The Ringworld is a science fiction construct - a huge world created in the shape of a ring. Originally described in Larry Niven's book of the same name, Ringworld, it also appeared in three sequels, The Ringworld Engineers, The Ringworld Throne, and Ringworld's Children. The concept of a Ringworld was subsequently used by other authors, in modified versions such as the small rings in the X-Box game Halo, or the orbitals of Iain M Banks.

So What's a Ringworld?

• Take a long strip of metal. A very long strip of metal. Make it about 600 million miles (960 million km) long - that's a little more than the distance from the Sun to Jupiter. Make the strip about 1 million miles (1.6 million km) wide¹.

• Bend it round into a circle so that it forms the shape of a wedding ring. It will have a radius of about 95 million miles (153 million km). Position it so that there is a star in the centre - a star similar to our Sun will do. The surface of the ring will be about the same distance from the star as we are from the Sun, so it will receive about the same amount of heat as the Earth does from the Sun.

• Spin the ring about its axis. Standing on the inside surface of the ring, you will feel centripetal force which will feel the same as the force of gravity. You'll be able to walk around on the surface, with the star directly overhead. Because the ring is so big, it will feel as if you are walking on flat ground, in the same way as the surface of the Earth seems flat.

• Put an atmosphere on the inner surface. You'll have to build big walls at the edges to stop the atmosphere flowing over the sides. A thousand miles (1,600 km) high should be enough to keep in the atmosphere.

You now have a Ringworld. So what do you do with it? You live on it. The inner surface has a living space three million times as big as the surface of the Earth, so there's plenty of room for people to live, to spread out, to have as much space as they want.

There's room to have mountains, seas, deserts, and oceans big enough to swallow a planet. There's room for 20 quadrillion people² before the population density gets to the same as it is currently on the Earth. Someone wants a million square miles to set up a polar bear sanctuary? No problem - there's plenty of space.

The Idea

Science fiction writer Larry Niven developed the idea of the Ringworld from Freeman Dyson's 'Dyson Sphere', a sphere big enough to contain a star and absorb all the energy emitted by the star. Dyson's original idea was of orbiting asteroids which would collect all the energy emitted by the star, generating enough power to feed any civilisation no matter how power-hungry. This idea eventually developed into a solid sphere, with people living on the inside surface. The problem with this is that there is no gravity on the inside of a sphere - no matter how massive the sphere is, all the gravitational forces inside it cancel out and the result is that anyone living on the inside of the surface would just float off into the centre of the sphere. Spinning the sphere will cause centripetal force to pin the person to the inside surface, feeling as if they are being thrown outwards away from the axis of rotation. Unfortunately, this force it will only be perpendicular to the ground in a band around the equator of the sphere, so people living at latitudes of 45°, for example, would feel the apparent gravity at 45° to the ground; it would be like living on a very steep hill.

Niven's idea was to use just a narrow band of the sphere around the equator, and to flatten it so that apparent gravity is straight down everywhere in the world. It no longer achieves Dyson's aim of capturing all the solar power, but it still provides more or less unlimited living space.

The Book

Larry Niven's book Ringworld was first published in 1970. It was very well received and the following year was awarded the Hugo and Nebula Awards. The plot involved an unlikely group of two humans and two aliens of different species travelling to the Ringworld to investigate it. They crashland on the construct and spend the rest of the book trying to find some way of getting home. Along the way, they travel across the surface and meet some of the residents, who turn out to be human or very human-like.

The plot of the book is mediocre. It is mainly a vehicle for explaining the mechanisms of the Ringworld and marvelling at the sheer size of it.

Some Engineering Details

Day and Night

Have you ever stood in a desert at noon when the sun was directly overhead? In the Ringworld, the sun is directly overhead all the time and at all points on the surface. So there would be no day or night without special measures being taken. Some way is needed to make it dark so that the residents can sleep and also presumably so that they don't expire of heat stroke. Niven's solution was 'shadow squares'. A series of flat plates strung together with gaps in between them circle around the star between it and the Ringworld, blocking off the sun from parts of the surface while letting the light through to other parts.

So the inhabitants of the world do experience a sort of day and night - the sun doesn't rise or set, it just suddenly gets cut off and it is dark, then day comes just as suddenly.

Building the World

Just how much mass is in the Ringworld? We don't know what type of metal it is made from, but if it were made of steel, and were about 50 feet (15m) thick, its total mass would be about 10% of the mass of Jupiter. This is insignificant compared with the mass of the star at the centre. This means that it would be far easier to build the Ringworld around the star than to build it in open space and to move the star to the centre of it. You could in theory build it in open space and then move it to the star, but the most likely source of construction material is a planet, and these are usually found already orbiting stars.

Presumably to create a Ringworld, you would find a large planet at roughly the right difference from the star, convert the mass of the planet into metal plates, tow them into place, join them together, then spin it up to speed: a vast undertaking far beyond the abilities of our present civilisation, but who knows what we'll be able to do in the future?

Space Travel to and from the Ringworld

To get the centripetal force on the inside to simulate normal gravity, you'll have to spin the Ringworld at a rate of about one revolution every 9 Earth-days. This doesn't sound so fast, but it means that the surface is actually travelling at a speed of 2.7 million miles per hour (4.3 million km per hour). This won't matter to the people living on the surface, but it makes it very difficult for anybody arriving by spaceship from other worlds. The pilot of the spaceship will have to get her craft up to 2.7 million mph to match the rotational speed of the Ringworld before falling into a landing bay. Conversely, when leaving the Ringworld, spaceships can be launched by opening a trapdoor under them and just dropping them into space. Such a spaceport would have to be outside the ring (underneath, from the inhabitants' point of view) or outside the atmosphere-retaining side walls of the ring.

The spaceport designers provided a magnetic linear accelerator consisting of a sequence of mag-lev hoops. The approaching spaceship would have to line up exactly with the first hoop. It would then pass through each hoop in turn and as it did so would be magnetically accelerated to the rotational speed of the Ringworld, before docking.

This is not as easy as it sounds - from the point of view of the spaceship, the first hoop would be approaching at 2.7 million mph - not much room for error there!

Topography

Life would be very boring if the world were completely flat. Ask any resident of Kansas. The Ringworld designers built mountains, valleys and so on into the surface to make it more interesting. But since the construction material is only tens of metres thick, these are impressed on the base structure of the ring. The mountains are not accumulations of rock, but places where the bedding metal happens to be higher.

This means that the mountains are unchanging. They don't wear down over millions of years, but remain the same for the lifetime of the ring.

Erosion

As rivers flow into oceans, they erode the land, carrying soil along and depositing it in the oceans. The Ringworld is made from metal, but there is a layer of rock and soil on top to make it appear like a real terrain. This too will be eroded by the rivers.

In our world, the land is continually and very slowly restored by the action of plate tectonics. Since the Ringworld substructure is fixed and unchanging, some other mechanism is needed to prevent the oceans filling up with silt, so there are giant pipes and pumps which move the silt from the oceans and deposit it in giant silt mountains at the base of the atmosphere-retaining rim walls.

Living on the Ringworld

Although the inhabitable surface of the Ringworld is curved, it is much flatter than the surface of the Earth, having a much bigger radius. It would appear completely flat to the inhabitants. There would be no horizon, because the surface curves the opposite direction to on Earth. Instead of distant places being below the horizon, they will be obscured by the haze of millions of miles of atmosphere. During the day, things would look very much the same as they do on flat parts of the Earth such as the prairies. At night, however, the rest of the Ringworld would be visible in the sky. Perspective would transform it so that it looked like a giant arch, going straight up, over the top of the sky and down the other side. Portions of the 'arch' would be lit, and other parts dark, as they experienced their own day and night.

There's no North on the Ringworld. If you want directions, the most obvious ones for inhabitants are towards the apparent bases of the arch. These are in fact 'along the ring'. Niven's explorers invent the rather cumbersome terms 'spinward' and 'antispinward' for the direction the ring is spinning and the opposite direction. They also use 'port' and 'starboard', borrowed from the nautical world, for the left and right directions when facing spinward. Locals would presumably have simple words in their own languages for these terms.

In Niven's story, the Ringworld was constructed a very long time ago, so long ago that civilisations have risen and fallen. The original makers of the Ringworld are no longer around, and the inhabitants are at various levels of progress. In some places, they live as simple farmers, totally oblivious to the fact that they are on an artificial construct. In other places, they have progressed to the point of mastering machines, with giant cities floating above the surface of the world by a form of magnetic levitation similar to mag-lev trains. In between are every form of society including ones ruled by priest classes who still know how to operate the machines but not to build them.

In the second and later books, Niven also took the opportunity to introduce the concept of a number of almost-human species living together. Here on Earth, there haven't been any almost-human species since the Neanderthals died out about 25,000 years ago. Some people think it was the arrival of our species, Homo sapiens, that wiped out Homo neanderthalensis. On the Ringworld, there would be room for many closely-related species to live. Sex always livens up a science fiction story, so Niven introduces 'vampires', a type of semi-intelligent almost-human that sucks a normal person's blood while having pheromone-induced sex with them.

Some Details that Niven Forgot

After the publication of Ringworld, it was pointed out to Niven that his vast construction was unstable. The Earth orbits around the Sun because of the mutual force of gravity between the two bodies. In the Ringworld, on the other hand, the star attracts all parts of the ring equally, so the net gravitational force is zero. The ring doesn't orbit around its star, it just sits in a position where the star is in the middle. But there's nothing to keep it in that position, so over time the ring would drift and the star would no longer be at the centre. If the ring didn't hit the sun and melt, it would at least get close enough to burn up all the inhabitants.

Niven's response was to publish a sequel, The Ringworld Engineers. In this, the designers of the Ringworld had been aware of the instability, and had installed a system of solar-powered rockets which would nudge the giant construct back to the safe position with the sun exactly in the centre. Unfortunately, the original engineers who built the ring were no longer around and subsequent civilisations had dismantled the rockets for use as other things.

A detail of the Ringworld's operation that was never mentioned or investigated was how the climate would operate when all parts of the surface are heated equally by the star and there is no Coriolis effect to cause wind or sea currents. Earth's climate is a complex system which is only now being figured out by scientists. We are offered no indication as to what the Ringworld's climate system would be like.

Although we are told about the mechanisms for removing silt from the bottoms of the oceans, we are never told how the soil and rock which are eroded by rivers are replaced - there would have to be something to repair the damage of erosion and prevent large areas of the world being reduced to bare metal.

Rings in Other Stories

Ringworlds also feature in modified form in the works of writers other than Larry Niven. Iain M Banks's 'Culture' series of books commonly feature 'Orbitals'. These are similar to the Ringworld but much smaller. An orbital typically has a radius of about 1.5 million km (0.95 million miles), making them about one hundredth the size of the Ringworld, with one ten-thousandth the surface area. They're not big enough to have a star in the centre. Instead, the whole ring orbits around a star in such a way that the centre of the ring traces a circle around the star. Because the star is on one side of the ring, a normal day and night cycle can be achieved as the ring rotates. Although orbitals are mentioned in many of Banks's books, they're not usually central to the plot.

Smaller again are the halo structures, Ringworlds only about 6,000 miles (9,700km) across, that feature in the Halo series of games for the XBox games console. There were not built for people to live on, but as bases for giant destructive weapons.

Ringworld was also the inspiration for an early Terry Pratchett book, Strata, which ultimately led to the Pratchett's Discworld, the setting for one of the best-selling series of books of our day. You can read about the details in The Origin of Terry Pratchett's Discworld.