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Thread: Dyson spheres

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    Quote Originally Posted by Mastikator View Post
    Even better, make a space station that build the drones that harvest minerals from the asteroid belt.

    Step one is to make a moon base as a refueling station, if we can manufacture fuel from materials on the moon then we can make drones go anywhere.
    From there we send drones to the moon base and then send them to the asteroid belt, we'll need different kinds of drones. Some for harvesting, some for hauling, some for placing, and an ever expanding space station to smelt, forge and assemble more drones.

    When the entire belt is converted into drones we'll need to disassemble Ceres, then Mars and convert them into drones. From there we'll have enough resources to start on a swarm that eventually devours the entire solar system. In a few thousand years we'll have our Dyson swarm.
    Tens or hundreds of millions of years I'd say. Disassembling a planet is a different project than disassembling an asteroid, and exponential growth doesn't help you if the things you're growing can't actually do the job (or if they cost so much to do the job, each new unit is a net loss). There's the issue of actually getting the thing broken up into drone-sized pieces, first. Then the problem of the cost of retrieve materials from their own gravity well... Maybe if you could deorbit a moon or something to just within the Roche limit so the moon and planet tear each-other apart, then you just have to harvest from the neck between them...

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    Quote Originally Posted by Lord Torath View Post
    You can't really do a swarm, though, due to orbital mechanics - satellites always follow 'great circle' paths, and if you do those in different orientations at the same radial distance, you're going to get collisions. If you do them at different distances, then you get satellites obscuring each other, which greatly reduces the efficiency of your collection process.
    I could be mistaken, but I was under the impression that the idea of a Dyson swarm, or at least one of the more practical means of implementing one, is that your satellites aren't so much orbiting the star as using solar sails and radiation pressure to counterbalance the star's gravity and maintain a constant position relative to it.

    Quote Originally Posted by Batcathat View Post
    On the topic of gravity, would it be (at least theoretically) possible to accomplish it by having the sphere itself be thick enough to generate gravity in the right direction (and assuming the sun is far enough away not to pull everything in the opposite direction)?
    Not really; the net gravitational force exerted by a spherical shell of uniform density on anything inside of the shell is zero, and while your Dyson sphere might not be either perfectly spherical or have completely-uniform mass distribution it's unlikely that there'd be enough deviation from the idealized model to matter, especially without causing your sphere any more problems than such a megastructure already has.

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    Quote Originally Posted by Anymage View Post
    My guess is both. The idea of dome cities was probably the most sensible plan back during the starry-eyed days when people thought that space habitats were right around the corner. As time went on geodesic dome = futuristic space habitat just became a part of the public consciousness even if scientists nowadays consider them impractical.
    A lot of the places where we would put an extraterrestrial settlement have issues with high winds (Mars) or precipitation of nasty stuff (Titan). Domes are a useful solution for that kind of environment.
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    Quote Originally Posted by Batcathat View Post
    Of course, I just meant that if people are gonna live on it, the massive amount of space seems like the only upside compared to just living on planets.
    Eh, you could cluster the living spaces together. Say, an inhabited band along the equator of the sphere, and the rest is covered in solar cells and giant microwave arrays.
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    Quote Originally Posted by Mechalich View Post
    A lot of the places where we would put an extraterrestrial settlement have issues with high winds (Mars) or precipitation of nasty stuff (Titan). Domes are a useful solution for that kind of environment.
    Wind is the least of your problem on Mars. (Long after the cold, the radiation, the toxic soil, the dust and the lack of air). The air is actually so thin you wouldn't even feel most of the storms, and they couldn't really blow anything over.

    Realistically, Mars settlements would be pretty far underground anyway, for radiation shielding.
    Last edited by Eldan; 2023-11-16 at 05:47 AM.
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    Quote Originally Posted by Eldan View Post
    Wind is the least of your problem on Mars. (Long after the cold, the radiation, the toxic soil, the dust and the lack of air). The air is actually so thin you wouldn't even feel most of the storms, and they couldn't really blow anything over.

    Realistically, Mars settlements would be pretty far underground anyway, for radiation shielding.
    The wind blows that toxic soil around as dust. A dome would keep it from getting all over everything, which is highly significant. A dome could also potentially provide significant radiation shielding, depending on what it was made from while at the same time providing greenhouse heating to everything inside.

    Sure, underground is the best bet, and it doesn't need to be far, even a very limited amount of regolith, like a meter or two, provides tons of radiation shielding on top of whatever your buildings are made from, but it would probably still be useful to put a dome over the top of the settlement anyway, to keep dust off monitoring equipment, greenhouses, vehicles, and anything else it's useful to keep on the surface.
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    How would a dome protect from dust? Surely, the dust will just stick to a dome, too.

    Edit: Oh, right. Things inside the dome are protected, of course.

    Could still have it not dome shaped, though.
    Last edited by Eldan; 2023-11-16 at 06:26 AM.
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    Quote Originally Posted by Aeson View Post
    Not really; the net gravitational force exerted by a spherical shell of uniform density on anything inside of the shell is zero, and while your Dyson sphere might not be either perfectly spherical or have completely-uniform mass distribution it's unlikely that there'd be enough deviation from the idealized model to matter, especially without causing your sphere any more problems than such a megastructure already has.
    Just to follow up on this a bit, if you want to live on the outside of the sphere (or with the sun in the 'down' direction), then there's no problem. Well, sort of. See below. You can treat the mass of the sphere and the mass of the sun as point masses to determine the gravitational force you'd feel walking on the sphere.

    The acceleration you feel due to gravity is G*M/r2

    G is the universal gravitational constant: 6.673×10−11Nm2/kg2
    Since a newton (N) is 1 kg m/s2, that's also 6.673×10−11m3/(kg s2)
    M is the combined mass of the sun and the dyson sphere (assuming it is concentric around the star) in kg
    r is the radius of the dyson sphere in meters

    On earth it's 9.807 m/s2.

    To get that same acceleration on the outside of a dyson sphere at 1 AU (150 million km or 1.5x1011m) you need a sphere mass of 3.3x1033 kg. The sun's mass is only 1.99x1030 kg, so you'd need 1,700 solar masses of material for your dyson sphere.

    If we assume 1 solar mass for the dyson sphere, that means you need a radius of 0.0347 AU or about 5.2 million km. Mercury's orbit varies between 46 and 70 million km from the sun, so this is very close in.

    If you use only the mass of Jupiter for your sphere, the radius shrinks to 3.7 million km if you want Earth-like gravity. I'm not certain what temperature your sphere will be at that distance, but I'm going to go with 'uncomfortably' hot, and probably in the range of 'reduced structural integrity' hot.
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    Quote Originally Posted by Mechalich View Post
    A lot of the places where we would put an extraterrestrial settlement have issues with high winds (Mars) or precipitation of nasty stuff (Titan). Domes are a useful solution for that kind of environment.
    Yeah but why a dome specifically? The buildings in Florida aren't domes and they get hurricanes constantly, so whatever geometric advantage there is apparently isn't worth it. Why wouldn't they just be building shaped?

    Quote Originally Posted by Aeson View Post
    Not really; the net gravitational force exerted by a spherical shell of uniform density on anything inside of the shell is zero
    I think that's only true at the exact center. Anywhere else you're going to feel proportionally more gravity from the part of the shell that's closest to you

    Quote Originally Posted by Eldan View Post
    Realistically, Mars settlements would be pretty far underground anyway, for radiation shielding.
    That strikes me as probably a lot more expensive than putting up a building, even a radiation shielded one.

    Maybe that's the point of the dome shape; reducing the needed radiation shielding material. That still doesn't completely work though because the round shape is going to leave you with a lot of impractical to use areas; it would save even more material to just square those areas off
    Last edited by Bohandas; 2023-11-16 at 11:40 AM.
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    Quote Originally Posted by Bohandas View Post
    Yeah but why a dome specifically? The buildings in Florida aren't domes and they get hurricanes constantly, so whatever geometric advantage there is apparently isn't worth it. Why wouldn't they just be building shaped?
    Are we still talking about the classic domed sci-fi city or just dome-shaped buildings? If it's the former, I'm assuming the point of having a dome instead of "just" buildings is to have an outside of sorts.

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    Dome shaped buildings. generally connected by tubes.
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    All this time and no one has talked about Buuthandi?

    So, Schlock Mercenary has a species that has moved its population of several trillion to several stellar enclosures; instead of a rigid Dyson sphere, it's essentially a balloon, with the ability to somewhat shift to accommodate the moods of the star. The population lives in stations that dangle from the sail.

    Now, the world of Schlock Mercenary has a few things that make this more viable, with artificial gravity being one of them (though the F'sherl-Ganni have also adapted themselves to microgravity to some extent), and teleportation, but if you don't need gravity, you can get a LOT of living space by dangling cities from your stellar balloon.
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    What happens to the heliosphere of a Dyson'd star?
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    Quote Originally Posted by Imbalance View Post
    What happens to the heliosphere of a Dyson'd star?
    As I understand it, you have to have a way to regulate it, or use the plasma. A Buuthandi can manage a bit better, since it can change shape, but a Dyson sphere would have to have collectors or something to prevent damage.
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    Quote Originally Posted by Aeson View Post
    ...the net gravitational force exerted by a spherical shell of uniform density on anything inside of the shell is zero,....
    Quote Originally Posted by Bohandas View Post
    I think that's only true at the exact center. Anywhere else you're going to feel proportionally more gravity from the part of the shell that's closest to you
    That's a standard calculus-level physics problem. It turns out, with the inverse-square law, while the part in one direction may be closer, the part in the other direction is larger, exactly enough to balance things. But don't take my word for it. Look it up.

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    Quote Originally Posted by DavidSh View Post
    That's a standard calculus-level physics problem. It turns out, with the inverse-square law, while the part in one direction may be closer, the part in the other direction is larger, exactly enough to balance things. But don't take my word for it. Look it up.
    The next question (ties into points made above) is if the gravitational effect of a hollow sphere (or say a really large hollow sphere with a star located in the center of mass within it) on someone standing on its outside surface is the same as if standing on a similarly massed solid object. My back of my napkin/off-the-top-of-my-head says "yes" (for much the same reasons).

    Er... I'm still having trouble seeing the whole "biome on the outside of the sphere" bit working though. Geez. The slightest misalignment or out of balance mass on the whole would cause things to slide to one side or the other (how "deep" would the atmosphere be releatively speaking?). I'm just seeing any sort of "wobble" in the system being absolutely catastrophic for anyone living inside. Maybe if you had sealed in areas with tall walls to keep the air in (well of souls like?). Could also have lots of different environments in such a setup as well. Hmm... Also think you'd have a heat problem (couple of them, actually).


    I think the main point here is that, at least as originally envisioned, the purpose of a Dyson sphere was not about creating living space (though you could in theory with sufficiently advanced tech). It's about capturing the energy output of the star. If you had the technology to even approach that needed to build an actual solid Dyson Sphere (which is almost ridiculous anyway), you should be able to trivially build a host of other artificial environments in a variety of configurations that would meet the same requirements and not be tied to the sphere itself. Use the sphere to collect energy and beam it to thousands of orbiting stations would work just fine too. There's just no reason at all to physically connect the living areas to the sphere itself, and doing so actually introduces additional problems IMO.

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    If the star is just a big fusion reactor, couldn't a sufficiently advanced society just harvest its material to make numerous smaller fusion reactors?

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    Quote Originally Posted by Maat Mons View Post
    If the star is just a big fusion reactor, couldn't a sufficiently advanced society just harvest its material to make numerous smaller fusion reactors?
    Making a scoop that can handle stellar temperatures will be a hassle. As will be lifting the hydrogen out of the star's gravity well. Plus it takes a lot of work to get smaller amounts of hydrogen to fuse, while stellar masses creating stellar pressures do that naturally and for free. I'm not seeing how any of that is easier than just having satellites to convert the energy into more usable forms.

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    I wasn't meaning to suggest it would be easier than building satellites to collect solar energy, which seems feasible. I was meaning to suggest it would be easier than building a complete rigid sphere encompassing a star, which seems borderline impossible.

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    Quote Originally Posted by gbaji View Post
    The next question (ties into points made above) is if the gravitational effect of a hollow sphere (or say a really large hollow sphere with a star located in the center of mass within it) on someone standing on its outside surface is the same as if standing on a similarly massed solid object. My back of my napkin/off-the-top-of-my-head says "yes" (for much the same reasons).
    Surface gravity is GM/R2 for idealized spherical masses, so the physical "size" of the object you're dealing with matters quite a bit; a solid object with the same mass as a hollow sphere will only have a similar surface gravity to that of a hollow sphere if both the solid object and the hollow sphere have similar radii. The surface gravity of the Sun is ~28g; a shell of negligible mass with a radius of ~3.7 million kilometers (~5.3 solar radii or ~0.025 AU) would have approximately Earth-normal surface gravity if centered on the Sun; a shell of negligible mass with a radius of 1 AU (~1.5e8 km) likewise centered on the Sun would have a surface gravity of about 0.0006g.

    Also, since we're talking implausible mega-structures here, it perhaps bears mentioning that the shape of the object matters. I don't know what the surface gravity of, say, a solar-mass dumbbell is off the top of my head, but I'm pretty certain it's neither equivalent to that of a spherical object of similar mass nor uniform over the surface of the dumbbell.
    Last edited by Aeson; 2023-11-17 at 01:49 AM.

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    Quote Originally Posted by Aeson View Post
    Also, since we're talking implausible mega-structures here, it perhaps bears mentioning that the shape of the object matters. I don't know what the surface gravity of, say, a solar-mass dumbbell is off the top of my head, but I'm pretty certain it's neither equivalent to that of a spherical object of similar mass nor uniform over the surface of the dumbbell.
    Assuming a very advanced civilization did want/need to build an artificial habitat on the same scale as the Earth or bigger, does anyone know what the ideal shape would be? Making some sort of fake planet seems like the obvious choice, but maybe there's a better one?

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    Personally it seems to me that Dyson spheres come after artificial gravity, which is to say probably a very long time in the future, and maybe not at all.

    Quote Originally Posted by Batcathat View Post
    Assuming a very advanced civilization did want/need to build an artificial habitat on the same scale as the Earth or bigger, does anyone know what the ideal shape would be? Making some sort of fake planet seems like the obvious choice, but maybe there's a better one?
    I don't like planets as a habitat at all, I'd much rather have houses and hotels in space than on the surface of Mars or Vernus.
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    Quote Originally Posted by Batcathat View Post
    Assuming a very advanced civilization did want/need to build an artificial habitat on the same scale as the Earth or bigger, does anyone know what the ideal shape would be? Making some sort of fake planet seems like the obvious choice, but maybe there's a better one?
    A planet is a terrible choice. The we, here on Earth, are only using around the top ~20 km or so of a object with a radius of 6371. That's really, really bad. You want to maximize surface area to produce as much space as possible with the least amount of matter.

    A hollow sphere does this, but that creates the problem of gravity. Since we don't have - and physics strongly suggests we'll never get - artificial gravity, you need an object that can be spun for gravity, but if you do that with a sphere, 'gravity' is going to be different depending on how far you are from the equator, which is not helpful. Instead you want either a ring or a cylinder design (which are ultimately the same thing, just a matter of which axis is being emphasized). And you want it to be as big as possible both to reduce the impact on centripetal acceleration on everyone living inside, provide the maximum of amount of shielding it can, and take advantage of certain scaling effects to hold in the atmosphere.

    The classic Niven Ringworld design delivers all these things, but unfortunately it's simply too big. The forces involved in the thing's rotation would rip apart known form of matter held together by chemical bonds, which is why Niven made up a material called Scrith that was impossibly strong (or possibly nuclear matter) to hold the Ringworld together. A Bank's Orbital, common in the Culture Universe, is basically a small version of a Ringworld but instead of being wrapped around a star its a big hoop that orbits the star while spinning on its own. note that small is very relative here. The average Bank's Orbital has a circumference of ~10 million km and a width ~3000 kilometers, giving one 30 billion sq km of surface area, or roughly 60x that of Earth. This is still sufficiently large that super-strong exotic matter with properties heretofore unknown to science must be postulated to allow one to exist, though it's orders of magnitude less ridiculous than a full Niven Ring.

    A Bishop Ring or McKendree Cylinder is a significantly smaller habitat design (note that the Halo rings, from the eponymous game series, are basically Bishop Rings with funny proportions) that is within the threshold of known materials technologies, primarily advanced carbon nanotube constructions. A McKendree Cylinder can, in theory, be 1000 km in radius and 10,000 km in length while maintaining a 1g surface 'gravity', giving it roughly 10% of the surface area of Earth, but if you chopped Earth up to make such cylinders, you could probably make thousands of them. Hundreds of millions of people could easily live in such a habitat in comfort, so the goal, if you really needed space, would be to build hundreds of them by sacrificing unimportant objects with a high carbon content like 10 Hygiea.

    Quote Originally Posted by halfeye
    Personally it seems to me that Dyson spheres come after artificial gravity, which is to say probably a very long time in the future, and maybe not at all.
    While a full sphere/swarm is impractical, aspects of a swarm can be taken into consideration. For example, if a civilization did build a few hundred 1000 km radius space habitats it would be useful to both coat their exteriors in solar panels on whatever side faces the star and to have additional energy gathering units positioned throughout the system for both backup power, shielding (putting your habitat behind a big solar collector helps it you're worried about flares and stuff), and to power any sort of big interstellar project like a solar sail based starship or a giant ball of computronium full of digital people.

    Space-based solar energy collection is even a potentially viable medium-term technology, especially if fusion doesn't work out.
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    Would it be possible to convert an entire planet into a huge many-storied spherical building several times the planet's natural size like in the sci-fi comedy Let's Visit the World of the Future? Or would something like that just collapse back to its original size in very short order?
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    Just using the materials the planet is already made out of? Almost certainly not. We already see what a planet-sized mass of planet material collapses into, it's a planet.
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    While you couldn't increase its size, you could increase its habitable area. If you have a planet or planetoid that's relatively volcanically inactive, you could hollow out its crust. There was even a sci-fi series that had folks living inside an essentially hollow Mars-sized planet, with a big fusion candle in the middle to serve as a sun (and smaller candles for richer habitats).
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    Quote Originally Posted by Bohandas View Post
    Would it be possible to convert an entire planet into a huge many-storied spherical building several times the planet's natural size like in the sci-fi comedy Let's Visit the World of the Future? Or would something like that just collapse back to its original size in very short order?
    Assuming sufficiently-advanced elemental transmutation and super-materials, it's at least plausibly possible, but it likely wouldn't be practical, especially if you're relying on natural gravity, and if your structure is large enough you might even run into issues with excessive air pressure on the inner/"lower" levels while also having generally the same temperature regulation and food/water supply issues and so on that plague more conventional ecumenopoleis.

    Quote Originally Posted by Eldan View Post
    Just using the materials the planet is already made out of? Almost certainly not. We already see what a planet-sized mass of planet material collapses into, it's a planet.
    You could, however, plausibly cover its surface in structures (or maybe one massive structure) ~150 stories tall using present-day materials - assuming, at any rate, that you have enough of those materials to work with and that the surface you're building on is or can be made strong enough to support such structures - and maybe get a few more levels carving into the crust.

    Granted, you may run into other issues doing that - structural limits aren't exactly the only, or largest, concern when it comes to ecumenopoleis.

    Quote Originally Posted by LibraryOgre View Post
    While you couldn't increase its size, you could increase its habitable area. If you have a planet or planetoid that's relatively volcanically inactive, you could hollow out its crust. There was even a sci-fi series that had folks living inside an essentially hollow Mars-sized planet, with a big fusion candle in the middle to serve as a sun (and smaller candles for richer habitats).
    Extending the habitable zone by tunneling into the crust is feasible within limits, but I don't see what practical benefit a fusion lamp in the center of the planet would bring - your artificial sun's under the floor, probably separated from you by several thousand miles of the stuff that makes up the mantle and core of the planet because you can't remove that without losing the mass that gives your planet its surface gravity and holds its atmosphere in place.

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    Default Re: Dyson spheres

    Quote Originally Posted by Aeson View Post
    Extending the habitable zone by tunneling into the crust is feasible within limits, but I don't see what practical benefit a fusion lamp in the center of the planet would bring - your artificial sun's under the floor, probably separated from you by several thousand miles of the stuff that makes up the mantle and core of the planet because you can't remove that without losing the mass that gives your planet its surface gravity and holds its atmosphere in place.
    Ah, that's because they weren't tunnelling.

    The interior was air-filled. The whole thing was microgravity, inside a planet-sized open atmosphere, though smaller than Schlock Mercenary's "Can full of sky", and without the same level of central power.

    The "tunnel into the planet" idea is separate.
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  29. - Top - End - #59
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    Default Re: Dyson spheres

    All of the calculations for gravity of spherically-symmetric mass distributions become a lot easier when you realize that Gauss's Law works just as well for gravity as it does for electromagnetism. Pick any arbitrary point inside of a spherical shell, and draw a spherical Gaussian surface at that radius. The mass enclosed is zero, and so the net flux of gravitational field through the surface is zero. Since the structure is spherically symmetric, the magnitude of gravitational field everywhere on that surface must be the same, and so it's zero everywhere.

    Similarly, if you're outside the sphere, all that matters for determining the gravitational field is the total amount of mass inside of you, and the fact that it's spherically symmetric, so the gravity outside of a shell is the same at the same point as the gravity outside of a concentrated star, or even a point mass.

    Gauss's Law can also be used to show that with a massive object shaped like a ring, gravity everywhere on the surface of that object always points towards the surface of the object. So even if it's not spinning, you could walk around on the inner surface of a ring.

    Oh, and we can also use Newton's Third Law. Put a mass anywhere inside of a symmetric spherical shell, even someplace off-center, and the net force on it is zero. Turn that around, and you see that, if you have an off-center spherical shell around a star, then the net force on the shell from the star is also zero. A complete solid sphere is neither stable, like a marble in the bottom of a bowl, nor unstable, like a marble on top of an upside-down bowl. It's neutrally stable, like a marble sitting on a flat tabletop.
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  30. - Top - End - #60
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    Default Re: Dyson spheres

    Quote Originally Posted by LibraryOgre View Post
    Ah, that's because they weren't tunnelling.

    The interior was air-filled. The whole thing was microgravity, inside a planet-sized open atmosphere, though smaller than Schlock Mercenary's "Can full of sky", and without the same level of central power.

    The "tunnel into the planet" idea is separate.
    Probably requires incredible structure strength to keep it from collapsing into a more compact body, but I can't be certain without a more detailed description. It's more plausible than the Can Full of Sky, which had both vaguely Earth-like levels of (rotational pseudo-) gravity and noticeable atmospheric pressure at a much greater range of altitudes than I believe possible.

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