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A ring can rotate at just the right rate to counter its self-gravity. No special requirement for the structural strength, you just need to maintain the shape precisely. A size limit would only come from engineering and dark energy, but the latter is still a small effect for a galaxy-sized object.

Would it have to be a ring though? I was thinking something like a galaxy sized 'metropolis' or like a solar system sized ship. Is that theoritically possible?

You could certainly engineer it to make it work but keep in mind you'd need many many galaxies worth of material. Something like a Dyson sphere would need a few planets worth of material so they become less probable on those grounds alone.

Multiple nested rings would work, too. Three-dimensional structures are more challenging. You could support them with rotating rings.

Thank you for humouring me. So a giant cube of hellish proportions with many billions of interlocking rooms is out of the question.

Billions of rooms are easy. If you make every room a 10 meter cube then the whole structure is just ~20 kilometers in diameter and gravity is still tiny. Each room would have as much space as the ISS, so the structure could potentially house the current world population (ignoring some practical concerns like heat management).

Well, that depends on what kind of structure do you need and how you build it...

Let's say it's a ring made of nanites (which sort of solves all material tension problems, because we assume that nanites can automatically rebuild any damages). Let's assume that the radius of the ring is 50000 light years (an approximation for the Milky Way radius, which equals 5*10^19 m), its width is 1 km (10^3 m) and its thickness is 100 nm (a size of a nanite, 10^-7 m). Let's assume that the nanites are made of carbon and have its density, which equals 3*10^3 kg/m^3.

The mass of this structure is going to be:

M = ro * V = ro * S * L = ro * h * w * 2* pi * R = 3*10^3 * 10^3 * 10^-7 * 2 * 3 * 5 * 10^19 = 9 * 10^19 kg.

Milky Way mass is 10^12 solar masses, and the mass of the Sun is 10^30 kg. In fact, the mass of the Moon is 10^22 kg. So one Moon is more than enough to build such a ring around the Galaxy...

Ok, so I just need FTL travel, many galaxies worth of materials, some way to heat the place and billions of workers and we can make a hellish galaxy sized Ikea flat pack colony. Excellent.

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I don't know where you're going with this, but you have my hexwrench in this battle, friend

Wouldn't your giant cube need cooling instead of heat? The heat is generated by the people and machines, so the tricky part is disappating the heat effectively. From what I understand it's harder to cool off in space than to heat up.

Since you're in a vacuum you lose all of that passive cooling we take for granted on Earth.

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Yes, and a compact shape like a cube makes that harder. But the main point is that any conceivable size is still much smaller than galaxy-sized. The death star for example, if it has a level of crew per volume that's comparable to a current ship, would hold several tens of trillions in personnel. And it's tiny, like a quarter the diameter of Ceres. But it could be built without requiring magic materials. Moving it would be a different issue.

No. Billions of workers don't get you anywhere. You'll need numbers of workers you'll have to look up the names for.

Aren't they all just called Bob?

A few planets is a tiny fraction of the mass of a start system. How exactly is that an example of a galaxy sized structure needing multiple galaxies worth of material?

If you want to go full sci-fi with this, look into the concept of the birch world

Now we're talking. Off down a little rabbit hole I go...

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It is an example to demonstrate how much mass is needed to accomplish even a small scale version of what their talking about. It's an analogy.

Galaxies are not dense on average, because of the space between stars. If you want something galaxy sized but denser, there is the serious issue of where you could obtain enough material, since the space between galaxies is quite vast.

>Galaxies are not dense on average, because of the space between stars.

I get that, hence the question as to whether it is theoretically possible to build a structure the size of a galaxy with the space filled in by 'structure' without it collapsing in on itself. It's a hypothetical question so sourcing materials isn't the object of the question; the point is whether, despite the fact it would take several billion years to build at current space traversal times, it would be possible with our current understanding of our universe.

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You need cooling, not heating. The heat loss scales with the surface of the structure, but heat production scales with the volume (if we just scale everything up). The volume grows faster than the surface. Even the ISS needs radiators already.

>Let's assume that the nanites are made of carbon

>the mass of the Moon is 10^22 kg. So one Moon is more than enough to build such a ring around the Galaxy...

Carbon is only available in trace amounts on the moon. Carbon makes up about 0.46% of the universe, but it's not like it's conveniently all in one place for mining.

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Not sure where you are going. Given the choice of any type of construct, of course it is possible. After all, if you could somehow move astronomical masses around, you could just create your own galaxy.

Tbf, at that point, you might be able to just re-arrange atoms to be whatever you want.

All materials have structural limits. A column, made of a certain material in a certain gravitational field, will either crush or buckle under its own weight if it is too high. This still applies if the gravitational field is the self-gravity of the structure itself.

Dynamic support might be used. One concept is you make a sort of long skinny particle accelerator and the particles going back and forth exert a reaction force on the endpoints supporting the far end. But even with that, the bottom end loop has to be able to support the load.

Based on known materials, even a solid Dyson sphere is impossible without dynamic support. A realistic Dyson sphere would instead be simply a swarm of satellites so dense it can absorb nearly all its star's light.

The ultimate limit is that the volume of a region of space scales with radius cubed, but the critical mass above which a black hole is formed scales linearly with radius. Thus the larger a structure is, the less dense it must be to stay below that critical mass. (Which in any case could only be approached by transporting matter in from other galaxies, since the starting galaxy was obviously not a black hole.)

Once you're at the point that the "structure" is a thin mesh of wires or tubes stretching over interstellar distances, well, why have the structure at all instead of just a bunch of free-floating spacecraft that can share power and data with lasers or something?

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