Surprisingly, Some Dyson Spheres and Ringworlds Can Be Stable (phys.org)
- Reference: 0176789001
- News link: https://science.slashdot.org/story/25/03/22/0414254/surprisingly-some-dyson-spheres-and-ringworlds-can-be-stable
- Source link: https://phys.org/news/2025-03-dyson-spheres-ringworlds-stable.html
> In the realm of science fiction, [sun-energy capturing] Dyson spheres and ringworlds have been staples [3]for decades . But it is well known that the simplest designs are unstable against gravitational forces and would thus be torn apart. Now a scientist from Scotland, UK has shown that certain configurations of these objects near a two-mass system [4]can be stable against such fractures ...
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> [A] rigid ring around a star or planet, as in Larry Niven's "Ringworld" series of novels, is also unstable, as it would drift under any slight gravitational differences and collide with the star. So [engineering science professor Colin] McInnes considered a restricted three-body problem where two equal masses orbit each other circularly with a uniform ring of infinitesimal mass rotating in their orbital plane. The ring could enclose both masses, just one or none... McInnes also investigated a shell-restricted three-body problem with the shell also of infinitesimal mass, again with the shell enclosing two masses, one or none.
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> For the restricted ring, McInnes found that there are seven equilibrium points in the orbital plane of the dual masses, on which, if the ring's center were placed, it would stay and not experience stresses, akin to the three stable Lagrange points where a small mass can reside permanently for the two-body problem... McInnes restricted this research to a planar ring (in the plane of the circularly orbiting masses) but says it can be shown that a vertical ring, normal to the plane, can also generate equilibria...
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> These results can aid the search for extraterrestrial intelligence, McInnes said, "If we can understand when such structures can be stable, then this could potentially help direct future SETI surveys." An important technosignature would be one bright star orbiting in tandem with an object showing a strong infrared excess. Shells around a sun-exoplanet pair or an exoplanet-exoplanet pair could also be possible. A nested set of Dyson spheres is also a feasible geometry.
In 2003 Ringworld author Larry Niven [5]answered questions from Slashdot readers ...
[1] https://www.slashdot.org/~Required+Snark
[2] https://phys.org/news/2025-03-dyson-spheres-ringworlds-stable.html
[3] https://entertainment.slashdot.org/story/04/09/09/1620233/ringworlds-children
[4] https://academic.oup.com/mnras/article/537/2/1249/7989465
[5] https://interviews.slashdot.org/story/03/03/10/167206/ladies-and-gentlemen-dr-larry-niven
Re: (Score:1)
I'm not worried. Trump just awarded Boeing the F-47 contract. Since successful execution of even this contract beggars belief, a Dyson swarm just won't happen.
OTOH, if Musk announces it as his next project ...
Neither is plausible in the traditional form. (Score:3)
The basic problem, not addressed, is the amount of mass required to build either a RingWorld or a Dyson Sphere with solid shells. But a Topopolis should be doable. It's not rigid and it can be build incrementally. It would probably need a diameter of at least 10 miles, and various segments of it would need to be separable (but that could be handled with magnetic links between the segments. This would allow some segments to be non-rotating so that, e.g., T-junctions would be possible.
OTOH, I'm not sure there's enough matter in the asteroid belt to build one, so you might need to disassemble a few moons...after awhile.
That said, a topopolis would never be self-maintaining. If civilization died, so would everyone living there. (OTOH, if you could build one, you could certainly build smaller habitats. And a working topopolis would make an excellent basis for an em-launcher for habitats headed for interstellar space. So it would be likely to spread a some very small fraction of the speed of light. Probably less than 0.001c for system to system spread. [This is assuming that controlled fusion is actually possible.])
Re: Neither is plausible in the traditional form. (Score:2)
You've either read Heaven's River (Bobiverse book 4), or you should.
What about comets etc? (Score:1)
A ring occupies every possible position in its orbit simultaneously so is roughly 3 x 10^8 times more likely to be hit by some random rock or comet in an elliptical orbit than if it was a satellite. A sphete on the other hand is guaranteed to be hit by everything that crosses its orbit. Just saying.
That's good, (Score:1)
because OUR world ain't currently stable.
Indistinguishable from magic (Score:2)
If you have the technology to build one of these structures, then keeping it from falling into the star wouldn't be a problem.
Niven's ringworld had bussard ramjets keeping it stable.
Actually I think what we might find is smaller ringworlds around red dwarf stars, because that type of star is more common and has a longer life.
Re: (Score:2)
There's a lot to be said for passive stability. As Niven illustrated when he had the residents of the Ringworld stealing the ramjets for spacecraft.
OT: Dyson Sphere is super stable (Score:2)
A friend and I were talking about how well it performs considering it's done with Unity.
Ringworld, maybe. But sphere? (Score:2)
It seems conceivable that a ring could be stable, because the centrifugal force would be more or less consistent across the ring.
But a sphere would always have gravity at the poles, with no centrifugal force to keep the structure from collapsing into the star.
So how exactly would this collapse be prevented in a spherical geometry?
Re: (Score:2)
The stability question isn't about the tensile strength and other mechanical properties of the structure itself, it's about the orbital mechanics. A ringworld orbiting a single star is stable on a "peak" position, where any movement away from that position results in net gravitational forces that move it further away. In contrast, the stable lagrange points are positions where small amounts of movement away from that position result in graivtational forces moving it back towards that position -- a "valley
Re: (Score:2)
> The stability question isn't about the tensile strength and other mechanical properties of the structure itself, it's about the orbital mechanics. A ringworld orbiting a single star is stable on a "peak" position, where any movement away from that position results in net gravitational forces that move it further away."
That is the definition of the word "unstable," when any movement away from equilibrium results in a force moving it further from equilibrium.
What you meant so say was, "A ringworld orbiting a single star is in equilibrium on a "peak" position." But it's an unstable equibrium.
> In contrast, the stable lagrange points are positions where small amounts of movement away from that position result in graivtational forces moving it back towards that position -- a "valley" by comparison.
Yes, the two Lagrange points L4, and L5 are stable equilibria. The colinear Lagrange points, L1, L2 and L3 are unstable.
Re: (Score:1)
Dyson swarms, or physically impossible material strength. The ring is unstable because if any one part of the ring has a higher pull/different mass that part of the ring experiences stronger gravity, and is pulled closer to the star. In an object that would result in a higher velocity, but in a ring the whole ring's speed balances out resulting in on change in speed but that part of the ring still being closer. This continues until the system collapses.
But then most orbits need station keeping, I'm not su
Re: (Score:2)
Let's put it like this: Dyson spheres should be called Yogi Berra spheres, in theory, practice and theory are the same, in practice, they aren't. Take the Solar system as an example. If we want to build a Dyson sphere at the orbit of the Earth, we would have about 10 metric tons of building material per square meter - for anything, including the sphere itself. And most of our building material would be hydrogen from the big gas planets anyway. Even if we manage to fusion all hydrogen into heavier elements t
Re: (Score:2)
> But then most orbits need station keeping,
Orbits in a two-body system (one central body, one orbiting body) do not need station keeping. They only need station keeping if you have some perturbation.
You need station keeping, for example, if you want to stay in an orbit of exactly 24 hours in a perfectly equatorial inclination (because of perturbations from the Earth's non-spherical shape, and perturbation from the gravity of other bodies, primarily the Sun and moon.). But even then, while the perturbations are enough to slightly change the shape and
Re: (Score:3)
Dyson spheres are inherently stable when it comes to gravity. It is a complex integral calc but if you work it out, any gravity body inside the sphere has a net 0 effect on the sphere. Now that said, that is a perfect sphere. In reality it would be an imperfect sphere and thus the calc doesn't give you a net 0, which I am guessing is what they are talking about here as it would allow the imperfections enough leeway to be truly feasible.
Re: (Score:2)
That's not true. If it were, you'd be floating around right now.
I think what you're referring to is that the gravitation from a perfect spherical shell is cancelled out in its interior. Basically the opposite of what you said. It's called the shell theorem.
This story is about the stability of a shell surrounding a mass like a star. That is definitely not stable with a single star inside. This story is about the discovery that there are stable configurations when two stars are involved.
Re: (Score:2)
No, that's not what I was talking about. Inside the dyson sphere, the sphere has no gravitational effect, and likewise the sun has no effect on it. Both will affect any body exterior to the sphere, but that's not relevant here. This has nothing to do with me floating, I'm not a dyson sphere, nor am I on the interior of any sphere. Again, this only true if it is a perfect sphere, which it won't be. Now if you want to live on the inside the sphere you're going to need a gravity like effect, produced by s
Re: (Score:2)
That's just not correct.
Re: (Score:2)
Yes, it is correct, and the very theorem you brought up, shell theorem, proves it. Here, directly from wikipedia: Isaac Newton proved the shell theorem[1] and stated that: 1. A spherically symmetric body affects external objects gravitationally as though all of its mass were concentrated at a point at its center. 2. If the body is a spherically symmetric shell (i.e., a hollow ball), no net gravitational force is exerted by the shell on any object inside, regardless of the object's location within the
Re: (Score:2)
Your math is correct but that's not the definition of the word "stability".
Stability means that there is a restoring force in response to a perturbation which returns the system to the initial position. The shell theorem says there is no restoring force.
Re: (Score:2)
Okay, that's fair enough, I was just thinking of the two bodies relative to each other, not external vectors.
Re: (Score:2)
Your calculations may be right, assuming that the sphere is perfectly rigid. Since there's no such perfectly rigid substance, certainly not at that scale, the poles would implode into the star, even if the rest of the structure stayed intact. At the scale of a Dyson sphere, the "shell" would be as flexible as cellophane.
Re: (Score:2)
Yeah, the theory may work, in practice, no chance in hell. There is all kinds of issues once you start to turn that theory into reality. Even assuming you could find the perfect substance to make the sphere from that had just the right rigid properties and such, and can perfectly place every last atom to be in a perfect spherical pattern, the reality is it still wouldn't make a perfect sphere, just a series of point masses that resemble a perfect sphere, and that's not good enough for the theory to work.
Neutral stability Dyson sphere (Score:2)
> Dyson spheres are inherently stable when it comes to gravity.
No, they're not. They are precisely neutral. A perturbation from the centered position results in neither a restoring force nor a increasing force. So, if you perturb it with velocity v, the perturbation velocity remains constant, until it crashes into its sun (at constant velocity).
Re: (Score:2)
The inverse square law is the problem. If one part of the ring gets close to the sun the gravitational attraction goes up more than linearly, while the attraction across the diameter goes down more as as that is farther away from the sun.
So soon the ring will be dragged way off center.
The fictional Ringworld had attitude jets to hold it in position.