Application of the Thermodynamics of Radiation to Dyson Spheres as Work Extractors and Computational Engines and Their Observational Consequences

Abstract

I apply the thermodynamics of radiation to Dyson spheres as machines that do work or computation and examine their observational consequences. I identify four properties of Dyson spheres that complicate typical analyses: globally, they may do no work in the usual sense; they use radiation as the source and sink of energy; they accept radiation from a limited range of solid angles; and they conserve energy flux globally. I consider three kinds of activities: computation at the Landauer limit; dissipative activities, in which the energy of a sphere’s activities cascades into waste heat, as for a biosphere; and “traditional” work that leaves the sphere, such as radio emission. I apply the Landsberg formalism to derive efficiency limits in all three cases and show that optical circulators provide an “existence proof” that greatly simplifies the problem and allows the Landsberg limit to be plausibly approached. I find that for computation and traditional work, there is little to no advantage to nesting shells (as in a “Matrioshka Brain”); that the optimal use of mass is generally to make very small and hot Dyson spheres; that for “complete” Dyson spheres, we expect optical depths of several; and that in all cases the Landsberg limit corresponds to a form of the Carnot limit. I explore how these conclusions might change in the face of complications, such as the sphere having practical efficiencies below the Landsberg limit (using the endoreversible limit as an example), no use of optical circulators, and swarms of materials instead of shells.