In 1960, visionary physicist Freeman Dyson proposed that an advanced alien civilization would someday quit fooling around with kindergarten-level stuff like wind turbines and nuclear reactors and finally go big, completely enclosing their home star to capture as much solar energy as they possibly could. They would then go on to use that enormous amount of energy to mine bitcoin, make funny videos on social media, delve into the deepest mysteries of the Universe, and enjoy the bounties of their energy-rich civilization.

But what if the alien civilization was… us? What if we decided to build a Dyson sphere around our sun? Could we do it? How much energy would it cost us to rearrange our solar system, and how long would it take to get our investment back? Before we put too much thought into whether humanity is capable of this amazing feat, even theoretically, we should decide if it’s worth the effort. Can we actually achieve a net gain in energy by building a Dyson sphere?

Spherical Dyson cows

I’ll state from the outset that I'm a theoretical cosmologist, not an engineer. I have absolutely no idea how to go about building a bridge, let alone a structure that reshapes the very face of our Solar System. But I’m willing to bet that nobody knows how to engage in these kinds of mega-engineering challenges. We can’t say for certain what kind of advances in which technologies would be necessary to build a structure that even partially encloses the sun. To speculate on that would be science fiction—fun, but not very meaty.

What I do know, though, is physics, and there are some things we can say about the physics of a Dyson sphere. We can use building one as a thought experiment to explore fundamental principles of energy, orbit, and motion. And this is important because no matter what technology-so-advanced-it’s-indistinguishable-from-magic our descendants come up with that allows them to rip apart planets, they still have to face the cold hard realities of our physics. They can’t get something for nothing. If they want to resculpt a planet, that takes energy. If they want to move a mountain-sized solar panel into a different orbit, that also takes energy.

For these and many other reasons, a Dyson sphere costs energy. So we’re going to see how long it will take to recoup the energy investment of building one and what the optimal design might be to minimize the initial investment.

To get at some numbers, we’re going to make a lot of assumptions. People like to poke fun at physicists for simplifying complex problems, sometimes beyond recognition. The old joke goes that dairy farmers reached out to a nearby university to help understand why milk production was low, and the response from the physicists began by assuming that the cows were spherical.

But there is something powerful about this simplifying approach, which is why physicists are trained in it from day one. First, it lets us answer questions when we’re not interested in precise numbers at the outset. Here, we just want a general sense of feasibility—will building a Dyson sphere take a (relatively) small, medium, or extreme amount of energy? Second, simplifying the problem helps cover up mistakes (either in calculations or our starting assumptions). If all we’re going after is a general ballpark, then a factor-of-two mistake (or even 10 or 100) won’t really change the overall intuitions our calculations enable.

Lastly, we literally don’t know how to build a Dyson sphere, so trying to go for anything more complex will simply lead to us introducing many more assumptions to handle all the small details. Each of those assumptions will increase the uncertainty of any numbers we produce, and that uncertainty will probably end up buried in the analysis rather than handily stated upfront.