What's a Dyson Sphere? The Ultimate Solar Power Station!In 1960, the great polymath Freeman Dyson published a scientific paper titled "Search for Artificial Stellar Sources of Infrared Radiation". In it, he explained why he believed that sufficiently advanced alien civilizations, if they exist, would eventually - after a long enough period of time - surround their star with energy-gathering structures to maximize the amount of usable energy they harvest, and minimize losses to the void (basically, a Type II on the Kardashev scale). Eventually, such structures could completely enclose a star in something that became known as a Dyson Sphere. This idea is just science fiction until we discover a Dyson Sphere, but as a recent article in The Atlantic pointed out, some astronomers from Penn State think Dyson's reasoning makes enough sense that it's worth specifically looking for signs of Dyson Spheres out in the universe. What's so cool about it is that the ultimate power station turns out to be a massive solar farm, the solar farm to end all solar farms, being so gigantic that no solar rays actually escape it when it is fully developed. How's that for clean energy?
So since I find the idea of Dyson Spheres so fascinating, I thought I'd quickly go through the different variations and talk a little bit about how astronomers are thinking of going about finding them, since after all the whole point is to keep light from escaping out into the universe...
Dyson RingAbove is a Dyson Ring, a swarm of energy-collectors circling a star. Because multiple collectors would be mechanically much easier to build than a solid sphere that completely encloses a star (yet still WAAAAY beyond our current technological level), this could be the first stage in the construction of a Dyson Sphere.
Dyson SwarmNext is a Dyson Swarm composed of many Rings, enough to encircle the star from all directions. Wikipedia lists the pros and cons of such a setup:
This construction approach has advantages: components could be sized appropriately, and it can be constructed incrementally. Various forms of wireless energy transfer could be used to transfer energy between components and earth.
Disadvantages: the nature of orbital mechanics would make the arrangement of the orbits of the swarm extremely complex. The simplest such arrangement is the Dyson ring in which all such structures share the same orbit. More complex patterns with more rings would intercept more of the star's output, but would result in some constructs eclipsing others periodically when their orbits overlap. Another potential problem is the increasing loss of orbital stability when adding more elements increases the probability of orbital perturbations. (source)
Dyson BubbleThe Dyson Bubble is similar to the full Dyson Swarm, but the main difference is that the collectors and space habitats wouldn't orbit the star. Instead, they would be "statites—satellites suspended by use of enormous light sails using radiation pressure to counteract the star's pull of gravity."
Such constructs would not be in danger of collision or of eclipsing one another; they would be totally stationary with regard to the star, and independent of one another. [...] In theory, if enough statites were created and deployed around their star, they would compose a non-rigid version of the Dyson shell mentioned below. Such a shell would not suffer from the drawbacks of massive compressive pressure, nor are the mass requirements of such a shell as high as the rigid form. Such a shell would, however, have the same optical and thermal properties as the rigid form, and would be detected by searchers in a similar fashion. (source)
Dyson Sphere/ShellNext is the full Dyson Shell, the most common in fiction but also the most mind-boggling in scale. It would completely enclose a star and absorb all light coming from it, and if the star is centered in the sphere, it would have no net gravitational interaction with its englobed sun.
A spherical shell Dyson sphere in our solar system with a radius of one astronomical unit, so that the interior surface would receive the same amount of sunlight as Earth does per unit solid angle, would have a surface area of approximately 28.1 Eha (Exa Hectare), or about 550 million times the surface area of the Earth. This would intercept the full 384.6 yottawatts (3.846 × 1026 watts) of the Sun's output; other variant designs would intercept less, but the shell variant represents the maximum possible energy captured for our solar system at this point of the Sun's evolution. This is approximately 33 trillion times the power consumption of humanity in 1998, which was 12 terawatts.(source)
How Do You Detect Dyson Spheres?The existence of Dyson Sphere is purely speculative, but while we're speculative, we might as well go all the way and see how such artifacts could be detected. Basically, it depends if you have a full shell or not. Swarms of collectors could be detected because of how they alter the light pattern of their star and by the heat radiation that they would emit.
Full shells could also be detectable, but in the infrared spectrum. All the energy that they would capture would heat them up and produce a signature potentially detectable at interstellar distances.
For more details on the Penn State astronomers and their search for Dyson Spheres, check out: Atlantic.