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The year is 3030 and the robots (who have replaced humanity as the dominant intelligent beings on Earth) have decided to build a Dyson sphere around the sun. They're planning on disassembling Venus for materials and building a sphere with a radius a bit less than the orbital radius of the planet they're consuming.

The sphere is opaque to all solar radiation, barring a few high-energy gamma rays that manage to punch through, and also blocks the solar wind. It's thin, so it will reach thermal equilibrium fairly quickly and radiate energy as a blackbody based on its equilibrium temperature.

Earth, prior to the construction of the sphere, has a climate similar to that of modern day Earth. Neither nuclear war nor pollution has catastrophically damaged the biosphere. The Earth of 3030 is about 3°C warmer than it is today and is home to a stable population of 11 billion humans.

How uninhabitable will the construction of the Dyson sphere render the Earth? It will block out all sunlight, but should ultimately radiate the same amount of energy it's absorbing, effectively replacing the Sun with a much larger, cooler star. What life on Earth would we expect to survive in the post-sphere era?

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    $\begingroup$ Surface area of sphere: 0.616 au^2 volume of Venus: .0000000000002773 au^3 Thickness of completed sphere: 4.501623376623376623376623377E-13 au =~ 2.65 inches -- Apogee of Venus: .771au Perigee of Haley's comet: .586au - I forsee problems for your sphere sometime in 2061... (depending on how far "inside" Venus' orbit it is) If not before... -- solar wind pressure: 100 nPa (at Earth, will be higher at Venus) * surface area of sphere = 1.379×10^15 kg m/s^2 (kilogram meters per second squared) Going to need to transmute all that rock into something stronger I think... $\endgroup$ – Perkins Feb 14 '17 at 22:28
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    $\begingroup$ @Perkins The robots have designed there sphere significantly thinner than that, so that the pressure from the solar wind plus solar radiation exactly counteracts gravity. The excess materials were used to build a system of satellites containing power conversion and manufacturing facilities, as well as huge banks of lasers to protect the sphere against comets and asteroids. It probably won't face any problems in 2061, though, since that's about a millennium before its slated for construction. $\endgroup$ – ckersch Feb 14 '17 at 23:06
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    $\begingroup$ This is your mandatory "Rigid Dyson Spheres are not stable" comment. $\endgroup$ – Schwern Feb 15 '17 at 0:11
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    $\begingroup$ What is the point of building a Dyson sphere if you have the energy to casually disassemble Venus? $\endgroup$ – Zxyrra Feb 15 '17 at 2:13
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    $\begingroup$ @Zxyrra More energy. $\endgroup$ – Jens Feb 15 '17 at 10:35
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Life on Earth will suffer severely. Instead of the energy coming mostly in visible spectrum, now it will be coming in the far infrared part of spectrum. Skies will be dark - no Sun, no Moon, no planets, except for the stars. Because Earth's atmosphere is much more opaque to far infrared radiation, temperatures will plunge. The vast majority of living organisms will perish. Some might adapt to sustain on far infrared light, but this would be less concentrated energy than, for example, that which a hot geyser produces. I think the only remaining organisms will survive on geothermal energy and chemicals.

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    $\begingroup$ Far IR doesn't seem like as far a fall as I would have thought, also I would think higher opacity would tend to raise absorption and so temperature. I thought I read that was what global warming is all about. $\endgroup$ – user25818 Feb 14 '17 at 21:44
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    $\begingroup$ @notstoreboughtdirt Global warming (specifically radiative forcing) is caused by the atmosphere being opaque to infrared, but transparent to UV light. The sun emits more in the visible/UV spectrum (since it's hotter) so solar energy penetrates the atmosphere, eventually being absorbed by the ground. The ground eventually re-radiates this energy as infrared radiation, and that re-radiated energy can't easily escape due to the opacity of the atmosphere at those wavelengths. With incoming infrared radiation, more would be reflected by the atmosphere without reaching the surface. $\endgroup$ – ckersch Feb 14 '17 at 21:53
  • $\begingroup$ @notstoreboughtdirt It's not that simple. While the radiation is absorbed, that isn't a stable state - the energy will mostly be re-radiated long before it heats the ground "directly". And that's the crux of both this and the so-called greenhouse effect - the re-radiated energy goes in a random direction, so you've got about 50% chance it would go out into space, and 50% it would go back to the ground, on every absorption-reradiation. So what this would cause would be the top layers of the atmosphere being warm, while the bottom would be a frozen hellhole - and no convective heat transfer. $\endgroup$ – Luaan Feb 15 '17 at 8:49
  • $\begingroup$ @Luaan this would make for an amazing world for airships and sky-cities $\endgroup$ – dot_Sp0T Feb 15 '17 at 11:23
  • $\begingroup$ @dot_Sp0T It might be a bit tricky to make the warm part habitable, but just thinking about it I got a dozen story ideas in a world like that :D It certainly sounds like a great setting - death above, death below, only a relatively thin (a few kilometers?) strip of habitable and "floaty" air... if you drop too low, you'll never get back again, and yet you have to get materials from the ground deep below once in a while... Huge floating islands made of renewable materials, captured hydrogen, nitrogen, carbon... It might be worth the effort :P $\endgroup$ – Luaan Feb 15 '17 at 12:42
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Frank Cedano's answer means that the robots building the Dyson sphere can use a tiny proportion of the energy it collects to power a giant space sun lamp to illuminate the Earth (if they want to).

If the 11 billion humans on planet Earth also have advanced technology in the year 3030 they can build giant space fusion power generators with a total power output equal to that which Earth receives from the Sun and use the power in an orbital space lamp to illuminate the Earth. Presumably they already use fusion power plants on Earth to power their civilization anyway.

Or the humans can use infrared sensitive Sun orbiting solar panels (orbiting outside the Dyson sphere) to collect some of the waste infrared emitted by the Dyson sphere which the robots have no use for. The solar panels will convert the infrared to maser or laser frequencies and beam them to Sun orbiting power receivers (orbiting outside the Dyson sphere) that will beam the power to a giant orbiting sun lamp orbiting Earth.

If the Dyson sphere or shell stops the solar wind the humans may also have to build some sort of defense against cosmic rays that will now penetrate the inner solar system in greater numbers.

Presumably both the robots and the Humans will have highly advanced technology in 3030. So if the robots start a vast and long project that might have a negative impact on Earth the humans will have a long time to persuade the robots to modify the project to keep Earth habitable, or fight a space war with the robots, or make their preparations to keep Earth habitable, or build and move to space habitats using fusion power for energy.

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    $\begingroup$ if the dyson sphere rotates just right, they could simply leave a small hole big enough to just illuminate the Earth (and possibly the moon, although it would be interesting if they did not) $\endgroup$ – Michael Feb 14 '17 at 21:16
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    $\begingroup$ The robots only disassemble Venus, not Earth, even though they could have gotten more building materials from Earth. This shows they care at least a little about humanity's survival, so they can be reasoned with. $\endgroup$ – vsz Feb 14 '17 at 21:34
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    $\begingroup$ @vsz And the trade-off they need is absolutely tiny - Earth only captures a tinsy-weensy fraction of the Sun's output anyway. Just put a few windows on the thing, and we would barely notice, apart from the starlight being blocked - but then again, you don't look at stars in the daylight all that often, do you? :D You could even easily see how late it is, based on how much of the sphere was visible at night (or, if it's cold/dark enough, how many stars it occludes). $\endgroup$ – Luaan Feb 15 '17 at 8:54
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    $\begingroup$ @Luaan: Let's fake it! Put tiny luminescent dots on the Dyson sphere so humans can "look at the stars". $\endgroup$ – Matthieu M. Feb 15 '17 at 11:44
  • $\begingroup$ If there are robots on Venus by 3030, then surely there are humans on Mars. If you're going to put a spotlight or hole in the structure or some other solution to illuminate Earth, then you'll need to do the same for Mars, and any other human outpost in the system. $\endgroup$ – Simba Feb 15 '17 at 15:24
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Have your robot's build a dyson yo-yo instead.

Imagine two Venus orbit wide hemispheres encapsulating the sun but leaving a wide swath of open space between them. Span the open space with rigid cables to hold each hemisphere in proper relation to each other and their captured sun.

Now align the open space with the orbital plane of Earth and adjust it as needed with hemisphere mounted rockets.

Your robots get a tonne of new real-estate while Earth gets to keep its place in the sunshine.

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Not an answer, but I'd like to correct:

effectively replacing the Sun with a much larger, cooler star

The amount of solar-derived radiation reaching the Earth should be unchanged. Consider the surface of the Dyson structure occluding the Sun: call it the Dyson-Sun. It will absorb the same amount of radiation as the Earth currently does, and then re-emit it in the Earth's general direction.

What about thermal conduction? Every other part of the Dyson structure will be absorbing solar radiation at the same rate per surface area, so the temperature should be relatively uniform.

What about scattering? Some of the re-emitted radiation from the Dyson-Sun may not be directed towards the Earth; the remaining surface area of the Dyson structure will also be re-emitting scattered radiation, some in the direction of the Earth.

Some radiation would be scattered back towards the Sun, but as the Dyson structure has turned it into a closed system, eventually it will be absorbed and re-emitted out away from the Sun.

I would expect that the future inhabitants, comparing the old Sun to the Dyson-Sun, would perceive the latter to be hotter (since more EM radiation would be shifted to infra-red), darker, slightly bigger (because of the scattering), and blurrier.

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  • $\begingroup$ Welcome to Worldbuilding, but I disagree. If the sphere is built to absorb and convert radiation, I doubt it will be this inefficient; light will inevitably be lost. Yes, temperature will be uniform, but that doesn't mean temperature won't be very hot - the sphere can continue to warm, removing energy that would otherwise escape the system. $\endgroup$ – Zxyrra Feb 15 '17 at 2:29
  • $\begingroup$ @Zxyrra one could calculate the temperature of the sphere based on its surface area. It will radiate the same amount of energy as the sun, at this lower temperature of black-body radiation, from a larger apparent source in the sky. $\endgroup$ – JDługosz Feb 15 '17 at 7:38
  • $\begingroup$ @Zxyrra I am assuming the structure is 100% efficient at converting solar radiation to infrared. I said that the Dyson-Sun would be perceived as hotter than the Sun. $\endgroup$ – Daniel Holz Feb 15 '17 at 14:27
  • $\begingroup$ "Slightly bigger"? I figure something approaching 180 times bigger, by linear dimension, or 32400 times bigger, by angular area. Wouldn't it be perceived as a disk having an angular diameter approaching 90 degrees of the sky? (The elongation of Venus's orbit, as perceived from Earth, is roughly 45 degrees from the Sun.) $\endgroup$ – Beanluc Feb 15 '17 at 18:32
  • $\begingroup$ @Beanluc the structure, yes. It would occlude a vast fraction of the sky, but the majority of its radiation would not be directed towards the Earth, so would not be significantly visible to the eye: consider the sky (which scatters radiation from the Sun) & how we perceive it as separate and cooler than the Earth, even though a patch of sky that appears cool to us would appear to be extremely hot to another observer who perceives the sun to coincide with that particular patch. $\endgroup$ – Daniel Holz Feb 15 '17 at 21:56
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As long as there is a radiator orbiting the earth that puts out the same energy the sun deposited on earth then nothing would change. This radiator would have to be big enough to radiate energy on the same surface area as well as put out the same electromagnetic frequencies. Since the earth consumes only a small part of the energy of the sun now, you can certainly release that much energy easily from the dyson sphere.

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    $\begingroup$ See Alexander's answer. With a bigger radiating area, power power per unit area at the radiating surface goes down, and peak wavelength increases. So your answer is very, very wrong. $\endgroup$ – WhatRoughBeast Feb 14 '17 at 18:43
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    $\begingroup$ @WhatRoughBeast I'm reading this answer as essentially "it would be trivial for Dyson sphere builders to spare an insignificant fraction of the energy and construct it in some particular way so that conditions on Earth don't change". On the other hand, it would also be trivial for them to ensure that Earth gets destroyed - it all depends on what they wish. $\endgroup$ – Peteris Feb 14 '17 at 19:42
  • $\begingroup$ "As long as there is a radiator orbiting the earth that puts out the same energy the sun deposited on earth...". Close. It would have to be radiating in the same wavelengths as the Sun. And it wouldn't orbit the Earth, unless the Earth stopped spinning. It would have to be at the L1 point between the Earth and the Dyson Sphere (ie. directly in line with where the Sun was). This would keep it in the same position in the sky as the Sun was as the Earth revolves, and it would keep up with the Earth as it orbits the Dyson Sphere. Bonus points if it has the same apparent size in the sky. $\endgroup$ – Schwern Feb 15 '17 at 0:06
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If we assume continued industrial growth and not an energy stall, the limiting factor of planet-based civilizations energy consumption in the near future is radiating waste heat away.

A Dyson Sphere surrounding the sun would shut off visible light and replace it with infrared. This would, as others have noted, significantly reduce the surface temperature of Earth, probably to ice-ball levels, and wipe out the non-geological based biosphere.

However, if the level of technology required to disassemble a planet and build such a sphere is a K type 2 civilization. Managing the entire energy budget of Earth is a trivial task to a K type 2 civilization; it is literally 0.00000001% (give or take a 0, I may have lost count) of their energy budget.

If Earth is anywhere near at that technology level, they will require significant cooling efforts to deal with the pre-solar-shutdown; possibly the reduced radiation from the sun would be a boon and allow more technological development and energy use on Earth.

At this level, the biosphere will have to be heavily managed regardless of the sun being shut off.

As an aside, note that the "right" way to capture entropy (well, enthalpy maybe) from the sun is to not only have 1 layer; you want to harness the heat emitted by the first layer and use it for more work before radiating it again. So stopping at one layer isn't long term.

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