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Originally posted on the web by The Restless Technophile
The Ringworld is a setting invented by famous science fiction writer Larry Niven. It is a circular megastructure surrounding a star, and rotating so that the perceived gravity and sunlight are similar to Earth. It is a truly gigantic object:
The ringworld is a ribbon with a radius of 1 astronomical unit (the distance between the Earth and the Sun), meaning 150 million kilometers. It is 1.6 million kilometers wide (40 times the circumference of the Earth), and has a circumference of 1 billion kilometers. So in total, it has an habitable surface 3 million times larger than the Earth. That means that it behaves like a miniature galaxy, filled with very different forms of life, that you can visit without interstellar travel!
To prevent the atmosphere from escaping, walls 1600km high are built on each rim of the ribbon. And to give it a day/night cycle, a series of light-blocking shadow squares is placed on an inner orbit around the star.
To give it a 1g gravity using centrifugal force, the ring rotates in 9 days, giving it a velocity of 1200 km/s.
In Niven’s novel, the ring was built has a kind of natural preserve, to harbour plants and animal, and so the builders put just a small layer of soil on it, with few minerals. That prevents technological civilization using metals from developping. But if the same mining resources that we have on Earth were there, how would advanced societies develop? How would they be different from what we saw on Earth?
On Earth, realizing that the world is not flat is very counter-intuitive. From the perspective of a person, it might well be. It took the Greek scientist Eratosthenes to measure the curvature of the Earth, by noting that shadows decrease in length the closer to the equator you are.
Earth diameter computation by Eratosthenes
On a ringworld, the shape of the world is easy for all to see, it is always hanging over in the sky. There is no denying it, whereas even today we have flat earthers. Even more, with a bit of perspective math, the diameter of the ringworld can be computed, by measuring how much thinner it appears at zenith. So the nature of the world would probably be commonplace knowledge even for prehistoric-level cultures.
Gravity also took a lot of time to be theorized on Earth, with Newton inventing the theory of universal attraction during the late 17th century. The idea of a “creepy action at a distance” faced a lot of skepticism then, but the fact that it explained the motion of planets and moons eventually convinced everyone. However, on a ringworld, gravity -as in the force that throws objects to the ground- is not due to universal attraction. It is due to centrifugal acceleration due to the rotation of the ring.
This is much easier to understand, because it is easy to create centrifugal force by spinning a rock on the end of a rope, or even to create a miniature model ringworld by spinning a wheel. So the link between gravity, the spin rate and the diameter of the world would probably be established quite quickly, maybe at a level of science similar to the classical Greek antiquity. The spin of the ringworld is very apparent by looking at the background of stars, which makes a full rotation every 9 Earth days.
The drawback of this is that universal attraction might be discovered much later, especially if there are no out-of-plane planets orbiting the same star as the ringworld (in-plane planets are hidden from observers by the ring). In the original novel, the builders used the mass of the planets in the system to create the ring, leaving only distant Oort cloud objects, not visible to the naked eye. So to discover gravity, the civilization might even need to wait for the first probes to be launched over the edge of the ringworld: after some time, their controllers will see that they do not keep a constant speed as expected in a world without gravitational attraction. Instead, they will realize there is an unexplained force slowing them down slightly and pulling them towards the star, almost like the Pioneer anomaly. It might be some time before the inverse square law attraction is theorized, and before it is, the many astronomical observations, such as the shape of the galaxies, or even why does the central star does not blow itself apart, will remain a puzzling mystery.
The curvature of the ring has a very important impact on long-distance communications. On Earth, the horizon is limits how far you can communicate by radio or by light. Lighthouses, for instance, have to be very tall in order to warn ships far away that there is a dangerous area. On a ring, the negative curvature does not hide them at all, only hill and mountains can block the signal, and atmospheric absorption and distance dim it. Since there are no mountains at sea, ringworld sailors would be able to assess their position even very far away from a lighthouse.
How radar works on a round world
Light-based communication might even play a much bigger role. Since all it takes to send a signal at hundreds of miles (at night) is to make a fire on a relatively tall mountain, a few beacons are enough to cover a large kingdom. With a bit of focusing optics around the light source, the range can be further extended, and secure point-to-point communications can be designed.
In the end, the range will be limited by atmospheric attenuation: if it is similar to Earth, the atmosphere will not be completely transparent even in the absence of clouds or haze, and the signal will dissipate if stays for too long in the atmosphere. Interestingly, this means very long-range communications along the ring, in which the signal is sent upwards to a very different section of the ring, are less affected by atmospheric scattering, but the extremely large distance involved dims the signal a lot.
Radio waves enjoy the same benefits as visible light, but since they go more easily through weather and suffer less atmospheric attenuation, they make for even better communications. A society with late 20th century technology might set up very large communication dishes for point-to-point communication between two cities. This would make high-speed communications at intercontinental (or even interplanetary) distances possible, without any satellite or cable. If the two cities are only a few degrees apart on the ring, the dishes would have to be placed on tall mountains, because the absence of curvature when going from one wall to the opposite wall makes terrain masking a much bigger problem. Since each degree on the ring represents a tremendous distance of 2.6 million kilometers, ie 65 Earth circumferences, any reasonably-sized civilization will consequently need to use such high points.
Some omnidirectional emitters could be set up around cities to provide a wide coverage areas to all neighbouring receivers. However omnidirectional emitters might create a serious challenge: because they signals are never blocked by the horizon, all omnidirectional receivers see the signal from all omnidirectional emitters. This creates an interference nightmare. On Earth, mobile phone communications use a “cellular” design, in which one antenna only serves a small area around it (a “cell”) and other antennas do not interfere much, and even that creates interference challenges.
Since radars also use radio waves, they too would have a much greater range. On earth, good radar coverage is achieved by spreading out multiple radars, to avoid terrain and horizon masking. The latter is not a challenge on a ring, so using one very big radar might be a better solution. Such a radar could see objects from very far away, and separate the echo of moving objects from the ground return by using the Doppler effect. So they would be able to see a flight taking off from a continent away or more. Nevertheless, the fact that they can see anywhere also means they can be jammed from anywhere, and at long distances jammers are at an advantage because their signal only has to make a one-way trip, whereas the radar’s has to make a two-way trip.
To overcome terrain masking of radars and communications systems, a solution could be to set up systems on the top of the ring’s walls. With a wall height of 1600km, that gives a good vantage position, but even that only sees the base of the opposing wall at a very small angle, since that is 1.6 million kilometers away. Still, a receiver 20 000km away from the wall would see the top of the wall at a 5° degree angle, which is the standard for acceptable terrain masking in terrestrial communications.
The top of the walls are a very good vantage point
Putting things on the walls is a difficult endeavour, since climbing a 1600km wall is hardly an option. Using rockets could be one way to do it. Sending one to that altitude requires 5.6km/s of speed after engine burnout, which is significantly less than the 7.8km/s required to achieve Earth orbit, so rockets reaching the top of the wall will have an almost twice better payload fraction. To get back from the top of the wall, atmospheric reentry will occur at a lower speed than on Earth but with an angle of 90°, whereas it is almost 0° on Earth. That will result in a much harsher reentry at around 40g, which would kill something as squishy as a human. So a crewed exploration needs to pack a booster to slow it down before hitting the atmosphere, making it much more complicated. A one-way trip to deploy robotic infrastructure is a better idea. Besides, since the top of the wall is a very hostile environement, with no atmosphere and a constant exposure to radiations, it’s probably better to just use robots. Like our current solar system probes, they could be powered with solar panels. Nuclear power is not necessary as there is always 1500W/m² of solar radiation, since the radius of the ring is constant. Alternatively, instead of carrying the solar panels, power can be beamed from the ground to a receiver looking across the top of the wall.
A further step could be to unspool a tether from the top of the wall, kind of like a space elevator on a planet. In fact, it is much easier to build than on Earth, where it needs to be more than 36 000km long. With a material of 360km breaking length like carbon fiber, the taper area ratio needed for the cable is exp(1600/360)=85, giving a taper radius ratio of 9 between the bottom of the cable and the top, which is not unreasonable. With the wall elevator deployed, there is no need for rockets and there is no problem for reentry anymore. A similar tether can be deployed to the outer side of the wall to get to the underside of the ring and explore it, for instance to get a better looks at the thrusters of the ringworld, after they have been detected by a probe dropped from the wall.
The top of the wall (which is only 70m wide in the novel) can further be exploited to deploy observatories to look at the sky in the plane of the ring, which you can never see from inside the ring. A sensible exploration plan would be to look for other structures in this plane, since there is already the shadow squares and the ring in it. It is even useful for looking at the rest of the sky, since there is no atmosphere to distub the telescopes, but build telescopes on the top on the mountains at the base of the walls, where there already is a quasi-vacuum, is more practical.
It is also a good place to build a transport system, because there is no air, so no friction. A solar-powered wheeled vehicle could reach high speeds, or even better a maglev train could keep accelerating indefinitely as it has no contact with the groud, so no friction, and the ambient temperature is the one of space, so superconductors work without needing a cooling system. Maglev requires infrastructure for the track, so it is less useful as an exploration system like a wheeled vehicle can be, but can be interesting to transport goods between to already inhabited points of the ring.
In Niven’s work, the builders left a maglev system atop of the wall, and left some elevators to get there.
While we are in the subject of transportation, what are the other options for long-range exploration of the ring? A first idea would be to use rockets. On Earth, you can get from anywhere to anywhere using a rocket with only 8km/s speed after burnout. However, that is only because rocket gets to orbit. There is no such thing as an orbit on the ringworld. Rockets behave like there are on a infinitely flat Earth (neglecting the Coriolis effect), and so the same 8km/s rocket only gives you a 6400 km maximum range. That’s nothing compared the ring.
So rockets are useless, except to get above the wall as seen before. What about planes? Let’s look a a plane going at Mach 3, like the SR-71 spy plane. Surely going at 3000 km/h will get you somewhere? In fact, that amounts to 72 000 km a day, meaning it takes 22 days of uninterrupted flying to go from one rim to another. To get to the opposite point of the ring, it takes 20 years of flying. The planes only covers 9° of ring per year. For a subsonic plane, you need to triple the numbers.
While you are within the footprint of your civilization, you can land and refuel, so a conventional plane using hydrocarbon-based fuel is an option. For exploration, even if you manage to land, refuelling will take a lot of time and energy. Ideally, the plane would carry all of its fuel. That is a problem for very long ranges, since fuel adds mass, so you need more fuel to carry it, and so on. The Breguet equation tells you the mass ratio of fuelled mass of the plane over its dry mass is exponential with range. So it’s not feasible for long distance to use chemical fuels. Instead, one could rely on nuclear fuel. Nuclear planes have been envisionned since the 1950s. Here is a primer from a previous article. One of the design was the US SLAM missile, a doomsday weapon going Mach 3 and outputting a whole not of radiation (which was not a bug but a feature).
There is also some recent Chinese work on small lightweight molten salt reactors, which can run for more than 10 year at full power. In any case, the best designs for a plane are lightweight so they use highly enriched uranium, and ideally no shielding. That is an issue both for passengers and for electronics. Electronics can be put in a small volume far away from the reactor, with its own shielding.
That might be enough for an uncrewed probe, but for passengers having to endure a 20-year trip, the required volume is much more important. A solution to increase the distance between the reactor and the crew, thus decreasing radiation dose, could be to have a first plane containing the reactor, towing another one containing the crew, like is done to launch gliders. Such a design was initially envisionned by a Soviet scientist in 1935, when they realized shielding the crew would be too heavy.
The glider can even be detached so that the crew can land and explore. It can carry a small reactor that can be activated on the ground to generate the fuel for takeoff, by making hydrocarbon fuel from plants for instance. Meawhile, the powered part of the system circles above, and eventually catches the glider after takeoff. It could even catch it directly from the ground with a skyhook-like system, eliminating the need for fuel generation on the ground.
In an exploration setting, there are no airfields to land on, and sufficiently flat surfaces might not be present. However, if there are enough lakes or oceans, a seaplane can do the job. Probes can also be parachuted to get some data from the ground.
Finally, we can look at what it takes to put a satellite in orbit “around” the ringworld. To do that, a rocket has to launch it with enough speed that it cancels out almost entirely the speed due to the rotation of the ring, which is 1200 km/s, so that the satellite ends up in an orbit around the sun 1 Astronomical Unit wide. That means the final speed as to be the same as the orbital speed of the Earth around the sun, so 30km/s. That’s impossible to achieve with 20th or 2st-century rocketry, where the maximum speed after burnout is on the order of 10 km/s. If the probe does not slow down enough, and it speed is higher than the sun’s escape velocity at 1 AU, which is only 42km/s, then it will be flung out of the system. That’s great for exploration of the outer system, or even interstellar exploration (at 1200km/s you cross 5 lightyears in “only” 1500 years), but only in the plane of the ring.
As we have seen, exploring the ring by physically getting there is complicated and takes a lot of time. Due to the distance involved, instead of an exploration problem, we can treat is as an astronomy problem. Like the Moon was mapped from the Earth long before any astronaut put his boots on the ground there, what can be achieved by building a big telescope and pointing it at the other side of the ring?
There are two things that limit the performance of a telescope: first, the atmosphere blurs out the image, but for our setting it is not a problem if the telescope is built on one of the tall mountains along the wall, since they reach above the atmosphere. Second, there is a diffraction limit which means that to see small details, you need a big mirror.
The biggest telescope under construction right now is the european Extremely Large Telescope, with a diameter of 40m. For this, the diffraction limit in the visible spectrum for a target at the other side of the ring, 300 million kilometers away, is 3km. That’s the smallest possible detail that can be resolved. This resolution makes for an acceptable mapping tool, but a lot more information can be extracted by splitting the light with a spectrometer to analyze the different wavelength. Then, composition of the materials of the 3x3km pixel can be analyzed very precisely, since each material has a specific spectral signature. So it is possible to detect the spectral signature of vegetation or buildings for instance, even if they are much smaller than 3km wide.
Complementary information can be achieved by using building larger telescopes using interferometric techniques. This is very complicated for optical telescopes, so it is mostly done with radio telescopes, where the signal can be recorded and combined digitally later. Going to radio waves means a much longer wavelength, so you lose some of the resolution you gained with the wider aperture. One such radiotelescope is the ALMA array, a network of repositionable antennas, which when combined can emulate a 16km wide aperture. With a minimum wavelength of 0.3mm for ALMA, the resolvable distance at 300 million km is 6km, so it is comparable to the ELT. However, it is much easier to improve it by moving the antennas farter away than building a larger mirror for an optical telescope. Combining antennas across intercontinental distances is possible, that is how the first image of a block hole was obtained:
The Event Horizon Telescope combined many radio telescope arrays, including ALMA
The limitation of radio interferometry is that it takes a long time to form an image, as the antennas have to be moved around to get all the signals, even more so in the case of ring-to-ring imaging since there is no relative motion of the source and the array. So all the changes in the scene that are shorter than the exposure time will be blurred out or create artifacts.
One thing to note is that Doppler-effect based imaging techniques do not work on the ring, as it is a rigid solid and there radial relative speed of any two points is zero.
There are lots of other domains to tackled regarding this idea of a modern society that has developed on a ringworld, but the ones above show it’s a very interesting settings that can be sufficiently different from our world to be interesting, while still being rooted in diamond-hard science fiction apart from the ring (which has to be much harder than diamond to not break up under its own weight)!
