Dyson Spheres
Freeman Dyson, the conceptualist behind the Dyson Sphere, hated the fact that the hypothetical engineering megastructure was named after him. In fact, to this day, he openly credits the origin of the idea to the 1937 novel Star Maker by Olaf Stapledon and wishes it had been named something else. Regardless of the origin of the concept or the name, Dyson Spheres remain a prominent subject in both astrophysics and engineering.
Just in 2015, Tabby’s Star (more formally known as KIC 846… blah blah, no one really cares about the number) exhibited a very curious dip in luminosity which could not be explained by the presence of a planet or a sun spot. Speculations abounded that this strange dip in luminosity, duplicated several more times in different magnitudes, was in fact the construction process of a stellar megastructure currently underway. Though the data was later more accurately explained by a rogue, evaporating Plutoid along an eccentric orbit, this event was a wake up call that Kepler may be detecting more than exoplanets out there.
Such a structure would be an astonishing engineering feat. In essence, a Dyson Sphere is an immense, solar energy collecting megastructure whose purpose is to power a space faring civilization during a period of rapid advancement. It is generally associated with a species’s achievement of becoming a Type II civilization on the Kardachev Scale, which is a species which collects and utilizes the power output of an entire star (~10²⁶ Watts). For comparison, our entire planet currently only uses an average of about 1.8*10¹³ Watts; more than 20 trillion times less. This is not a structure we would endeavor to accomplish within the century, or maybe even the next millennium.
In his adaptation of the idea, Dyson imagined a future human civilization whose energy demands were skyrocketing. As our power consumption approached the power output of the sun, it would become essential that we harness the full potential of our home star instead of trying to duplicate that power elsewhere. In his proposal, Dyson suggested three main different types of Dyson Spheres which could serve to harness the energy output of the Sun.
Dyson Swarm
The Dyson Swarm is the simplest Dyson megastructure. In essence, a Dyson Swarm is just an array of solar panels placed in orbit around the sun. To collect an appreciable flow of power, these solar panels would have to be really, really big. Most designs utilize a radius for each panel which is multiple times larger than the distance from the Earth to the Moon.
The construction of such a swarm could begin with just one satellite, transmitting energy wireless back to the Earth. More satellites could eventually be added to form a ring around the host star; a Dyson Ring. Because these constructs are in orbit, the configuration of these satellites becomes very difficult when more satellites are added beyond the first ring. Orbits would intersect, and would either have to be timed well or placed in slightly different orbits to avoid collisions. The second option, however, would induce eclipse periods on other satellites lowering the volume of power collected.
Material for these structures could come from the asteroid belt. Given a satellite radius of 10⁷ meters (25x the Earth-Moon distance), and a thickness of a few meters, several hundred satellites could be constructed from the mass of a large asteroid, such as Ceres. Thousands could be built by utilizing the remainder of the asteroid belt. The real trouble here would be reorganizing the asteroid matter in order to make a material which is both capable of collecting the solar output of the sun, and counteracting the gravitational stresses of it’s own mass.
Relative to their immense surface area, each satellite would be incredibly thin, making such structures extremely fragile. Collisions with comets, asteroids, and meteors would be detrimental to such an array’s success, and even gravitational interactions with the other inner planets could tear an array apart. Care would have to be taken in deciding specific orbital configurations as well as implementing asteroid defense systems.
Dyson Swarms would be difficult to use as a biosphere for humans because they would provide little to no gravity. A Dyson Swarm could instead be constructed within the orbit of Earth (so long as there are not a lot of eclipses with Earth which could cool the planet substantially) and provide a power source from abroad.
Dyson Bubble
Similar to the Dyson Swarm, the Dyson Bubble would consist of a large array of solar power collecting satellites around a host star. The difference here is that instead of placing these satellites in orbit around the star, they are placed at a Goldilocks distance where the solar pressure from solar wind and radiation is exactly equal to the gravitational force of the sun on the satellite. This makes each satellite completely stationary.
This greatly reduces the extreme complexity exhibited by Dyson Swarms, and allows for a much greater yield of power collection. Theoretically, a civilization could just continue to add more satellites at this Goldilocks distance until the entire star was covered, essentially creating a Dyson Shell. It also eliminates any chance of eclipses from other satellites because each satellite remains still.
Construction of a Dyson Bubble would probably be more difficult than a Dyson Swarm, however, because gravitational interactions and collisions would be even more severe for the array. In a Dyson Swarm, there is almost a 0% chance that a gravitational effect or impact would actually render the satellite completely useless, possibly knocking it out of alignment or into a slightly eccentric orbit. However, in a stationary Dyson Bubble, a very small interaction could destabilize the precise configuration which allows the vessel to remain in place, possibly causing it to fall into the sun. Such an array would require very advanced attitude control systems.
Dyson Bubbles suffer from the same inability of Dyson Swarms of not providing a very suitable biosphere for humans. The only gravity on a Dyson Bubble would be from the sun, which would be tiny unless the bubble was constructed very close to the star (which would probably be dangerous). This means that Dyson Bubbles would have to be built with the orbit of Earth in mind, leaving enough sunlight to reach our planet’s surface.
Dyson Shells
The most commonly depicted Dyson megastructure in science fiction is the Dyson Shell. It is often incorrectly used synonymously with the term ‘Dyson Sphere’. A Dyson Shell is a complete, solid sphere constructed around the entirety of a star. Dyson Shells have the advantage of capturing 100% of the solar output, which Swarms or Bubbles cannot claim. They also provide the largest surface area for any potential human biosphere. Despite being so iconic and popular, this is actually the least feasible of the Dyson megastructures.
A Dyson Shell would require more solid material than is present in the solar system to make a shell even a hundred meters thick. But even at this thickness, the structure itself would likely not be able to withstand the compressive force of its own gravity (if constructed with a 1 AU radius). If you wanted a Dyson Shell to have artificial gravity, it would have to rotate around the sun at more than 1200 km/s, which is 0.4% of the speed of light. This is equivalent to completing a solar orbit in a little over a week, as opposed to a year. The stresses on the structure would be unimaginable. If you wanted to build it by just expanding a Dyson Bubble like was suggested earlier, you would be forced to block all incoming light from the Earth, which wouldn’t really matter because you would need the material from the Earth to build it anyways.
Dyson Shells may not be practical for our sun, but perhaps a species living around a smaller star, such as an M class red dwarf, would reap the benefits of a Dyson shell better than our species. With such a small energy output, a much larger percentage of their host star’s power would have to be captured in order to properly serve a rapidly advancing Type I alien civilization around such a star. Furthermore, habitable distances from such stars often allow for orbits which precess in a matter of days anyways, making artificial gravity potentially possible.
The Search for Dyson Spheres
If stellar megastructures are a natural stage in a Type II civilization’s development, then there could be hundreds or even thousands of them throughout the galaxy. If they do exist, they would exhibit some telltale properties which we could identify to unveil their existence.
One property of Dyson Spheres, which was highlighted by the Tabby’s Star incident, is eclipses. If we witness massive eclipses which suggest a planet larger than Jupiter, but without the associated gravitational wobbling that such a planet would induce on the host star, this may be a sign that we are seeing a Dyson Swarm. Similarly, if we witness an eclipse which occurs extremely slowly due to a large, dark body, it may be a sign that we are seeing the transiting effects of a Dyson Bubble. Because Dyson Bubble satellites aren’t in orbit, they would only eclipse us with the very slow relative motion of the actual host star itself with respect to our planet.
Dyson Shells may be more difficult to detect. Because they completely absorb all of the visible light from a star, they would appear invisible to any traditional telescope. However, there may be an emission from a Dyson Shell that we can detect; infrared radiation.
Energy collection occurs at the inner surface of a Dyson Shell. If a species didn’t use any of the power they collected, the outside surface of the shell would reach an equilibrium temperature and re-emit 100% of the radiation in the infrared band at that temperature. However, if the species is frugal, they will likely use some, if not most, of the power from their host star. There will always be waste heat, however; heat that either cannot be captured, or heat generated by whatever they are using the power for. This waste heat will be radiated through the shell at a lower frequency than if they had not captured/used any solar power at all. This means that if we detect a very large, low frequency infrared source, we may be seeing the effects of a Dyson Shell.
Conclusion
A Dyson sphere, in any form, would represent the turning point in a species’s road to becoming a truly intergalactic race. Whether or not it is ethically correct to completely rearrange the matter of a several billion year old solar system for one’s own purposes may be up for debate in the modern day. But one thing that is certain is that our energy demands as an advancing species are growing, and don’t appear to be slowing down. As we approach an interstellar stage of development, it may be necessary to utilize stars, the powerhouses of nature, in order to fuel our means of discovery and exploration. The universe doesn’t need us to survive, but we sure as hell need it, both now and likely in the distant future.
