The material factors which ultimately limit the expansion of a technically advanced species are the supply of matter and the supply of energy. At present the material resources being exploited by the human species are roughly limited to the biosphere of the earth, a mass of the order of 5 X 10^19 grams. Our present energy supply may be generously estimated at 10^20 ergs per second. The quantities of matter and energy which might conceivably become accessible to us within the solar system are 2 X 10^30 grams (the mass of Jupiter) and 4 X 10^33 ergs per second (the total energy output of the sun).

The reader may well ask in what sense can anyone speak of the mass of Jupiter or the total radiation from the sun as being accessible to exploitation. The following argument is intended to show that an exploitation of this magnitude is not absurd. First of all, the time required for an expansion of population and industry by a factor of 10^12 is quite short, say 3000 years if an average growth rate of 1 percent per year is maintained. Second, the energy required to disassemble and rearrange a planet of the size of Jupiter is about 10^44 ergs, equal to the energy radiated by tlhe sun in 800 years. Third, the mass of Jupiter, if distributed in a spherical shell revolving around the sun at twice the Earth's distance from it, would have a thickness such that the mass is 200 grams per square centimeter of surface area (2 to 3 meters, depending on the density). A shell of this thickness could be made comfortably habitable, and could contain all the machinery required for exploiting the solar radiation falling onto it from the inside.

It is remarkable that the time scale ozf industrial expansion, the mass of Jupiter, the energy output of the sun, and the thickness of a habitable biosphere all have consistent orders of magnitude. It seems, then, a reasonable expectation that, barring accidents, Malthusian pressures will ultimately drive an intelligent species to adopt some such efficient exploitation of its available resources. One should expect that, within a few thousand years of its entering the stage of industrial development, any intelligent species should be found occupying an artificial biosphere which completely surrounds its parent star.

The energy and time required to dismantle a planet and turn it into a biosphere surrounding a sun.

In summary, the circumstellar shells of Dyson civilizations-at temperatures ~300 degrees K and radii ~1 a.u.--can be detected with existing telescopes and state-of-the-art infrared detectors in the 8-13-u window out to distances of several hundred parsecs. But discrimination of Dyson civilizations from naturally occurring low-temperature objects is very difficult, unless Dyson civilizations have some further distinguishing feature, such as monochromatic radio-frequency emission.

How difficult would it be to detect their heat signature?