used a variant of generated gravity in his tales of the Okie cities.

	His "spindizzy" motors used a little-known law of physics (still undiscovered) to create their own gravity and their own motive force. Because the spindizzy motors worked better for higher mass, his vehicles tended to be big. Most of the stories centered around Manhattan Island, which had been bodily uprooted from its present location and flown intact to the stars. Two of the stories involved whole worlds fitted out with spin dizzies. They were even harder to land than the flying cities.

	But we don't really need spindizzies or generated gravity to build flying cities

	In fact, we don't really need to fill out Heinlein's "Universe" ship. The outer hull is all we need. Visualize a ship like this:

1.	Cut a strip of Los Angeles, say, ten miles long by a mile wide.

2.	Roll it in a hoop. Buildings and streets face inward.

3.	Roof it over with glass or something stronger.

4.	Transport it to space. (Actually we'll build it in space.)

5.	Put reaction motors, air and water recycling systems, and storage areas in the basement, outward from the street level. Also the fuel tanks. Jettisoning an empty fuel tank is easy. We just cut it loose, and it falls into the universe.

6.	Use a low-thrust, high-efficiency drive: ion jets, perhaps. The axis of the city can be kept clear. A smaller ship can rise to the axis for sightings before a course change; or we can set the control bridge atop a slender fin. A ten-mile circumference makes the fin a mile and a half tall if "the bridge is at the axis; but the strain on the structure would diminish approaching the axis.

	What would it be like aboard the Ring City? One gravity everywhere, except in the bridge. We may want to enlarge the bridge to accommodate a schoolroom; teaching physics would be easier in free fall.

	Otherwise it would be a lot like the multigeneration ship. The populace would be less likely to forget their destiny, as Heinlein's people did. They can see the sky from anywhere in the city; and the only fixed stars are Sol and the target star.

	It would he like living anywhere, except that great attention must he paid to environmental quality. This can be taken for granted throughout this article. The more thoroughly we control our environment, the more dangerous it is to forget it.

Inside-Outside

	The next step up in size is the hollow planetoid. I got my designs from a hook of scientific speculation. Islands in Space, hv Dandridge M. Cole and Donald W Cox.

Step One: Construct a giant solar mirror. Formed under zero gravity conditions, it need he nothing more than an echo balloon sprayed with, something to harden it, then cut in half and silvered on the inside. It would be fragile as a butterfly, and huge.

Step Two: Pick a planetoid! Ideally, we need an elongated chunk of nickel-iron, perhaps one mile in diameter and two miles long.

Step Three: Bore a hole down the long axis.

Step Four: Charge the hole with tanks of water. Plug the openings, and weld the plugs, using the solar minor.

Step Fire: Set the planetoid spinning slowly on its axis. As it spins, bathe the entire mass in the concentrated sunlight from the solar mirror. Gradually the flying iron mountain would be heated to melting all Over its surface. Then the heat would creep inward, until the object is almost entirely molten.

Step Six: The axis would be the last part to reach melting point. At that point the water tanks explode. The pressure blows the planetoid up into an iron balloon some ten miles in diameter and twenty miles long, if everybody has done his job right.

	The hollow world is now ready for tenants. Except that certain things have to be moved in: air, water, soil, living things. It should be possible to set up a closed ecology. Cole and Cox suggested setting up the solar mirror at one end and using it to reflect sunlight back and forth along the long axis. We might prefer to use fusion power, if we've got it.

	Naturally we spin the thing for gravity.

	Living in such an inside-out world would be odd in some respects. The whole landscape is overhead. Our sky contains farms and houses and so forth. If we came to space to see the stars, we'd have to go down into the basement.

	We get our choice of gravity and weather. Weather is easy. We give the asteroid a slight equatorial bulge, to get a circular central lake. We shade the endpoints of the asteroid from the sun, so that it's always raining there, and the water runs downhill to the central lake. If we keep the gravity low enough, we should be able to fly with an appropriate set of muscle-powered wings; and the closer we get to the axis, the easier it becomes. (Of course, if we get too close the wax melts and the wings come apart  ...)

Macrolife

	Let's back up a bit, to the Heinlein "Universe ship. Why do we want to land it?

	If the ship has survived long enough to reach its target star, it could probably survive indefinitely; and so can the nth-generation society it now carries. Why should their descendants live out their lives on a primitive Earthlike world? Perhaps they were born to better things.

	Let the "Universe" ship become their universe, then. They can mine new materials from the asteroids of the new system, and use them to enlarge the ship when necessary, or build new ships. They can loosen the population control laws. Change stars when convenient. Colonize space itself, and let the planets become mere way-stations. See the universe!

	The concept is called Macrolife: large, powered, self-sufficient environments capable of expanding or reproducing. Put a drive on the Inside-Outside asteroid bubble and it becomes a Macrolife vehicle. The ring-shaped living city can be extended indefinitely from the forward rim. Blish's spindizzy cities were a step away from being Macrolife; but they were too dependent on planet-based society.

	A Macrolife vehicle would have to Carry its own mining tools and chemical laboratories, and God knows what else. We'd learn what else accidentally by losing interstellar colony ships. At best a Macrolife vehicle would never be as safe as a planet, unless it was as big as a planet, and perhaps not then. But there are values other than safety. An airplane isn't as safe as a house, but a house doesn't go anywhere. Neither does a world.

Worlds

	The terraforming of worlds is the next logical step up  in size. For a variety of reasons, I'm going to skip lightly it. We know both too much and too little to talk coherently about what makes a world habitable.

	But we're learning fast, and will learn faster. Our present pollution problems will end by telling us exactly how to keep a habitable environment habitable, how to keep a stable ecology stable, and how to put it all back together again after it falls apart. As usual, the universe will teach us or kill us. If we live long enough to build ships of the "Universe" type we will know what to put inside them. We may even know how to terraform a hostile world for the convenience of human colonists, having tried our techniques on Earth itself. Now take a giant step.

Dyson Spheres

	Freeman Dyson's original argument went as follows, approximately.

	No industrial society has ever reduced its need for power, except by collapsing. An intelligent optimist will expect his own society's need tor power to increase geometrically, and will make his plans accordingly. According to Dyson, it will not be an impossibly long time before our own civilization needs all the power generated by our sun. Every last erg of it. We will then have to enclose the sun so as to control all of its output.

	What we use to enclose the sun is problematic. Dyson was speaking of shells in the astronomical sense: solid or liquid, continuous or discontinuous, anything to interrupt the sunlight so that it can be turned into power. One move might be to convert the mass of the solar system into its many little ten-by-twenty-mile hollow iron bubbles as will fit. The smaller we sub-divide the mass of a planet, the more useful surface area we get. We put all the little asteroid bubbles in circular orbits at distances of about one Earth orbit from the sun, but differing enough that they won't collide. It's a gradual process. We start by converting the existing asteroids. When we run out, we convert Mars, Jupiter, Saturn, Uranus ... and eventually, Earth.

	Now, aside from the fact that our need for power increases geometrically, our population also increases geometrically. If we didn't need the power, we'd still need the room in those bubbles. Eventually we've blocked out all of the sunlight. From outside, from another star, such a system would be a great globe radiating enormous energy in the deep infrared.

	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 1030 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 ninety-three 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 gases.

	The surface area inside a Dyson Sphere is about a billion times that of the Faith. 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 lor 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 10μ 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 part 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.

Ringworld

	I have come up with an intermediate step between Dyson Spheres and planets. Build a ring ninety-three million miles in radius—one Earth orbit—which would make it six hundred 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 it over. 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, mountain as dents. Riverbeds and river deltas would be sculptured 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.)

2.	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 tern 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 disassembled. 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 II

	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 Alderson, PhD, we can build 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 head. They're so flipping fog. 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 can make). The mathematics of a Ringworld are those of a suspension bridge with no endpoints. OK, go on with whatever you were doing.

The Disc

	What's bigger than a Dyson Sphere? Dan Alderson, designer of the Alderson Double Dyson Sphere, now brings you the Alderson Disc. The shape is that of a phonograph record, with a sun situated in the little hole. The radius is about that of the orbit of Mars or Jupiter. Thickness: a few thousand miles.

	Gravity is uniformly vertical to the surface (freshman physics again) except for edge effects. Engineers do have to worry about edge effects; so we'll build a thousand-mile wall around the inner well to keep the atmosphere from drifting into the sun. The outer edge will take care of itself

	This thing is massive. It weighs far more than the sun. We ignore problems of structural strength. Please note that we can inhabit both sides of the Alderson Disc.

	The sun will always be on the horizon—unless we bob it, which we do. (This time it is the sun that does the bobbing.) Now it is always dawn, or dusk, or night.

	The Disc would be a wonderful place to stage a Gothic or a sword-and-sorcery novel. The atmosphere is right, and there are real monsters. Consider: we can occupy only a part of the Disc the right distance from the sun. We might as well share the Disc and the cost of its construction with aliens from hotter or colder climes. Mercurians and Venusians nearer the sun. Martians out toward the rim, aliens from other stars living wherever it suits them best. Over the tens of thousands of years, mutations and adaptations would migrate across the sparsely settled borders. If civilization should fall, things could get eerie and interesting.

Cosmic Macaroni

	Pat Gunkel has designed a structure analogous to the Ringworld. Imagine a hollow strand of macaroni six hundred million miles long and not particularly thick—say a mile in diameter. Join it in a loop around the sun.

	Pat calls it a topopolis. He points out that we could rotate the thing as in the illustration—getting gravity through centrifugal force—because of the lack of torsion effects. At six hundred million miles long and a mile wide, the curvature of the tube is negligible. We can set up a biosphere on the inner surface, with a sunlight tube down the axis and photoelectric power sources on the outside. So far, we've got something bigger than a world but smaller than a Ringworld.

	But we don't have to be satisfied with one Loop! We can go round and round the sun, as often as we like, as long as the strands don't touch. Pat visualizes endless loops of rotating tube, shaped like a hell of a lot of spaghetti patted roughly into a hollow sphere with a star at the center (and now we call it an aegagropilous topopolis). As the madhouse civilization that built it continued to expand, the coil would reach to other stars. With the interstellar links using power Supplied by the inner coils, the tube city would expand through the galaxy. Eventually Our aegagropilous galactotopopolis would look like all the stars in the heavens had been embedded in hair.

The Mesgasphere

	Mathematically at least, it is possible to build a really big Dyson Sphere, with the heart of a galaxy at its center. There probably aren't enough planets to supply us with material. We would have to disassemble some of the stars of the galactic arms. But we'll be able to do it by the time we need to.

	We put the biosphere on the outside this time. Surface gravity is minute, but the atmospheric gradient is infinitesimal. Once again, we assume that it is possible for human beings to adapt to free-fall. We live in free-fall, above a surface area of tens of millions of light-years, within an atmosphere that doesn't thin out for scores of light-years.

	Temperature control is easy: We vary the heat conductivity of the sphere to pick up and hold enough of the energy from the stars within. Though the radiating surface is great, the volume to hold heat is much greater. Industrial power would come from photoreceptors inside the shell.

	Within this limitless universe of air we can build exceptionally large structures. Ringworld-sized and larger. We could even spin them for gravity. They would remain aloft for many times the lifespan of any known civilization before the gravity of the core stars pulled them down to contact the surface.

	The Megasphere would be a pleasantly poetic place to live. From a flat Earth hanging in space, one could actually reach a nearby Moon via a chariot drawn by swans, and stand a good chance of finding selenites there. There would be none of this nonsense about carrying bottles of air along.

Final Solution

	One final step is to join two opposing life styles, the Macrolife tourist types and the sedentary types who prefer to restructure their home worlds.

	The Ringworld rotates at 770 miles/second. Given appropriate conducting surfaces, this rotation could set up enormous magnetic effects. 'These could be used to control the burning of the sun, to cause it to fire off a jet of gas along the Ringworld axis of rotation. The sun becomes its own rocket. The Ringworld follows, tethered by gravity.

	By the time we run out of sun, the Ring is moving through space at Bussard ramjet velocities. We continue to use the magnetic effect to pinch the interstellar gas into a fusion flame, which now becomes our sun and our motive power.

	The Ringworld makes a problematical vehicle. What's it for? You can't land the damn thing anywhere. A traveling Ringworld is not useful as a tourist vehicle: anything you want to see, you can put on the Ringworld itself... unless it's a lovely multiple star system like Beta Lyrae; but you just can't get that close on a flying Ringworld.

	A Ringworld in flight would he a bird of ill omen. It could only he fleeing some galaxy-wide disaster.

	Now, galaxies do explode. We have pictures of it happening. The probable explanation is a chain reaction of novas in the galactic core. Perhaps we should he maintaining a space watch for fleeing Ringworlds ... except that we couldn't do anything about it.

	We live on a world: small, immobile, vulnerable and unprotected. But it will not be so forever.

The End