Star system mechanics, astronomy, & Dyson Spheres

General Off-Topic Discussion
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Yep, that’s fair indeed and entirely accurate. If the material for the Dyson Sphere is sourced entirely from within the solar system, what would matter is the total mass and the distance, none of which would have changed.

The challenge is that it wouldn’t fully work, which I was sort of going on, but it certainly would have helped if I had clarified that. Disassembling the planets, for example, could provide enough material to build a Dyson Sphere with a thin shell – we’re talking about one meter or so – at 1 AU. (Which is an assumption right there. This would be a shell that covers the star plus out to the orbit of our planet.)

However, for thicker shells, the mass of the planets alone wouldn’t suffice at all. Maybe breaking down all of the Kuiper Belt and the Oort Cloud would do the trick, but only up to about ten meters of shell material. Most of the icy material would be of no benefit whatsoever.

The aliens would actually be better off mining their own star if they were going to pull only from their own solar system. Or pulling from interstellar material, which is more what I was doing. It’s in the latter case where you might get some perturbations.

Realistically, we’re talking about some elements that would likely change mass distributions as well. A Dyson Sphere around a star would experience gravitational forces pulling it toward the star. The sphere would need to resist collapsing inward due to this pull. Additionally, the sphere’s own gravity would pull it together, creating stress that the material would have to withstand. Those are formidable problems!

Even the strongest materials known today, like graphene or carbon nanotubes, would likely not be strong enough to resist such forces. So even if all the material from the solar system or the interstellar medium were harvested and used, building a rigid, solid Dyson Sphere that holds up under its own gravity is essentially impossible without materials or technology far beyond what we currently understand.

Using exotic materials to construct a Dyson Sphere – and that is what we would have to do, at least so far as we know – would likely involve significant extra mass, which would, in turn, have observable effects on the star system and its surroundings.

As you can see, that’s a lot of context for what I originally said obliquely but didn’t clarify!

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Not to mention the problem of the sphere remaining in place. It would be unstable and eventually crash into the star without immense propulsion.

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Definitely! A solid Dyson Sphere surrounding a star would not naturally stay centered around it. Any slight perturbation, maybe caused by uneven construction, external gravitational forces, or solar activity, would cause the star to pull unevenly on the sphere.

So let’s say these aliens have thrusters distributed across the sphere’s surface to apply corrective forces and keep it centered. First, the propulsion systems would require colossal amounts of energy, which would result in waste heat detectable as infrared radiation as we all talked about earlier. Also, you would likely get shifts in the star’s barycenter. (Granted, these would be longer-term shifts.)

Interestingly, as I started doing some calculations, I realized that harnessing power from a black hole would probably be a more efficient and stealthy alternative for a civilization that values both energy generation and avoiding detection. Granted, the intense gravity and radiation from the accretion disk and black hole would require extremely advanced, durable materials capable of withstanding incredible tidal forces, high temperatures, and radiation but (1) all this depends on the orbit and (2) it would count on many other civilizations perhaps not wanting to get to close to black holes anyway!

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Interesting. How would you capture significant amounts?

I had no idea I was going to precipitate so interesting a discussion :smiley:

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What’s interesting to consider is that the energy output from a black hole’s accretion disk can be up to around forty percent or so of the mass of the material falling into it. This would be vastly more efficient than the modest one percent or so of the energy conversion rate of nuclear fusion in stars.

Okay, so with that, I could imagine a civilization that builds energy-harvesting structures around the black hole, such as orbiting stations that extract power from the accretion disk. Or, perhaps even better, tap into the black hole’s rotational energy via the Penrose process. (Currently entirely theoretical. But maybe not to the aliens!)

Also, a rotating black hole generates a strong magnetic field, and this magnetic field interacts with the surrounding plasma in the accretion disk or the black hole’s jets. There’s something called the Blandford-Znajek process which would essentially tap into the rotational energy of the black hole itself by converting this energy into outgoing electromagnetic radiation. That could be used for lots of power.

There’s challenges to all of this, of course! But it’s interesting to think of how an advanced enough civilization may try to compensate for the challenges. For example, if we imagine massive electromagnetic funnels, designed for energy harvesting around a black hole, those could be difficult to detect from a distance given the nature of a black hole and how spacetime is warped near it.

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Whether a Dyson sphere deliberately vents or not, it would eventually radiate away all of the star’s output as some form of low-grade energy – likely what we would consider heat. It’s just a matter of thermodynamics. There are other stellar objects whose primary output is heat, such as brown or black dwarf stars, but they would be much smaller and dimmer than a Dyson sphere, as they both release far less total energy.

Possibly you might wish to consider a Dyson swarm – a collection of objects that collectively orbit a star at a variety of distances and speeds – rather than a sphere. Dyson spheres require hypothetical materials with tensile strengths greater than physics may actually allow. Dyson swarms require some careful engineering but are much, much easier to construct and don’t rely on physics we don’t understand.

The star system of a Dyson swarm would be easier to detect, but the swarm itself might be harder, depending on the precise structure. It might make a star appear to twinkle even in the emptiness of space, and might be mistaken for a particularly dense asteroid belt or ring formation. A civilization that invested deeply in astronomy would likely notice the peculiar behavior of that system very rapidly, but one at our own level of advancement would have to look very, very carefully to pick it out.

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This was in fact Dyson’s original concept. The name “Dyson’s sphere” has resulted in a misconstruction of what the idea originally was.

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A Dyson SPHERE, as opposed to a swarm, is still imaginable. It would indicate a very, very advanced alien species, though, as we’re not even sure the physics of our universe would permit such a structure to exist, and even if it’s permitted it would require astounding advances in construction and material sciences.

I am reminded of the fictional TARDIS, and how some components necessary for its function aren’t physically possible, so the Gallifreyans generated them in a mathematical space where physical law was more flexible.

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What’s interesting is that Dyson thought the idea of a full sphere was impossible. He very much disliked the idea. The problem with “requiring a very advanced alien species” is that (1) we’re making a huge assumption [that any level of advancement could achieve it], and (2) we might as well just call it magic. (Insert appropriate Arthur C. Clarke quote here. We all know it!)

But all of this goes back to something I mentioned upstream: motivation. What would be the purpose of creating something that, even if it could work with extremely exotic materials, could be achieved in far easier ways (at least easier for these very advanced alien species)?

There is also the idea that technology tends to begat technology. So, if our hypothetical race got to the point where they could engineer matter on the scale of an actual Dyson sphere, they would presumably have access to exotic power solutions that would negate the need for what such a sphere is generally proposed to be useful for in the first place!

It is a different context, but it reminds me of the 1996 alien invasion movie Independence Day where the aliens raided planets for resources. But it made no sense in that if an alien species could (1) travel between the stars and (2) create ships that could negate gravity and pressure, they could easily harvest unpopulated worlds for any materials they would need. Unless, of course, the aliens are proposed to be jerks that just like blowing up other cultures! Still, it’s a valid consideration: we tend to imagine alien motivation based on our needs. But the civilizations we talk about are usually framed such that they would have far outpaced our needs. Consider the aliens in Childhood’s End, The Alien Years or even Timelike Infinity where the “invasions” were more pragmatic.

Anyway, back on track, in terms of aliens that wanted to hide, needed power, and used extreme technology to do both, the Heechee in the Gateway series and the Shrouders in the Revelation Space series are both interesting examples. (Utilizing technology related to black holes in both cases.)

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The classic “alien invaders” were usually metaphors or allegorical representations of how ‘advanced’ human societies acted during the Age of Empires. As you point out, most material resources are more easily acquired in outer space than by invading a planet like Earth, and the exceptions (like biological resources) would require much more elegant and delicate handling than an invasion would permit.

The exceptions, such as Half Life and Half Life 2, involve alien societies that aren’t much more advanced than we are in terms of space travel but can easily generate portals between ‘dimensions’, making it both possible and feasible to (for example) remove most of Earth’s water for their own use. Aliens who could travel in space could get all the hydrogen-and-oxygen they wanted from harvesting the Oort Cloud!

The reasoning of traditional economics is starting not to apply to our own civilizations, it certainly doesn’t apply to spacefaring societies. We don’t really know how to think about post-scarcity societies, as the absolute failure of Star Trek to explain how the future is supposed to actually function makes clear.

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Okay I like this. My system generator doesn’t generate black hole systems yet but improving that is a priority. Simply having them creating “energy harvesting” stations to take advantage of that energy solves my problems. It also makes the system perceptible without making it necessarily interesting. For the player, black holes are likely to be mostly a nuisance.

Thanks for the lateral thought.

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Let alone for the physicist.

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Fortunately I don’t have to worry about them too much :smiley:

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I’ve always thought that Larry Niven’s ringworld concept was considerably more probable than a Dyson sphere. (If you’ve not read Ringworld: a circular ribbon of unobtainium approximately 1AU in radius, 1 million miles across, not very thick, rotating around a sun-like star at a speed sufficient to provide about one gee on the inner surface.) The amount of unobtainium required is enormously less than what would be needed for a full Dyson sphere. In his telling, the motivation is not particularly energy collection, but to provide a vast amount of earth-like livable surface for a huge population.

The connection to the current discussion is that such a structure would be pretty hard to detect, I think. If you happened to observe from not quite edge-on, maybe it would cause surprising transient occlusions of its star. Otherwise, it seems like it’d be much less noticeable than a full sphere, because it’d intercept and re-radiate much less energy.

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That means the ring would need to spin at 0.4% the speed of light. :grimacing:
It would also have the same instability problem of the sphere and need ridiculous propulsion just to stay centered.

I think any species capable of doing that would find a simpler solution.

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Niven was aware of this: he does mention giant propulsion engines there just to stabilize the ring’s position. The energy expense would be staggering, of course.

More rigorous designs have been described: the Orbitals in Iain Banks’ Culture Saga, which are smaller and placed at the L₄ - L₅ Lagrange points between a planet and its single moon, to name one. There are some downscaled versions of these hyperscale structures that have been seriously proposed by NASA as concept ideas for orbital settlements in past decades. They would be of course out of current engineering reach and far too costly, but the science is likely to stand up to scrutiny. A bit of research into this would probably unearth tons of material - it depends on how “hard scifi” the author wishes to go.

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He wasn’t aware of this when he wrote the original book. Fan lore is that fans worked out the math and chanted “The Ringworld is unstable!” at him at cons until he put the stabilizer engines in the sequel.

Of course the Banks people have the ridiculous technology, and could build a full Niven-style ring or Dyson sphere if they felt like it. They just don’t generally bother because Orbitals are easier. (And can be constructed incrementally, which is convenient.) (You start with two Plates spinning around a common center, and add more as needed.)

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If you were going to build a giant ring around the sun at 1AU, you’d be better off with it not spinning. You would still have to stabilize it, but you’d get 0.6g (Edit: 0.0006g, I seriously misplaced a decimal) gravity for free on the outer surface, and have built-in protection from all the nasty radiation.

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Now that you mention it, I remember hearing this story before, though I don’t know how substantiated it is. I checked that Niven does not mention the thrusters until the second book, so it’s at least plausible. If so, this looks like a case of Author’s Saving Throw…

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I think this is the idea I am going to run with for now. It solves two problems for me:

  • it’s just a black hole system and so not inherently special (except in so far as Black Holes are) from outside
  • it’s pretty clear when you are in it that shit is going down so no mistaking it when you find it

Since Black Hole systems aren’t intrinsically useful or interesting to the player (no human colony is going to be camped on one) it means they’re not likely to make a beeline for the Beyonder system before the plot requires them to.

If they accidentally mount a mission to the outer rim and encounter the system, well that’s different. That ship is going to be “lost with all hands”. Up to the player if they want to risk another on such a foolhardy endeavour.