# Dyson Sphere Update

Notes from a panel at Chicon 7. 12 noon, Friday, August 31, 2012. Weblinks and various comments and expansions added.

##### Dyson Sphere Update

What is a Dyson sphere? When can we make one, or at least part of one, and how? As a society, would we want to? What could one do with a Dyson sphere? Can we detect one now, or in the near future? What would be the environmental ethics of a Dyson sphere maker?

Jordin Kare, Allen M. Steele, James L. Cambias, David L Clements, G. David Nordley

While the usual concept of a Dyson Sphere is a solid sphere surrounding a star, that was not Freeman Dyson’s original idea. He was thinking of a system of many orbiting structures. His idea was based on a concept by J.D. Bernal. The point of this is to capture and use the entire energy output of the star, rather than the very small fraction that reached an individual planet.

If they exist, current infrared telescopes could find Dyson spheres at the range of nearby stars. Look for a Black body with a temperature around 300oK with a total radiation like a star, i.e. a solar luminosity object in the far infrared. This is a testable prediction.

Matrioshka spheres: Imagine a Dyson sphere that captures stellar radiation at a temperature T0 and reradiates it at T1 < T0. (e.g. T0 = 6000oK and T1 = 300oK). You then imagine another Dyson Sphere outside the first one catching the radiation at T1 and reradiating it at T2 < T1, and yet another one outside that …. It is difficult to think of why such a system might be built.

A uniform spherical shell is neutrally stable with respect to the gravitational forces of objects inside it, e.g. the parent star. This follows from Gauss’s law or (equivalently) the Shell Theorem. Larry Niven’s Ringworld would not be stable, since it is essentially a 2-D circle rather than a sphere.

I thought I heard one of the speakers say, 1 GW of solar power needs 720,000 km2 at a distance of 1 AU. Let’s check this: The total power output of the sun is 3.839×1026 W = 3.839×1017 GW. 1 AU = 93,000,000 miles = 150,000,000 km = 1.5×108 km. Area of a sphere 1 AU in radius = 4π x 2.25×1016 km2 = 2.83×1017 km2. Dividing the area by the power gives 0.736 km2/GW, which is way off. However 0.736 km2=736,000 m2, which is reasonably close as astronomical estimates go. So I suspect the speaker said, or at least meant to say 720,000 m2.

Coronal mass ejection could be a hazard for a Dyson sphere. However it could also be source of materials for building one.

Think really big: Build a Dyson sphere around the black hole at the center of a galaxy. It should be possible to extract about 30% of the total energy of the mass (E = mc2) falling into the black hole. I would like to see how this is calculated, but I suspect it is a fairly serious exercise in General Relativity. This seems like the most energy you can extract without having access to large amounts of antimatter.

The amount of energy a civilization can extract gives its place on the Kardashev scale.

Imagine 1/2 of a Dyson sphere (Dyson hemisphere). Think of using it a a giant solar sail. On galactic time scales it is actually not too slow!