Dyson sphere

A **Dyson sphere** is a megastructure that completely encompasses a star and captures a large percentage of its power output. It's used by a spacefaring civilization to meet its energy requirements once those requirements exceed what can be generated from the home planet's resources alone. Only a tiny fraction of a star's energy emissions reach the surface of any orbiting planet. Building structures encircling a star would enable a civilization to harvest far more energy. By definition it is regarded as a Class B stellar engine.

The collected energy can be beamed to remote locations for use in life support, industry, computational processing, or transport; these beams can also be used for defense or communication. Energy can also be stored using antimatter.

The concept was popularized by Freeman Dyson who speculated that such structures would be the logical consequence of the escalating energy needs of a technological civilization and would be a necessity for its long-term survival. He proposed that searching for such structures could lead to the detection of advanced, intelligent extraterrestrial life.

Following the publication of Dyson's proposal, it was pointed out that a solid circumstellar sphere would be subject to intolerably high stresses. Dyson replied that what he actually envisaged was a loose collection of over 100,000 objects traveling on independent orbits in a shell about 1 million kilometers thick. This Dyson Swarm arrangement is a Type I Dyson sphere and a solid shell is Type II.

A Dyson shell can support vast amounts of processing power, especially if arranged in concentric shells as a Matrioshka Brain.

T. Y. Hsiao has proposed to place a Dyson sphere around a black hole instead of a star. Around the black holes are hot accretion disks, hot gas coronas, and sometimes ultra-relativistic jets, releasing large amounts of potentially usable energy far more efficient and productive than the nuclear fusion of stars. Accretion of material as it falls into a black hole entering the event horizon releases up to 40% of the energy of the material falling in, whereas fusion is only up to 3% efficient.

Black hole energy collection would initially be for Type II and at the upper limit of energy collection, even a Type III civilization could have its energy needs met by a sufficiently active Dyson-encased black hole, without the necessity of commandeering all the star-supplied energy from an entire galaxy. Such civilizations are not likely to be located on the structure, but at a safe distance away, with the energy beamed to them as needed.

The ultimate Dyson sphere is one that traps dark energy inside and reverses the expansion of the universe, or the Big Rip. By manipulating extra dimensions, a Type IV civilization would create a gigantic sphere where dark energy can reverse polarity so that the cosmic expansion is reversed. Outside the sphere, the universe might continue expanding exponentially, but inside the sphere, galaxies and stars evolve normally.

A Megasphere is the ultimate feat in gigascale engineering. It is a Dyson sphere 200 light-years in diameter, with a surface area of more than 130,000 square light-years. The base structure is 10 km thick, and both sides are equally habitable. About 1300 galactic masses are needed as raw materials, with 1056 J required for assembly, and another 1056 J to maintain the Megasphere at a comfortable 25 ºC for a billion years. This is a Type IV endeavor.

Examples:

#### Configurations and variations:

It would have a problem of dynamical instability. Even if the hollow sphere was rotated fast enough to support the equatorial zone of the structure against collapse, the polar regions would fall in and destroy it. To solve this, the star can be surrounded with several layers of ring-shaped tubes. They contain a fluid flowing fast enough to create the centrifugal force required to support the tubes and loads above them. One set of parallel tubes holds up the equator and other sets support the regions near the poles. The tubes are also heat engines, with the fluid driven by energy absorbed from the sun and flowing through generator stations which supply power to all the inhabited levels. A Matrioshka brain can adjust the velocity of flow to fit the loads.

The Sphere is about 15 solar masses. The shell has a wall of solid steel more than 3 km thick, but the hull is more than 60 km deep. The total energy needed to assemble the sphere would probably run on the order of 1043 joules, but to propel it through space at 17%c will require an additional 4 x 1046 joules of energy.