The purpose of this report is to point out other possibilities which ought to be considered in planning any serious search for evidence of extraterrestrial beings. We start from the notion that the time scale for industrial and technical development of these beings is likely to be very short in comparison with the time scale of stellar evolution. It is therefore overwhelmingly probable that any such beings observed by us will have been in existence for millions of years, and will have already reached a technological level surpassing ours by many orders of magnitude. It is then a reasonable working hypothesis that their habitat will have been expanded to the limits set by Malthusian principles.

We have no direct knowledge of the material conditions which these beings would encounter in their search for lebensraum. We therefore consider what would be the likely course of events if these beings had originated in a solar system identical with ours. Taking our own solar system as the model, we shall reach at least a possible picture of what may be expected to happen elsewhere. I do not argue that this is what will happen in our system; I only say that this is what may have happened in other systems.

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 the size of Jupiter is about 10^44 ergs, equal to the energy radiated by the 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 of 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.

If the foregoing argument is accepted, then the search for extraterrestrial intelligent beings should not be confined to the neighborhood of visible stars. The most likely habitat for such beings would be a dark object, having a size comparable with the Earth's orbit, and a surface temperature of 200 deg. to 300 deg. K. Such a dark object would be radiating as copiously as the star which is hidden inside it, but the radiation would be in the far infrared, around 10 microns wavelength.

An extraterrestrial civilization grown to a certain population will need to harvest the full resources of it's environment. By converting a planet like Jupiter into a shield around the system's star, the civilization could maximize the use of the sun's output.

The mathematician Freeman Dyson, of the Institute for Advanced Study, offers a scheme in which the planet Jupiter is broken down piece by piece, transported to the distance of the Earth from the Sun, and reconstructed into a spherical shell – a swarm of individual fragments revolving about the Sun. The advantage of Dyson's proposal is that all of the sunlight now wasted by not falling upon an inhabited planet could then be gainfully employed; and a population greatly in excess of that which now inhabits the Earth could be maintained. Whether such a vast population is desirable is an important and unsolved question. But what seems clear is that at the present rate of technological progress it will be possible to construct such a Dyson sphere in perhaps some thousands of years. In that case, other civilizations older than we may have already constructed such spherical swarms.

A Dyson sphere absorbs visible light from the Sun. But it does not continue indefinitely to absorb this light without re-radiating; otherwise, the temperature would become impossibly high. The exterior of the Dyson sphere radiates infrared radiation into space. Because of the large dimensions of the sphere, the infrared flux from a Dyson sphere should be detectable over quite sizable distances – with present infrared technology, over distances of hundreds to thousands of light-years. Remarkably enough, large infrared objects of roughly Solar System dimensions and of temperatures less than 1,000 degrees Fahrenheit have been detected in recent years. These, of course, are not necessarily Dyson civilizations. They may be vast dust clouds surrounding stars in the process of formation. But we are beginning to detect objects that are not dissimilar to the artifacts of advanced civilizations.

A hypothetical sphere surrounding a star, harnessing all of its power.