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I'm imagining an impossibly massive, diverse world that could only fit on the inner surface of a Dyson Sphere/Ring or similar megastructure. Would it be possible to design a system such that from the habitable area, it roughly imitates a natural terran environment? I'm specifically looking for emulation of terrestrial features, as opposed to simple habitability.

Obviously the sun would be in a fixed position relative to the surface, but there could at least be an inner rotating filter to induce day/night cycles. But what about the conditions on the surface? How could you create a persistent atmosphere, climate, and geology that appears natural from inside? What might the view look like at day and night?

Assume that sufficient energy is harnessed from the star to sustain any passive artificial systems, but those should not be apparent from the habitable area. Also assume that its inhabitants are native, so nothing needs to be convincingly Earth or otherwise deceptive, just relatably similar results.

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    $\begingroup$ Related: Is A Solar Dyson Sphere Habitable? $\endgroup$ – DaaaahWhoosh Aug 19 '15 at 15:20
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    $\begingroup$ More than related, an essential prerequisite if this question is meant to add nuance and not be a duplicate. Those answers should be read first. $\endgroup$ – JDługosz Aug 19 '15 at 16:29
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    $\begingroup$ That question is specifically about temperature equilibrium in a closed system. $\endgroup$ – Travis Christian Aug 19 '15 at 16:32
  • $\begingroup$ If this concept interests you, I'm sure you'd find the book Bowl Of Heaven by Gregory Benford to be of interest! amazon.com/Bowl-Heaven-Gregory-Benford/dp/0765366460 $\endgroup$ – Dan Aug 19 '15 at 18:00
  • $\begingroup$ Bowl of Heaven was very disappointing. $\endgroup$ – JDługosz Aug 20 '15 at 1:12
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World Trees are massive, advanced Elder Tech biological constructs that replicate Dyson Sphere technology in a living framework. They replicate many of the existing properties of planets, but on a stellar scale.

Following is a short summary of simulated or replicated planetary systems.

Construction

Despite the word "tree", World Trees have only a cursory resemblance to Terran lifeforms. They make heavy use of Carbon Allotropes, creating much stronger and useful structures than out of wood. The primary support is graphene layers, grown in "branches", which are then covered by something analogous to a planetary crust. Viewed from a distance, World Trees appear as tightly woven webs, with only small gaps and openings. Note that these terms are relative - the smallest branch known is still over one million kilometers wide.

Other allotropes, such as lonsdaleite, are used where non-tensile strength is required. Use of non-carbon is minimized. The seed source of World Tree material is unknown, but is suspected to be harvested nebulae.

Gravity

World Trees spin, but at a rate which would provide only small fraction of planetary gravity. Through a currently not fully understood mechanism, World Trees convert and amplify this into a simulated gravity effect of roughly .78G.

Polar regions are left uninhabitable, with no gravity or atmosphere, but the majority of the World Tree maintains comfortable levels.

Atmosphere

Simulating biological processes, the World Tree regulates Oxygen, Nitrogen, and Carbon Dioxide on a massive level to maintain a fairly normal mixture (there is evidence that other World Trees exist for different biological requirements). This is supplemented by actual flora and fauna living on the interior of the tree.

Gravity, combined with walls on the branch sides, is used to contain atmosphere. Trace losses are replenishes by new material transmuted from the solar wind.

Geological

Over long periods of times, branches move together or apart, with some process to regulate crusts. This appears to be an attempt to simulate plate tectonics, and the end result is very similar.

Instead of subduction, when two branches merge the material from one is instead harvested and extruded elsewhere, to provide "fresh" crust material where needed.

Power & Day/Night cycle

Giant "leaves" extend out into the areas between branches. These harvest solar energy, as well as the solar wind for raw materials. A second set of leaves is used to create a day/night cycle, slowly closing over populated areas and then re-opening.

Excess power is provided by special "trees" that grow on the inside, extruding from the World Tree branches through the crust. Power is generally sufficient to maintain a Tier 1 civilization.

Reclamation & Pollutants

The World Tree actively filters its water systems and atmospheres to maintain the original levels of trace elements. Some storage is assumed, but excess trace elements are incorporated back into fresh crust material, making them - in some cases literally - gold mines of raw material.

Station Keeping

The World Tree manipulates its leaves to maintain position around the star. By trapping the wind in one direction but venting in the other, this produces a small but significant thrust that can be used to counter impacts or other accelerations.

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  • $\begingroup$ The tree is a spinning clump, or completely enclosing Dyson shell? $\endgroup$ – JDługosz Aug 19 '15 at 16:02
  • $\begingroup$ @JDługosz: Think close to a full shell, with only a few small gaps between branches. $\endgroup$ – Dan Smolinske Aug 19 '15 at 16:12
  • $\begingroup$ So the equator is like a ringworld. If terressed, could have floors into high latitudes, with lower angle of sun and higher spin-gravity. But as a complete shell, how do you keep it in position around the star, since it won't be orbiting and the star will not feel net gravity from the shell or vice-versa. $\endgroup$ – JDługosz Aug 19 '15 at 16:16
  • $\begingroup$ @JDługosz: I'll add a section on that, I'd forgotten to address it. My assumption is that it would use the solar wind normally as a station-keeping mechanism (capture on one side should provide thrust), with some sort of more powerful engine as a backup if necessary. $\endgroup$ – Dan Smolinske Aug 19 '15 at 16:19
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    $\begingroup$ How does this solve any of the problems of a traditional dyson sphere? It seems all you've done is make the sphere out of branches rather than some other material. $\endgroup$ – Samuel Aug 19 '15 at 16:44
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Actually, in theory if you built the inner filter correctly (using lenses instead of vacuum) you could simulate the sun moving across the sky. The sun will never go fully from horizon to horizon but you will see it track across the sky rather than always being noon.

For the terrain you can either build the surface features right into the underlying support layer and effectively fix the mountains and suchlike in place. Alternatively you can have a flat supporting layer and make mountains just by piling rocks on top of each other.

Either way would look the same from the surface, the difference would only become apparent if you started digging downwards.

Climate can self regulate in the same way it does here on earth, however there is a big complication with dyson spheres: There is no way to give them gravity.

You can't spin them, it's a sphere so you end up with a ringworld and masses of wasted space.

The mass of the sphere can't generate gravity - inside a sphere gravity cancels itself out.

So you need some sort of artificial gravity field to keep everyone in place. There are no alternative options really. That's the main thing that would need to be artificially regulated and maintained.

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    $\begingroup$ For gravity I was assuming rotation of either a wide ring or an equatorial band in a sphere, bounded by some kind of insurmountable "natural" barrier. That would still yield millions of times the Earth's surface area. $\endgroup$ – Travis Christian Aug 19 '15 at 16:27
  • $\begingroup$ Artificial gravity field still doesn't work, in general. It would need to fall off faster than the square, or be "aimed", neither of which gravity does no matter how it is generated. $\endgroup$ – JDługosz Aug 19 '15 at 16:37
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    $\begingroup$ @JDługosz Yes, it would need to be a localized artificial gravity effect - not a traditional gravity well. $\endgroup$ – Tim B Aug 19 '15 at 18:29
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    $\begingroup$ @TravisChristian Then 90% of your Dyson Sphere is wasted. Just build a ringworld, you'll use a fraction of the material and get the exact same living area. $\endgroup$ – Tim B Aug 19 '15 at 18:29
  • $\begingroup$ IOW, magic. Gravity as a spacetime stress tensor doesn't stay in a zone like a bad smell. Waving it away would undermine the "hard" in hard sf. $\endgroup$ – JDługosz Aug 20 '15 at 1:19
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The common idea of a Dyson Sphere is a rigid shell around the star. The inner surface will not have any gravity, so it cannot be planted with a natural habitat.

However, Dyson's actual idea was for a swarm of individual bodies that allow all light from the star to be capured. This has gravity if invidual bodies are compact and separated enough, but a worldlet is ot whst you want.

How about a ribbon of superdense material, larger than a conventional planet? Build on one side. Bulk of swarm is lightweight solar collectors so dense plate has net gravity.

You can have a roof made from transparent material several miles over the ground, to keep air in. Plate can be thicker in places to allow gravity to change from place to place. Roof can attenuate more or less light, and heat sinks can be added on the underside, so temperature can vary substantually as well.

Imagine a world like a scrap of paper among a dyson swarm, facing the sun. It is curved and undulating like a scrap of paper in the wind, which doesn't affect the gravity but does allow changing sun angles, as waves pass through the material the ground is tilted one way than the other. There may be decorative curlies on the ends which also support habitat.

The climate and gravity varies over large expanses, mto provide biomes from numerous host worlds and some imagined ones. If the air can remain partitioned, it could change from place to place as well.

Being superdense material,, you should not have too much of it. But it is very thin, so uses less matter than a normal planet.

more realistically

Such a civilisation will have moved beyond physical bodies. The whole reason for building such a thing is to harvest power for computing. So, the sphere is actually a Matroska Brain, running as a Minecraft server. The population lives "on the inside" in the deeper sense.

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