Dyson spheres

[halfeye]

Of course, most of the sun is Hydrogen with some Helium which I don't think would be useful to build a Dyson Sphere with.

73% hydrogen, 25% helium, 2% others. You probably don't mess with Dyson spheres before you've worked out making new elements out of old anyway.

[Trafalgar]

73% hydrogen, 25% helium, 2% others. You probably don't mess with Dyson spheres before you've worked out making new elements out of old anyway.

Making elements like that would require being able to create and control supernova level fusion reactions. Again, if your civilization can do that, they don't need a Dyson Sphere.

[Seppl]

It is the similar to the Ringworld discussion: A more modest approach just scales much better than a full Dyson Sphere. If a civilization invested the same amount of material into smaller, conventional fusion reactors, they would achieve a much, much higher energy yield per metal cost compared to solar collectors. They could build billions of such reactors for the same cost, allowing the civilization to spread out much further and yielding way more energy than a star. And, most importantly, they could start mass producing fusion reactors at a technology level just slightly above our own, whereas the Dyson Sphere requires crazy speculative future tech, that we do not even know is possible.

The only reason for such a civilization to eventually build a Dyson Sphere would be just for fun. Maybe some hobbyists proving that an ancient sci-fi idea is actually possible, similar to us building Leonardo's flying machines.

[Cikomyr2]

A cool YouTuber made a video explaining the most straightforward way you'd start building a Dyson Sphere (swarmstyle rather than shell).

Basically, simpler is better. Just have the swarms be acting like mirrors, built on Mercury via robotized industry and then slingshot into space with magnetic cannons.

You have key collecting points around the solar system that receives the reflected sunlight, which is then transformed in electricity. On earth, or anywhere you want.

[Bohandas]

So my question whenever something like a Dyson sphere comes up is "What is your civilization doing that requires that amount of energy?" If your civilization has the technology to build a Dyson sphere, it probably doesn't need to.

What wouldn't they be doing? The more technology you have the more things there are that need energy.

EXIT:
Actualoy, I know what they wouldn't be doing. They wouldn't be conserving electricity.

[Mechalich]

What wouldn't they be doing? The more technology you have the more things there are that need energy.

Well, there are probably limitations on how much electrical power any civilization is going to use per capita. It's not going to increase ad infinitum. Additionally, electricity use produces heat and that heat needs to be radiated back into the vacuum, which is a significant limitation if you're beaming power from very large dyson swarm elements to much smaller rotating habitats (similarly, this is why building a 'city planet' is problematic, since it would create a heat buildup that would melt its components).

It's important to note that the Dyson Sphere concept was popularized by Freeman Dyson in 1960, and was very much apiece of the science fiction of the 1960s and 1970s, when endless exponential population growth, and the implications of said growth, were rather important in the popular zeitgeist (The Population Bomb was published in 1968). As such, authors played with concepts based on the idea of trillions or even quadrillions of humans. Recent events have made it clear that such an outcome is probably unlikely, barring some kind of immortality tech, and therefore speculation has broadly dropped it as a consideration.

[Trafalgar]

Well, there are probably limitations on how much electrical power any civilization is going to use per capita. It's not going to increase ad infinitum. Additionally, electricity use produces heat and that heat needs to be radiated back into the vacuum, which is a significant limitation if you're beaming power from very large dyson swarm elements to much smaller rotating habitats (similarly, this is why building a 'city planet' is problematic, since it would create a heat buildup that would melt its components).

Transmitting all that power is going to be it's own problem, especially if you are converting it to electricity. Would you use microwave? I can see some problems with that, not least of which is that it's line of site. I imagine if you are building a dyson sphere, you want to collect all that power at one location. So a microwave transmitter on the fall side of the sphere would have to go through several relay stations to get around the sphere its self.

Probably easier to just focus the sunlight itself. I remember Kim Stanley Robinson's "Red Mars", "Green Mars", "Blue Mars" series had a gigantic lens that focused the Suns light on Mars as part of the terraforming project.

[halfeye]

Transmitting all that power is going to be it's own problem, especially if you are converting it to electricity. Would you use microwave? I can see some problems with that, not least of which is that it's line of site. I imagine if you are building a dyson sphere, you want to collect all that power at one location. So a microwave transmitter on the fall side of the sphere would have to go through several relay stations to get around the sphere its self.

On the inside of the sphere, all of the lines of sight that don't intersect the star are clear.

Probably easier to just focus the sunlight itself. I remember Kim Stanley Robinson's "Red Mars", "Green Mars", "Blue Mars" series had a gigantic lens that focused the Suns light on Mars as part of the terraforming project.

That might work, but you'd want a freznel lens, or multiple lenses.

[Mechalich]

I imagine if you are building a dyson sphere, you want to collect all that power at one location.

Why? A dyson sphere - or more accurately a swarm - would probably be powering an entire solar system's worth of stuff. Hundreds of habitats, thousands of industrial installations, countless other automated facilities. There are only a couple of things you actually need to concentrate massive amounts of power for: boosting a starship and powering interstellar communications, which both involve pointing really big lasers at faraway things. And you still wouldn't need anything like 100% of solar output for that at any given time.

[gbaji]

No, what is meant by "neutrally stable" is that there is NO force exerted between a hollow sphere and an object located inside of it, even if the sun isn't centered inside that sphere.

I don't think that's actually correct (well, not fully correct anyway). The sphere itself is gravitationally neutral on all objects within it. However, the star sitting in the middle of this otherwise empty space absolutely does exert gravity and will exert a pull on any unattached objects floating around inside the sphere.

A Ring should also be just as equally gravitationally neutral with regard to an object floating within it's "width" as well. For pretty much the same reason a sphere is. I mean, we can imagine a sphere is just a collection of attached rings, gradually decreasing in diameter as we reach the poles, so the math *should* be the same.

The innate instability with both models is that there are going to be external forces on the outside surfaces (gasses, meteor impacts, gravitational effects from other stars, etc), that will impart slightly different momentum on the ring or sphere than is exerted on the star itself, which will gradually cause the star to drift towards the surface of the object. Heck, Even internal pressures will be uneven (stellar gasses, solar flares), which will cause this drift effect as well. The other forces involved, while perhaps small relatively speaking will certainly not be equal over time, and will always cause the system to drift. Unlike an object in orbit, in which velocity and gravity are in balance, there's nothing holding these objects "in place" relative to the center of gravity of the star.

Both types of structures would require some form of station keeping system. Honestly though, if you have the tech to build something like this in the first place, the station keeping requirements should be trivial (well, unless someone scavenges them to make ships or something).

[NichG]

Neutrally stable would mean that even if you displace the object slightly from the point at which all forces on it balance, the resultant forces still exactly balance. Stable would mean that the forces point back towards the fixed point, whereas unstable means that (at least some component) of the forces resulting from a small displacement point away from the fixed point.

In order for a ring to be neutrally stable, I think you'd need gravitational force to fall off as 1/r not 1/r^2. It's been awhile since I've done the integral explicitly though. Here's a bunch of different ways of proving it for a sphere and 1/r^2 though: https://en.wikipedia.org/wiki/Shell_theorem

Basically, since a point experiences a zero net force from within a uniform spherical shell of mass, it also exerts a zero net force on a uniform spherical shell of mass that contains it. Meaning that a sphere around *any* distribution of masses that completely contains them is gravitationally neutral - you can move the center of the sphere around anywhere and it still feels zero net force from those masses (as long as they remain contained). With a ring however, there isn't the material above and below to make this book-keeping work in 3d space with 1/r^2 gravity, so for the ring if you're closer to one side than the other it does change the balance of forces on the ring. If everything were 2d then gravity would go as 1/r and rings would be neutrally stable but spheres would not be.

[The Glyphstone]

Since it's tangentially related, what if you're trying to build a Nicoll-Dyson beam weapon?

[Mastikator]

Since it's tangentially related, what if you're trying to build a Nicoll-Dyson beam weapon?

Great way to blow up planets for sure. What did Kepler-186f ever do for us?

[Anymage]

Since it's tangentially related, what if you're trying to build a Nicoll-Dyson beam weapon?

Related.

Although while a weapon like that would be devastating on multiple levels, in practice I see less value to a weapon where the targets might well establish a significant off-planet presence and/or wipe themselves out by the time your attack finally landed.

[Mechalich]

Although while a weapon like that would be devastating on multiple levels, in practice I see less value to a weapon where the targets might well establish a significant off-planet presence and/or wipe themselves out by the time your attack finally landed.

There's also a targeting issue. It's really, really hard to determine a planet's location to within a single planetary radius several dozen years in the future from several dozen light years away. In fact, given the nature of orbital perturbations there's probably a theoretical maximum distance from which this is possible at all. Targeting a star is much easier, but it's rather more difficult to do any significant damage that way.

[NichG]

There's also a targeting issue. It's really, really hard to determine a planet's location to within a single planetary radius several dozen years in the future from several dozen light years away. In fact, given the nature of orbital perturbations there's probably a theoretical maximum distance from which this is possible at all. Targeting a star is much easier, but it's rather more difficult to do any significant damage that way.

Well, the perturbation from a star 1 solar mass and 1 light-year away is about 10^-13 m/s^2. With a travel time of 10 years that's a maximum displacement of about 10km uncorrected for (~3e8 seconds squared * 1e-13 m/s^2 = 1e4 meters). So not so difficult. But it's quadratic with travel time, so a travel time of 100 years means a displacement of 1000km, at which point you'd probably want to at least start accounting for nearby stars.

Within system, Jupiter's effect for example is bigger at around 10^-8 m/s^2, but it's very predictable since one orbit of Jupiter is already 12 years...

[Jimorian]

I don't think that's actually correct (well, not fully correct anyway). The sphere itself is gravitationally neutral on all objects within it. However, the star sitting in the middle of this otherwise empty space absolutely does exert gravity and will exert a pull on any unattached objects floating around inside the sphere.

A Ring should also be just as equally gravitationally neutral with regard to an object floating within it's "width" as well. For pretty much the same reason a sphere is. I mean, we can imagine a sphere is just a collection of attached rings, gradually decreasing in diameter as we reach the poles, so the math *should* be the same.

The innate instability with both models is that there are going to be external forces on the outside surfaces (gasses, meteor impacts, gravitational effects from other stars, etc), that will impart slightly different momentum on the ring or sphere than is exerted on the star itself, which will gradually cause the star to drift towards the surface of the object. Heck, Even internal pressures will be uneven (stellar gasses, solar flares), which will cause this drift effect as well. The other forces involved, while perhaps small relatively speaking will certainly not be equal over time, and will always cause the system to drift. Unlike an object in orbit, in which velocity and gravity are in balance, there's nothing holding these objects "in place" relative to the center of gravity of the star.

Both types of structures would require some form of station keeping system. Honestly though, if you have the tech to build something like this in the first place, the station keeping requirements should be trivial (well, unless someone scavenges them to make ships or something).

If you would have also quoted my 2nd paragraph, you could have saved yourself some trouble, because you just repeated what I said: "That does mean that if the sun or the sphere start drifting because of some outside force, though, momentum would eventually carry them into a collision, but there is absolutely zero feedback to accelerate or decelerate the process."

[halfeye]

If you would have also quoted my 2nd paragraph, you could have saved yourself some trouble, because you just repeated what I said: "That does mean that if the sun or the sphere start drifting because of some outside force, though, momentum would eventually carry them into a collision, but there is absolutely zero feedback to accelerate or decelerate the process."

If that's the ringworld you are talking about, it's worse than that, once it drifts off centre it is accelerated by gravity further away from the central position, which means the collision is faster.

[Jimorian]

If that's the ringworld you are talking about, it's worse than that, once it drifts off centre it is accelerated by gravity further away from the central position, which means the collision is faster.

Correct, but I was talking about the sphere. The reply to me, however, did get this part about the ring wrong.

Ringworld: Dynamically unstable. As soon as the sun is no longer perfectly in the center, it accelerates to collision.
Dyson Sphere: Neutrally stable. There's no force between sun and sphere at all, so only outside forces can create a collision situation assuming no relative motion to begin with (which is the caveat I included in my original post which was then ignored as if I didn't say it).

One of the other constructs in the Niven-verse was a Rosetta World that the Puppeteers were traveling on. 5 planets without a sun in a stable orbital situation where they all orbited around a common center point. I can't quite recall the math of this one, but probably counts as dynamically stable to complete the above set.

[Seppl]

One of the other constructs in the Niven-verse was a Rosetta World that the Puppeteers were traveling on. 5 planets without a sun in a stable orbital situation where they all orbited around a common center point. I can't quite recall the math of this one, but probably counts as dynamically stable to complete the above set.

The Klemperer Rosetta is unstable, too. Any minor perturbation of a body would lead to increased attraction with the other bodies on one side and a decrease on the other side, destroying the whole formation in no time.

Not that much of a problem for the Puppeteer worlds in the book, as the planetary engines (that left even the Kzin awed by the Puppeteer's power), used to build the rosetta, are presumably still in place and could correct any misalignments.

[Maat Mons]

Somewhat tangential, but would 6 planets sharing a single orbit around a star be stable? Each planet would be in the L4 and L5 points of its neighbors.

[halfeye]

Somewhat tangential, but would 6 planets sharing a single orbit around a star be stable? Each planet would be in the L4 and L5 points of its neighbors.

My rule of thumb, if it doesn't happen in the wild, it's probably unstable. I don't think any cases of shared planetary orbits are known.

[Chronos]

For a Trojan situation to be stable, you need one planet to be significantly more massive than the other (IIRC, about a dozen times more). And of course the star needs to be significantly more massive than either. You might be able to make a hierarchical sextet, with one primary mass in the center, a secondary mass orbiting it, tertiary masses 60º ahead and behind it, quaternary masses ahead and behind those, and a quintiary mass opposite the secondary one... but I suspect that if you tried that, you'd find that the secondary still had a greater influence than the tertiaries, and it'd still end up unstable.

[Trafalgar]

Somewhat tangential, but would 6 planets sharing a single orbit around a star be stable? Each planet would be in the L4 and L5 points of its neighbors.

Would the six planets have the same mass? Because I think any difference would cause the system to fall apart.

[Lord Torath]

I'll just leave this here:
Octonary stars and 'Broken Pieces': Can the eight-star system from Star Trek: Picard actually exist?
Spoiler Alert: Maybe