The most realistic design for a Dyson sphere is that of a Dyson swarm ([32, 33]): a collection of independent solar captors in orbit around the sun. The design has some drawbacks, requiring careful coordination to keep the captors from colliding with each other, issues with captors occluding each other, and having difficulties capturing all the solar energy at any given time. But these are not major difficulties: there already exist reasonable orbit designs (e.g. [34]), and the captors will have large energy reserves to power any minor course corrections. The lack of perfect efficiency isn’t an issue either, with 3.8×1026 W available. And the advantages of Dyson swarms are important: they don’t require strong construction, as they will not be subject to major internal forces, and can thus be made with little and conventional material.
The lightest design would be to have very large lightweight mirrors concentrating solar radiation down on focal points, where it would be transformed into useful work (and possibly beamed across space for use elsewhere). The focal point would most likely some sort of heat engine, possibly combined with solar cells (to extract work from the low entropy solar radiation).
The planets provide the largest source of material for the construction of such a Dyson swarm. The easiest design would be to use Mercury as the source of material, and to construct the Dyson swarm at approximately the same distance from the sun. A sphere around the sun of radius equal to the semi-major axis of Mercury’s orbit (5.79×1010 m) would have an area of about 4.21×1022 m^2.
Mercury itself is mainly composed of 30% silicate and 70% metal [35], mainly iron or iron oxides [36], so these would be the most used material for the swarm. The mass of Mercury is 3.3022×1023 kg; assuming 50% of this mass could be transformed into reflective surfaces (with the remaining material made into heat engines/solar cells or simply discarded), and that these would be placed in orbit at around the semi-major axis of Mercury’s orbit, the reflective pieces would have a mass of:
0.5×3.3022×10234.21×1022=3.92 kg/m2.
Iron has a density of 7874 kg/m^3, so this would correspond to a thickness of 0.5 mm, which is ample. The most likely structure is a very thin film (of order 0.001 mm) supported by a network of more rigid struts.
They go on to estimate how long it’d take to construct, but the punchline is 31 years and 85 days.
A thing I’m still not sure about reading that is “what percent of the light is getting through?”. Like, how dense are the reflector modules?
Later in the paper it says “The Dyson sphere is assumed to have an efficiency of one third”, which could mean “realistically you only capture about 1/3rd of the energy in the first place” or “the capturing/redirecting process” loses 2/3rds of the energy.
They’re probably basing their calculation on the orbital design discussed in citation 34: Suffern’s Some Thoughts on Dyson Spheres whose abstract says
According to Dyson (1960), Malthusian pressures may have led extra-terrestrial civilizations to utilize significant fractions of the energy output from their stars or the total amount of matter in their planetary systems in their search for living space. This would have been achieved by constructing from a large number of independently orbiting colonies, an artificial biosphere surrounding their star. Biospheres of this nature are known as Dyson spheres. If enough matter is available to construct an optically thick Dyson sphere the result of such astroengineering activity, as far as observations from the earth are concerned, would be a point source of infra-red radiation which peaks in the 10 micron range. If not enough matter is available to completely block the stars’ light the result would be anomalous infra-red emission accompanying the visible radiation (Dyson 1960).
Bolded for your convenience. Presumably they justify that assertion somewhere in the paper.
Imperfect efficiency isn’t because it’s transparent (as everyone keeps trying to say, it doesn’t have to let through any sunlight at all) - it’s because of Carnot efficiency. If you want to convert sunlight into electrical energy, you can’t do it perfectly, which means your Dyson swarm heats up, which means it radiates light in the infrared.
So if 2⁄3 of the sun’s energy is getting re-radiated in the infrared, Earth would actually stay warm enough to keep its atmosphere gaseous—a little guessing gives an average surface temperature of −60 Celsius.
So if 2⁄3 of the sun’s energy is getting re-radiated in the infrared, Earth would actually stay warm enough to keep its atmosphere gaseous—a little guessing gives an average surface temperature of −60 Celsius.
That is, until the Matrioshka brain gets built, in which case assuming no efficiency gains, the radiation will drop to 44% of its original, then 30%, then 20%, etc.
A shell in a Matrioshka brain (more generally, a Dyson sphere being used for computation) reradiates 100% of the energy it captures, just at a lower temperature.
Yeah, the energy radiated to infinity only gets reduced if it’s being used for something long-term, like disassembling the sun or sending off energy-intensive intergalactic probes.
Armstrong & Sanders answer many of these questions in Eternity in Six Hours:
They go on to estimate how long it’d take to construct, but the punchline is 31 years and 85 days.
A thing I’m still not sure about reading that is “what percent of the light is getting through?”. Like, how dense are the reflector modules?
Later in the paper it says “The Dyson sphere is assumed to have an efficiency of one third”, which could mean “realistically you only capture about 1/3rd of the energy in the first place” or “the capturing/redirecting process” loses 2/3rds of the energy.
They’re probably basing their calculation on the orbital design discussed in citation 34: Suffern’s Some Thoughts on Dyson Spheres whose abstract says
Bolded for your convenience. Presumably they justify that assertion somewhere in the paper.
Imperfect efficiency isn’t because it’s transparent (as everyone keeps trying to say, it doesn’t have to let through any sunlight at all) - it’s because of Carnot efficiency. If you want to convert sunlight into electrical energy, you can’t do it perfectly, which means your Dyson swarm heats up, which means it radiates light in the infrared.
So if 2⁄3 of the sun’s energy is getting re-radiated in the infrared, Earth would actually stay warm enough to keep its atmosphere gaseous—a little guessing gives an average surface temperature of −60 Celsius.
That is, until the Matrioshka brain gets built, in which case assuming no efficiency gains, the radiation will drop to 44% of its original, then 30%, then 20%, etc.
A shell in a Matrioshka brain (more generally, a Dyson sphere being used for computation) reradiates 100% of the energy it captures, just at a lower temperature.
Yeah, the energy radiated to infinity only gets reduced if it’s being used for something long-term, like disassembling the sun or sending off energy-intensive intergalactic probes.