Dyson spheres are not dark

Dejan Vucinic (dejan@mit.edu)
Tue, 24 Jun 1997 13:35:33 -0500

Meisner@ACT.ORG wrote:

> Why, after infinite past time, with Some Big Smart Entity having
> surely evolved by now, do we find ourselves in a universe with more
> than a sextillion stars wastefully dissipating their energy; i.e. a
> universe full of chaos and entropy?
> This is, perhaps, an extrauniversal extension of Fermi's Paradox: a
> typical intergalactic civilization should be able to do a great deal to
> control cosmic entropy, slapping Dyson spheres around all those stars
> to prevent their waste of energy and whatnot. So why do we live in such
> a wasteful galaxy, among so many other wasteful galaxies? As Fermi
> asked, "Where are they?" And why aren't they cleaning things up?

I know of no evidence that the stars we see are _not_ inside Dyson
spheres. Let's say we put an opaque sphere around a star. The star
is radiating away a certain energy per unit time, and your sphere
has to deal with that power. I suppose you could reflect that energy
back toward the star, but soon you'd end up with a tremendous energy
density inside the sphere. A good designer would probably try to
balance radiation and gas pressure with gravitational stress on the
sphere (the exact ratio depending on how hot they like it), but once
they reach an equilibrium they'd let all the incoming energy out.
Remember that energy can't reduce entropy, you need a _flux_ of
energy, a thermodynamic engine. Therefore, a well-designed Dyson
sphere would look just like a star from here. A paranoid designer
might even go to some length to ensure that the spectrum of the
outgoing radiation looks "natural" to a casual observer.

In fact, I can hardly see how else can a Dyson sphere be built.
One would start from seeds that would "sail" near the star and
expand themselves in such a way that the gravitational stress at
any point during the construction is kept at an absolute minimum.
Even the finished sphere is gravitationally unstable, it needs
some sort of distributed propulsion mechanism or lots of
controllable openings to keep the star in the middle and to
dampen oscillations. It's a two-dimensional bridge without
endpoints of pylons---the mother of all engineering nightmares!