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There is this setup that I've seen a few times of a ring-shaped world where people live on the inner side. My question is: what is the minimum advancement of technology for such a thing to happen?

So first:

  • can it exist by itself without any crafting of any sort?
  • if no, how close? is a ring shaped planet something ever heard of?

And then to live on it, I'm assuming that you'd need to:

  • create / maintain an atmosphere (can it work without enclosing?)
  • create / maintain gravity (what happens close to the edge?)
  • ...other things that will come up...
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    $\begingroup$ Do you mean the technology to build such a thing, or the technology to live there? Ringworld is a classic tale about a primitive group living on such a ring. $\endgroup$ – Bobson Oct 7 '14 at 21:46
  • $\begingroup$ I mean the technology for such a thing to exist. So maybe not build it from top to bottom (if it so happens that it's physically possible that it exists in the wild), but to do what it takes to have people live on such thing. $\endgroup$ – Sheraff Oct 7 '14 at 21:53
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    $\begingroup$ Do you mean Ringworlds or Orbitals? en.wikipedia.org/wiki/Ringworld vs en.wikipedia.org/wiki/Orbital_(The_Culture) $\endgroup$ – Tim B Oct 7 '14 at 23:18
  • $\begingroup$ Yeah that makes a big difference. AFAIK Orbitals aka Halos are much easier to build in theory. $\endgroup$ – Zan Lynx Nov 28 '14 at 2:37
  • $\begingroup$ I think the biggest problem is the material strength. Ringworlds very probably cannot be built by any ordinary material. You need something like the magmatter. $\endgroup$ – Irigi Dec 3 '14 at 8:15
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My question is: what is the minimum advancement of technology for such a thing to happen?

  • As long as it's not a ring around a star, you might be able to start with today's technology - if you don't mind it only being small (a kilometer, or maybe a few Km) in diameter, and floating in Earth orbit. The hardest part might be getting it to be self-sustaining.

  • With somewhat later near-future tech, you might be able to build the same thing but larger, like Elysium, a Stanford Torus-like space habitat.

Elysium Stanford Torus interior

  • If you want a ring around an entire planet, you need much more material and technology to build it, keep it in place, and keep it from falling apart. You might want to start with a planet that already has rings, or break the moon of an earth-like planet into pieces, since they are already about the right orbit, though even that is a ridiculously enormous engineering project, and one wonders why one would want to do this other than that it seems like a cool idea.

  • If you want a proper "ringworld" around a star, good luck. That's absurdium-level technology, because not only do the materials need to be impossibly strong, but the amount of absurdly-strong material you need, is probably far more than the total material that even exists at that star, so you need FTL shipping capacity to import from other star systems light-years away. Though, if you already have the technology to have it hold together in the first place, maybe you can design something that will just capture solar wind and have it accumulate where you want it, so it just accretes in place from solar material... and somehow gets turned into impossibly strong stuff, though then you probably also need to be very patient for solar wind to amount to that much, though maybe you can divide that time down by somehow pulling most of the solar wind from all directions there, or even suck up some solar eruption material.

can it exist by itself without any crafting of any sort? if no, how close? it a ring shaped planet something heard of?

No. Closest are planetary rings and asteroid belts.

And then to live on it, I'm assuming that you'd need to: create / maintain an atmosphere (can it work without enclosing?) create / maintain gravity (what happens close to the edge?) ...other things that will come up...

The low-tech versions need enclosure.

Gravity you get artificially by rotation. Close to the edge, nothing. Over the edge, you fall off. But you need to keep the air in, so you probably have walls, cliffs, mountains, or whatever your atmosphere enclosure is on the sides.

For the vastly larger and higher-tech planet-encircling and star-encircling versions, you may as well say you have force fields that can keep the atmosphere there, if you have the other super-tech abilities. Or you could just enclose those too, assuming your impossibly-strong material has a transparent version.

Other things to consider are the energy required to keep everything in place, not falling apart, powered, heated, cooled, and repaired when things hit it.

Encircling a star of course has the night/day issue, but again, if you're able to actually build a star-circling world, no doubt you can arrange for whatever sort of sky display you want.

To me, the largest barrier to an actual star-scale ringworld is, if anyone ever had that much technology and resources, why would they ever choose to do that with it? It seems there would be far more efficient ways to use those god-like resources. I'd simply terraform existing lifeless planets and make them really nice and full of nice healthy lush environments where people and animals idyllically coexist, and I'd have done it thousands of years sooner than anyone could do a ringworld.

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  • $\begingroup$ Simply a perfect answer. $\endgroup$ – Sheraff Jan 15 '15 at 1:03
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Orbital stability:

Ringworlds are unstable. There is a net gravitational pull towards the near surface, meaning that a light source in the middle will eventually fall onto the ring (or, for something star-sized, the ring will fall into the star). You need an active stabilization system (magnetic manipulation, solar sails, attitude jets) to keep this from happening.

Surface gravity:

A non-spinning ringworld has a net gravitational pull towards the surface of the ring, but it's not very strong for anything a reasonable person would consider a "ring". In order for a practical civilization to live on the inside surface, you'll need to spin the ring at a pretty good speed to generate gravity through centrifugal force.

Material strength:

The material of the ring needs to be strong enough to hold itself together against the forces generated through spin gravity. Using the thin-wall formula for hoop stress (a reasonable assumption for something people would call a "ring"), a whole lot of things cancel out, leaving

stress = density * acceleration * radius

density is the density of the material the ring is made out of, acceleration is the effective surface gravity due to spin, and radius is the radius of the ring. Here's the bad news: radius is huge. For a ring only a thousand kilometers in radius, you're looking at hoop stresses on the order of 10-100 gigapascals, requiring exotic materials such as carbon nanotubes to hold things together.

Atmosphere containment:

Your ringworld needs walls to keep the atmosphere from falling off the edges. The height of the atmosphere is described by a parameter called "scale height", which depends on the temperature and molecular composition of the atmosphere, and the effective surface gravity. As a rule of thumb, the walls should be at least ten scale heights tall; for an Earth-like environment, that's about 80 kilometers.

Sediment handling:

If you've got any sort of soil-like surface (as opposed to a purely rocky or technological surface), you need a way to counteract erosion. Water flow will tend to wash soil into the nearest body of water, and a ringworld doesn't have the geologic cycles to bring it back up through mountain-building.

Conclusion:

A natural inhabitable ringworld isn't going to form. Even ignoring that there are no known processes that can form one, the material strength and orbital instability issues will keep it from forming. A high-tech civilization can build one, but the larger it is, the higher the required technology level.

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  • $\begingroup$ Just a note on smaller, planet sized rings: they don't need a light source in the middle. Place them at an angle to the star and they get a day/night cycle equal to their spin. Just like a spherical planet except on the inside instead of the outside. $\endgroup$ – Zan Lynx Nov 28 '14 at 2:42
  • $\begingroup$ @ZanLynx Actually, an orbital does not even need to be angled, if the radius is large enough compared to the width. This has the benefit of keeping the tidal forces simpler. $\endgroup$ – Ville Niemi Jan 15 '15 at 6:49
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Gravity, as far as I remember from what I've read on ring-worlds, should work through rotation - as the ring rotates around the star, the centrifugal motion keeps everything stuck to the inner surface. That means that if you walk off the edge, you probably are shot straight into space (might be convenient for spacecraft though). The effect would be pretty sharp too.

For the atmosphere to not evaporate completely, you'd need gravity and enough material for it to hold together. I'd guess the average atmospheric height would have to be close to Earth's to maintain a similar environment. The issue is, how do you keep it from escaping off the sides?

Spillage

Before the atmosphere problem is tackled, lets see what other problems could exist:

  • Since we're talking about a full world, we'd need biomes for the ecosystem to make its cycle - that means we also need minerals and stuff since not everything's a person with technology available. Even people however would need land that's cultivatable and doesn't waste stuff to space. The whole point of creating a ring-world is to take advantage of the star's power for production and energy - unless we made some sort of ring-cityscape and all our needs are met by energy to matter transmutation, we can't skip out on production activities we perform here on an Earth-like planet.

  • That means we also would need elevation, rivers etc. as well as compensate for some missing elements, such as oceans, volcanism and the like. We can't waste water either, but we need some reasonable underground depth.

From all of these, I don't see how capping the sides can be avoided. You might not have to enclose it on all directions, but you would have to enclose the sides - otherwise the spillage will get rid of most of the atmosphere and other environmental elements pretty quickly. We can't get around this through artificial gravity, unless it works around the rim as well, which would create an inner perpetual-day world and an outer perpetual-night world with some transitional regions over the edge.

Day and night

An extra issue would be the aforementioned day-night cycle - perhaps a solution would be to have an inner half-circle ring, turning everything under it into night, orbiting closer to the star. The problem however, is that you might lose heat very very quickly under shadow - it's possible that the "underground" surface would require heating at least during the artificial night.

There might also be a big issue with visibility of the sky. You might be able to reflect the celestial surroundings using the inner ring, but that's a confusing solution. You might also skip over the need for it by using satellites or rim-mounted telescopes. The ideal solution would be to make the sides transparent to the same wavelengths as the atmosphere.

The tech needed

From what I can tell, you need enough technology to create the scaffolding for two rings, cap the outer ring appropriately, then terraform it and control its environment carefully enough to maintain it stable. This requires both knowledge and equipment that would mean whoever attempts to create such a thing or maintain it would have a very deep understanding of both the physics but also the biology, ecology and geology involved in stabilizing it as a habitat. I haven't crunched the numbers for this kind of thing yet, but they're bound to be astronomical :)

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There are two pieces of reference for "ring-worlds": Niven's Ringworld obviously, and the Stanford Torus. Ringworld is quite outlandish due to its scale but it does describe the ring's inner workings and inhabitants in detail. The Stanford Torus however is very much grounded in reality and current technology, and would have been achievable using then-current technology (but also a lot, A LOT of money).

To build something like the Stanford Torus (a 1.8km diameter habitat that could house 10k-140k permanent residents), you pretty much need one thing: rocketry. Construction required getting to the moon, setting up a mining base, and mass-driving (a.k.a. catapulting) materials via L2 into assembly at L5. The engineering required was relatively simple; existing materials and science could build such a colony. Building something like Ringworld would require much much more technology though; it might have required strip-mining all the planets in a system for materials, maybe more. Definitely out of reach for a sub-Kardashev-type-2 civilisation.

As for living and maintaining such a thing, for the Stanford Torus you could possibly get by for a while (say decades) being completely self-sufficient, but eventually you'll need off-habitat maintenance, raw materials and such for repairs. For the Ringworld however, it would require similarly outlandish technology levels, as such a structure is mechanically unstable, the slightest perturbation will cause it to lose its position relative to its star, so it will require constant course corrections.

As for such a thing to come into existence, no it cannot arise naturally. It may be possible to contrive one that has so many autonomous systems that it is completely self-sufficient and requires no third-party maintenance, however, similar to the Axiom in WALL-E.

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    $\begingroup$ There is also the Culture Orbitals which are somewhere between the Stanford Torus and the Ringworld $\endgroup$ – Tim B Oct 8 '14 at 8:12
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The physics of one just happening is entirely unconvincing as other answers have indicated. So as we are in the realms of soft-sf at this point, here is a way that you might create one:

I would start from an asteroid belt in a convenient part of the habitable zone. Perhaps this asteroid belt resulted from the destruction of a planet with life on it, that helps with the next part.

The next implausible requirement is for life to persist on this asteroid belt- my idea for this would be something like a plant that grows filaments out around any asteroid it grows on. Then these filaments find a neighbouring asteroid they bind to it and the plant grows around it. Obviously we're in a vacuum here, so it's not going to be much like the plants that we understand, but we've already dispensed with a bunch of physics in the interests of creating an interesting world, so I'm not going to worry too much about this- consider the space trees in the Hyperion novels as a good example of this kind of thing.

Over time, presumably lots of it, our vine grows through as much of the asteroid belt as it can ( assuming it is powered by sunlight, it may not reach far onto the dark side ) and is binding the asteroids together very convincingly. Other asteroids and comets striking from the outside end up caught in the vine system and held there. This gives us a solid "ring" surface that has arisen in a natural way.

If you want a day/night cycle, you could place it around a planet's rings rather than an asteroid belt, that might also make the scale a little more manageable.

As for atmosphere, water, and consequent ecosphere, you might need to create secondary life forms for that or give our vines even more remarkable properties, but I think this would be directed by the kind of story you want to tell. It would certainly make more sense for a vine that had so many life-assisting properties to be bio-engineered rather than evolved, for example.

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In the very remote and unlikely chance that this can naturally form, collisions from other bodies such as asteroids and comets would quickly tear it a part early in it's formation and leave an asteroid belt at best. Nor can I see a natural setup that involves bodies of water and a stable atmosphere from forming. And another note...Earth's magnetic field is the basis for it's protection from space weather and to have a magnetic field you need a moving liquid iron core to generate the field strong enough to protect vs space weather events...this makes me strongly doubt life would ever be possible as a 'natural' event (if life seeding is ever natural).

From a creation standpoint...you are more looking at the dyson sphere scenario. This would start off as a space station in orbit around a planet and grow piece by piece until it's a considerably sized structure...and even then the expansion of it remains very compartmental and is added as it's needed/produced. We are quite a bit off this on a technological level, but it's possible that your ring world was originally created by a hyper advanced race that eventually lost control of the ring and billions of years later, here we are. Of course, you'd have to implement 'artifical grav generates' and the sort of tech that an extremely advanced race could create.

If you want a ring a species on a tech level of about ours could create...You'd start off with a nearby asteroid belt and have the initial precursors of colonization. Single large asteroids, potentially mined hollow and housing 'cities' would be our first step. Gravity is a bit of a luxury at this point and would be created locally (spinning that hollowed out asteroid for example). This would expand as solar generators can be placed in the area's between asteroids...and in the same scenario for the dyson sphere above, we'd start connecting asteroid to asteroid until the full ring is inhabited (throw in a reason that the majority of the population needs to get off Earth and onto this structure ASAP and you can increase the production speed of this. It wouldn't be close to a ring world and more a connection of structures...doubtful if an atmosphere would ever exist, nor would you have enough rotation to have standard gravity outside of spinning segments of the structure.

Should note that the distances here become a bit silly. The Earth has a circumference of around 41 thousand km and a surface area in the 510 million square KM range. However it's orbit is massive compared to this at about 940 million KM. In earths orbit, a 10 KM wide ring (just tiny) would have a square KM of 9.4 billion square KM, or about 20x the surface area of earth.

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No material we can think of could withstand the stress of providing spin gravity to anything that would qualify as a "world". The best that could be managed would be a big space station.

The less the curvature, the faster you need to spin to get the same level of effective gravity, and the faster you spin, the more tensile strength needed to keep the whole thing from flying apart.

Matter as we are used to it is held together by the electromagnetic forces between atoms and that simply can't produce enough force to withstand the stress of even a small ring world like one of Iain Banks's Orbitals, let alone the stress on Larry Niven's Ringworld. The forces within an atomic nucleus are strong enough to support at least the Orbital, but building a single atomic nucleus in the form of a ring 3 million km across wouldn't work. So you need something magic like the "Skrith" that Niven used.

An "active" structure that requires power to maintain its shape might help. This is one idea which has been proposed to solve the issues with space elevators and vacuum airships. I don't think there's a way to make it work for ring worlds, particularly not a Niven scale one so you end up with something like the "Structural Integrity Field" in Star Trek which is pretty much just magic. You also have a world that houses hundreds of times as many people as Earth (at a bare minimum for the smallest thing that might count as a ring world) which will fall apart catastrophically if there's ever a power outage.

If you have magical science fiction force fields, you probably have artificial gravity. Doing away with the need for spin gravity eliminates all the tensile strength issues. You don't really need a ring shape though; you could make it any shape you wanted: disc, strip, Moebius loop, etc. (A Klein bottle would be pushing it without some even weirder magic technology though)

Needless to say, if nothing we can think of would let us build one, then they won't occur naturally.

To put it in perspective, the sense in which we don't know how to build these things is much the same way that we do know how to build a planet: Put enough matter in one place and its own gravity will force it into a sphere shape. How you move that much matter is just a detail. How you get it together without building up so much heat that it takes hundreds of millions of years to be habitable, or how you get rid of the heat, is just a detail.

Building ring worlds is a way of showing that a species is "sufficiently advanced".

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