Back in 1960, the physicist Freeman Dyson publish an unusual paper in the journalScience entitled “Search for Artificial Stellar Sources of Infra-red Radiation.” In it, he outlined a hypothetical structure that entirely encapsulates a star to capture its energy, which has since become known as a Dyson sphere.
The basic idea is that all technological civilizations require ever greater sources of energy. Once the energy of their home planet has been entirely exhausted, the next obvious source is the mother star. So such a civilization is likely to build a shell around its star that captures the energy it produces.
Of course, such a sphere must also radiate the energy it absorbs and this would produce a special signature in the infrared part of the spectrum. Such a source of infrared radiation would be entirely unlike any naturally occurring one and so provide a unique way of spotting such as advanced civilization.
Because Sun-like stars seem the most obvious homes for advanced civilizations, most studies of Dyson spheres have focused on the properties these kinds of systems would have when built within the habitable zone at a distance of about 1 astronomical unit.
These studies have revealed well-known limitations, however. Such spheres tend to be unstable and require huge volumes of material to build. But most problematic of all, anything or anyone on the surface of these spheres would experience low levels of gravity, a problem that could not easily be solved with known physics.
Today, Ibrahim Semiz and Salim Ogur at Bogazici University in Turkey, define an entirely new class of Dyson sphere. Instead of thinking about a sphere around a Sun-like star, Semiz and Ogur consider a sphere built around a white dwarf.
They say that such a sphere would avoid some of the most severe problems and that there are good arguments to think that they might be more common than the ones Dyson originally imagined.
Semiz and Ogur begin by discussing the life-cycle of most stars. Stars spend most of their lives at a stage of their life cycle known as the main sequence. As they get older, however, they swell up and the temperature of their outer atmosphere cools as they become red giants.
Finally, these red giants explode leaving behind either a black hole, a neutron star, or a white dwarf. Every star with a mass less than about four times our Sun’s is destined for this final option. So as time goes on, a significant fraction of the stars in the universe should be white dwarfs.
Semiz and Ogur argue that any civilization that evolves during its sun’s main sequence and then finds a way to survive the red giant and supernova stages, will also probably find a way to create a Dyson sphere around the surviving white dwarf. For that reason, they suggest that these stars may be more likely to host such a structure.
What’s more, a white dwarf is a better host for a Dyson sphere. Semiz and Ogur point out that the habitable zone around a white dwarf is closer to the star, so such a sphere would be smaller. They calculate that a one-meter-thick sphere built in the habitable zone around a white dwarf would require some 10^23 kilograms of matter, just a little less than the mass of our moon.
And because the sphere is smaller, the gravity that anybody on the surface would experience would be stronger as well, almost Earth-like. That makes these kinds of Dyson spheres ideal homes for technologically advanced civilizations with at least a passing resemblance to our own.
There is one apparent disadvantage, however. Since white dwarfs emit less energy than Sun-like stars, a Dyson sphere around one would be much less luminous. And that would make it more difficult to detect. So if any civilizations in the Milky Way have reached this stage, it is going to be much harder for us to spot them.
That’s an interesting extension to the many analyses on Dyson spheres that have already been completed. And if it does map out a potential future for humanity, at least we have time on our hands. The Sun will eventually swell to form a red giant and ultimately explode leaving behind a white dwarf but we have about five billion years to come up with a survival plan.
Ref: arxiv.org/abs/1503.04376 : Dyson Spheres around White Dwarfs
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