I've been reading about real big structures built around stars like ring worlds and dysons spheres and stuff. I was wondering how big can a structure get, not just ring worlds but any kind of structure.

There are a few main factors which would limit a structures size,
- The resource available
- The strenth of the material
- The effect of gravty from stars as well as the structure

Any thing larger then a dysons sphere would require the resources of more then one solar system.

Would things like massive wall be possible, e.g if to alien races are at wall and wanted to keep each other out of their teritory. Maybe not soild structure but a mine field that would span lightyears.

What other structures could exist? Would they be possible to build?

The univese would have an enormous amount of resources available, the only problem is that its spread out too far. Unless some sort of FTL travel is possibe, structures like this will only stay in sci-fi.

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I think the only reason an alien race would need to build a structure larger then a dysons sphere would be for military purposes. But I would think a large fleet of ships would be alot better then a big structure, unless it housed some kind of weapon.

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Dyson spheres as generally described are impossible; the gravity of the star will pull everything of the inside of the sphere into the star; and in fact there is no material strong enough to make either a dyson sphere or a ringworld.

But there are alternatives; ring worlds could be built up to 2000km in diameter using buckytube; a dyson swarm of habitats could entirely surrond a star, but you have to control their orbits very carefully so as not to have them collide with one another;

and the biggest structures of all can use dynamic compression members as described by Paul birch- this technique involves using rotation beams of magnetic particles to support structures in a gravity field- given enough of these and you could build a structire around Jupiter, or around the Sun; gravity would still be towards the central planet or star, so you live on the outside.

http://www.orionsarm.com/tech/Supramundane_Worlds.html

Taking Birch's dynamic compression member techique to its limits, you could set up a loop of magnetic particles between stars; then enclose this loop in magnetic hoops and hang structures of the loops- eventually you could build a web between the worlds, and fill space with strands of megastructures- not exactly a wall, but just about as close as you can get without using impossibly strong materials.

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Thanks,

I always thought that there would be not material strong enough to build a type 2 dyson sphere.

How much stress would be produced in a ring world?

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It depends on how different the ring-world's spin is from the orbital velocity of a particle in the same orbit. If the difference is zero, there is not much stress, and also not much apparant gravity, or forces to hold in the atmosphere. If it spins so there is one-gee, every ton of material has a ton of force trying to break the ring apart.

As a side note about mega structures, take a few minutes and calculate the largest diameter that a very long [an AU or more] Iron bar in deep space could have before it would collapse simply beacuse of its own gravity. It is surprising! Giant structures are hard to design without giant parts.

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How would one go about making that calculation antoniseb?

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Approximate the length as infinity. Pick a central point and integrate the total force, this gives you a formula based on density and diameter. Find the diameter that produces a force greater than the compression strength of Iron [or steel if you prefer].

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