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In my novel, an alien species whose name is temporarily “Alphas” is a Kardashev Type II Civilization and has an entire galaxy under their control. My novel focuses on the interaction between the Alphas and the lesser both inside and outside their galaxy. In one particular arc, an Alpha has been tasked by their galactic polity to build a ring-shaped megastructure to house a large population of humanoid aliens whose homeworld was lost to a supernova. This Ringworld will not be as large as the iconic Ringworld of Larry Niven’s book but it will still be very large. The Ringworld is around 2.5 million kilometers in diameter and 30,000 kilometers in width. Now, the question is, what kind of construction material should the Alphas use to build this megastructure and others like it?

Note: The Alphas possess extremely advanced technologies but they still follow the laws of physics. They are capable of building Dyson Spheres but the spheres are considered too wasteful by their standards so the Alphas prefer building Ringworlds and planet-sized Shell Worlds instead.

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  • $\begingroup$ Just to make a point, an advanced intergalactic civilization capable of building mega structures would consider any and all materials available to us today or even imagined by us as jokes. Further, please take the time to read the tag wikis. The science-fiction and hard-science tags are mutually exclusive. I deleted the hard-science tag because you're asking a lot to get a hard-science answer for a technology that doesn't exist and hasn't been hypothesized to the level required by that tag. (Please read the wikis.) $\endgroup$
    – JBH
    Sep 24, 2022 at 2:35

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If you want something realistic you can use the current wonder material carbon nanotubes, or something more exotic sounding but not as well known (but potentially stronger) diamond nanothreads (also known as carbon nanothreads). Another material you might want to look into are aerogels. Some properties of aerogels are that they are extremely lightweight, excellent thermal insulators, and fairly good at load bearing. There are lot of other technical properties to them that make them good at various applications depending on how technically correct you wish to be.

But obviously you can create your own wonder materials in your world.

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  • $\begingroup$ Carbon nanotubes are strong and maybe even real. But are they strong enough to build a ringworld without it collapsing? $\endgroup$
    – Daron
    Sep 23, 2022 at 16:20
  • $\begingroup$ @Daron - no, they're not. Nothing we know of is. $\endgroup$
    – jdunlop
    Sep 23, 2022 at 17:12
  • $\begingroup$ There is also nested carbon nanotubes. Tubes within tubes. Also, I have a vague memory of a popular science article that wrote about theoretical exotic materials that could only be fabricated in zero G. I think they called one GassSteal? If this jogs someone's memory I'd like to hear more about it. $\endgroup$
    – Gillgamesh
    Sep 23, 2022 at 17:16
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    $\begingroup$ @jdunlop I remembeer reading about Nothing. Apparently, it is super-strong, very rare and has mystical properties. The civilization that discovered it, revered it like we do Gold. It played a prominent part in their religious ceremonies. In fact, we owe the phrase 'Is Nothing Sacred' to this material and this civilization. $\endgroup$ Sep 23, 2022 at 23:27
  • $\begingroup$ @JustinThymetheSecond - deadly poison, though. During one particularly vicious famine, millions of their people died from eating Nothing. $\endgroup$
    – jdunlop
    Sep 23, 2022 at 23:29
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I agree with the comments, nothing exists; to our knowledge such a material is impossible.

I would suggest as somewhat more believable a beaded necklace; you can put many Earth-like worlds in the same perfectly circular orbit, with some magical thrusters that keep them in perfect position, and ready transport between them.

A Galaxy sized civilization necessarily must have FTL travel and communications; or it is not a "civilization" in any useful sense.

FTL communications could keep the beads together, and perhaps the beads specialize differently, like States in the USA, with a common language but some farmers, some manufacturers, some in business or entertainment, etc.

The Earth's orbit is roughly 584 million miles. Venus gets closest to Earth at 38 million miles. Presume with tech, we can cut that to 4 million miles between beads, you could have 146 Earths in the same orbit. Call it 128 for a power of 2 balancing act; about 4.5B miles between planets.

Those can be constructed of just the other (lifeless) planets, moons and asteroids in your hypothetical system; just presume there are enough of them. In fact the beads can be constructed and added to the necklace one at a time; using your unexplained propulsive forces and powers that can keep them close without colliding.

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I guess the same material which they can build dyson shpere - will do, wink wink

  • man really, dyson swarm, not sphere, why do you tow that old mistake from 70's which wasn't even original design.

And dyson swarm is quite efficient in therms of mass energy extraction, not necessarly the best out there, a good fusion reactor can be better, but it is okay.

Shellwords are a waste, however, they may look nice on paper, but too much dead mass in reality, which does nothing but gravity.

Ring world is more or less okay however, but not necessarly of that size.

Just rough estimation, as order of things, but a ring made of 50GPa carbon nanotubes(not a 100 for different reasons, including safety margin) will break once rotated in the way to have 1g at its surface, at a diameter of that ring being about 3200km, which is far cry from 2.5 million you requesting.

There is a way to fix that, which brings the structure in category of active supporting ones, having rotating internal structure and stationary external structure which the rotating one is pressing to with a help of some sliding interface.

This way limits defined by material strength may be significantly extended, and shifted to limits of such sliding interface, and mass of the shell and its strength ofcourse.

Scaling up that 3200km ring to a 2.5million km one, will require putting 1000 times more material to outer shell that one in rotating one.

  • for fun, an iron ring, 1GPa one as will be 20km ring and to scale it same way will require to put 125'000 times more material to outer shell than internal shell. And mass of it will be a mass of earth for each about 1000km of width at least(with 3m deep soil, 8t per m2 mass of internal shell)

But the whole thing(if scaled to 2.5kk km) will require some nanomachines son stuff, for purposes of keeping it repaired top notch state it requires to be for its operation, so as for purposes of that sliding interface to have proper properties and specs, so as to balance all the stuf for stability, so as to keep it spinning, so as for noise reduction etc.

That nano-stuf does not have to be actual nano, or nano in all 3d dimensions, like a typical gray goo is imagined, it can be micro 2d structure stuff, meaning a smallest unit is microns sizes in 2 dimensions, with some nanostructure internal stuff, like made of carbon nanotubes internaly, typical molecules and stuff and have macroscopic length like cm's or meters. Such 2d micromachines still is a magic by our modern standarts, but it a lesser blackbox or a handwavium than a gray goo, and what can be useful in reality, imho.

So if you smear that ringword bearing with some micromachines, then variety of materials which can be used is quite wide, high strength sure are preffered, but at least you are not limited by some indistructable adamantium stuff.

Such 2d micromachines can't be considered as a handwavium (which your typical gray goo still can be one, even with high tech) in a high tech setting, as we do have examples of micron size machines which are cells and bacterias, they are just made out of poor materials, which flaw a carbon nanotubes can fix to a great degree.

So I would put 2.5kk km ringworld in a viable category, it just not necessarly such a mass efficient one in the first place, so as considering plenthy of carbon required then it makes sense to make it a smaller ring and maybe extend it to be a torus (if you like to have no edge of the world, continiuous internal surface) with a diameter of what is rotating to be around few thousand km's max but diameter of the torus itself can be those millions km's you require. So internal volume will be a closed system, not like ringworld stuff half open, and ligthing problem has to be solved in a different way.

PS

you can actually have a ringworld as grand-pa requested, meaning scaling it down is not necessarly makes it simplier, that iron scaled up bearing ring with 125000 mass on its stationary shell, if wrapped around sun at 1 a.u. will have reduced proportion of external mass, like just 2000 times more mass in external shell than in the internal rotating thing, due the gravity of the sun, which is about 5 mm/s2 at that orbit if I recall correctly. So building one around actual star may be in some sense simplier as material choices, but it will require plenty of those material, which sort of a problem here. Also it requires more from sliding interface, more speed difference, however less compression requirements, which may extend it variety of those interface solutions, but micromachines will still do, speed difference it their lesser concern.

PPS

Ah, yeah, materials in general. Advanced does not mean magic, so creating materials from nothing, and(or) transporting them in a solar system in reasonable time requires great deal of energy, if we talk on scsle of planet masses. Moving those masses in a single star system also requires plenty of energy, but considerably less of it.

So as element composition, considering that the guys like to use available materials sparingly, they will have to stick to the elements present in the system.(lets ignore converting all the hydrogen to heavy elements)

So even if they can make gold to become an unobtanium of strength of adamantium, is does not mean they have enough of that element in the system.

If they can bend space and time and convert all the matter in a system in to some specific element they sure they may do that, but can't tell about those guys that much, so the guys I was talking about aren't that space time benging level, but if they were then why bother to ask, rigth?, lol

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