A Dyson Sphere is a hollow sphere around a sun with a circumference of 1 AU, with a hollow space inside. A Dyson Sphere of this size would have a surface area 600 million times that of the Earth. It was named after Freeman Dyson, who proposed it as a way to capture all of the energy radiated by a star.

Structural elements of Dyson Spheres are interesting - the curvature of the inside of a Dyson Sphere is even less than that of the Earth. Also, there is a problem with gravity - everything wants to fall into the sun. To get gravity, you must spin it, but that makes only the equatorial regions habitable. This problem is what caused Larry Niven to cut out the top and bottom, leaving him with just a ring (ringworld).

Another way of handling the gravity problem is to have a habitable space between two Dyson Spheres, heated and given gravity from below.

There is also a way to create day and night - inside the sphere, if there were large square light-blocking materials rotating around the sun, the shadow they cast would work as night.

Dyson Spheres are popular with people who think that the earth is too small: people who like SF, Fantasy, Flat-earthers, and really anyone who wants to imagine billions of civilizations living in the same space.

In SF, the earliest reference to a Dyson Sphere was in Robert Silverberg's Across a Billion Years.

information from http://www.d.kth.se/~nv91-asa/dysonFAQ.html

Technically, Dyson's original 'sphere' was actually a huge cloud of energy-collecting devices in orbit around a star, implemented by an advanced alien race. It's a little implausable, but Dyson proposed the concept because such an apparatus would generate an enormous amount of black body radiation, measured as energy in the infra-red spectrum, and could be easily spotted by a telescope on Earth.

Always hot for a new idea, SETI tried scanning the sky in the infra-red range, and subsequently discovered several infra-red transients in the sky that fit the description of a dyson sphere. I can't remember if they were able to come up with an astrophysical explanation for these points.

Dyson Spheres have appeared several times in Science Fiction. I believe the first appearance was a photo finish between Cuckoo in the book "Farthest Star" by Jack Williamson and Frederik Pohl, and "Orbitsville" by Bob Shaw. The novels both appeared in 1975 and I believe they were both serialised (Only part of "Farthest Star" as "Org's Egg") in the SF magazine Galaxy the year before (Orbitsville starting in the issue that "Org's Egg" finished in).

Though there have been several others the main one that springs to mind is the Cageworld sequence by Colin Kapp from the mid-80s. Set in our solar system this posited a series of concentric Dyson spheres at each of the planetary orbits (I don't remember them dealing with where all the material came from).

And of course expert navigator and engineer Montgomery Scott (Scotty) succeeded in crashing into one in the episode Relics of Star Trek:TNG.

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