wowt explains: what is a dyson sphere?

The massive structure designed to capture the energy of an entire star envisioned by Freeman Dyson is popular in science fiction. But can we build it? And have alien civilizations tried a similar idea?
dyson sphere in progress

Imagine that you live on an ancient, distant planet orbiting a dying star. Your world relies on its light to survive and as it fades, it gets colder and more hostile to live in the place your species once used to call home. But you have the technology to save yourself. By surrounding your sun with solar panels to absorb and redirect it energy, you can concentrate its power output for your own ends. Sounds like pure fantasy, doesn’t it? Well, it isn’t. In fact, that’s more or less the scenario proposed by astronomer Freeman Dyson in 1959 as one potential answer to the Fermi Paradox and it lit up the imagination of countless sci-fi writers.

Amazingly, building what’s become known as a Dyson sphere is not impossible, and not only have we seriously thought about how to tackle its construction, there was even a very small chance we saw evidence of one around a not too distant star. But there are still a lot of question about both the feasibility and returns on investment from such a project, and a real-life version wouldn’t be an actual orb of metal alloy or carbon fiber surrounding a star like some sort of enormous Poke Ball. So, with all that in mind, let’s tackle some of the more frequent questions about Dyson spheres.

why would you want to build a dyson sphere?

Because you want to capture as much energy radiating from a star as possible for any reason that makes sense to you. Consider that the Sun produces 3.84 × 1026 Joules every second, the equivalent of roughly 455 million explosions that destroyed Hiroshima and a million times the energy we use every year. If you can control just one percent of that output, you could easily power 500 modern Earths in the time it takes you to take a deep breath. From there, what you do with it is up to you. You could use it to power interstellar missions using solar sails, turn it into a vast offensive weapon for solar system defense, or use the energy to run the biggest supercomputer design ever conceived.

Every second, you would get just as much power at your disposal again, and keep in mind that one percent is a conservative estimate. But there’s a catch. Gathering all that energy will be relatively easy after the sphere will be built. Using it will be a lot more difficult, especially if you’re using it on a dying star as frequently envisioned. Light bouncing around for tens, if not hundreds of millions of miles loses a lot of power thanks to the inverse square law, and unless you’re planning on hitting your receivers with death beams, you’re going to need to diffuse it even further. Overall, it’s an interesting thought experiment, but one which still needs a lot of little details further addressed.

how much energy could a dyson sphere produce?

As mentioned above, the figure really depends on its diameter and the efficiency of the panels being used. Here, 93 million kilometers from the Sun, we receive 1,380 Watts per square meter, but 1.4 billion kilometers away by Saturn, it’s closer to 14 Watts per square meter. Therefore, the bigger the Dyson sphere, the less energy it will have available in the first place, although it would gather a decent fraction of the power of a small one instead of orders of magnitude less since it would have a much greater surface area to gather it. That would, of course, come at the expense of requiring orders of magnitude more resources and energy to construct the sphere in the first place.

As an example, a sphere reaching all the way out to Saturn would receive 3.44 × 1020 J per second while a version that just encloses the inner solar system would see 6.55 × 1020 J per second, or just under twice the energy. With modern solar panels being roughly 35% efficient in space, compared to a peak of 22% on Earth thanks to a distinct lack atmospheric diffusion, we’d see as much as 2.29 × 1020 J per second captured by an inner solar system sized Dyson sphere. However, again, beaming all that power around without melting the receivers is a major challenge, which is why some ideas for these structures see the species using them living in the permanent night of its external shell, tapping directly into the power generated by them.

red dwarf star

A Dyson sphere’s output also depends on what kind of star it encloses. Our sun is a relatively large and powerful star, statistically speaking. Three out of four stars in the universe are red dwarfs and they’re are most likely to be home to a habitable world where intelligent life would have a chance to evolve and advance to the point where it could build such a megastructure. That’s where they’d run into two issues. The first is that only 10,000th of the energy of our sun would be available. The second is that the structure would have to be much smaller, just under 50 million kilometers across, which is a net positive because it would be in better position to capture as much of that energy as possible.

Assuming similar conversion efficiency, this works out to 3.02 × 1014 J per second, or 2.72 ×1022 J per year compared to the 7.22 × 1027 J we could collect with a Dyson sphere around our inner solar system over the same time frame. Basically, we’re looking at just over 50 times the Earth’s current energy appetite vs. more than 10 million. The biggest bang for your buck would be found around blue supergiants shining with up to 40,000 times the power of the Sun. A similar megastructure could gather 50 trillion times the energy we currently need as it orbits the star Rigel while being reasonably protected from its powerful plasma winds.

what would a dyson sphere look like?

A Dyson sphere couldn’t be a solid shell. Such a design wouldn’t be gravitationally bound to a star and drift, causing catastrophic disasters as it does. Instead, the best approach is a vast swarm of satellites with ion engines using lasers to maintain a stable orbit. The two primary challenges they’d face would be to keep from drifting into each other and to resist the push from the photons they’re trying to capture because they’d each be designed the same way as a solar sail. Neither is likely to be a serious problem because they could use a small amount of the energy they’ll capture to keep themselves stable, but this would require a very high degree of coordination over a long term.

However, this does make it much more difficult to see an alien species living on the outside of a Dyson sphere to take advantage of plugging directly into the photovoltaic grid and avoiding the problem of beaming the energy to where it’s needed, unless those satellites were the size of a megacity each, which would actually be somewhat plausible for a civilization capable of actually building such a structure. Life in these orbiting cities would be quite bizarre to us, potentially lacking gravity and populated with intelligent machines and cyborgs to whom gravity is very much irrelevant, or seeing residents in wheel-like structures rotating to produce a centrifugal force to simulate gravity and manipulating artificial light to mimic the night-day cycle.

have we found a dyson sphere in space?

tabby's star comets

Some astronomers thought that we may have found an indication of one around a star known as KIC 8462852, or Tabby’s Star. It was always a long shot, but current consensus holds that it was really a cluster of comets blocking our view, not the beginning stages of a Dyson sphere. It’s not that the star wasn’t a good candidate for such a megastructure, but there are a lot of steps between occasional strange patterns in its light and incredibly advanced alien societies which need to come with iron clad proof and so far, we have none. So far, there are no observations leaning towards the objects blocking its light being anything other than comets or dust as the patterns are fairly irregular and change very little, the opposite of what we’d expect to see if we were witnessing the construction of a Dyson sphere.

how soon could we build a dyson sphere?

Nowhere near soon. There are simply too many questions about how we’d approach it and how we’d beam the power back from it to Earth and other outposts, as well as how big we’d want to make it and what effects it would have on the rest of the solar system, if any. And this is before we try to get into the thorny question of how we’d ever be able to pay for it. Even with the best technology we have today, we’d be looking at millions of rocket launches carrying hundreds of billion tons of material and racking up a bill in the quadrillions of dollars. Even if we could seize every last bit of currency on Earth for this task, a grand total of just over $80 trillion, we’d only be able to pay for a rounding error of the balance.

could you create a dyson sphere around objects other than stars?

Yes, you absolutely could. Another potential type of Dyson sphere could be built around a black hole. Since black holes are much smaller than stars, the structure could be as small as a few hundred kilometers across and would be much easier to build. However, instead of collecting light and radiation from the accretion disks of black holes, you’d use mirrors and lasers to bombard the objects’ ergosphere and amplify its feedback to produce vast amounts of energy. This strategy could be used by aliens trying to escape the heat death of the universe, allowing them to spend the end of literally everything in virtual worlds powered by a gravitational dynamo until it finally evaporates into nothing.

# space // astronomy / dyson spheres / space solar power

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