4 minute read

I am currently reading the earliest SETI papers to get a better understanding of the core ideas in the field and how these have evolved. These are my notes on a report by Freeman Dyson published in Science in 1960. This paper first formalised the concept of searching for evidence of megastructures built by advanced extraterrestrial civilisations [1].

The structures proposed by Dyson later became known as Dyson spheres. A series of letters in a later issue of Science from the same year presented some criticisms and clarifications of the idea [2].

I am tracking all of the papers I’ve read and want to read on this topic in this roundup of the technosignatures literature.

Alternative strategies for SETI

The proposal by Cocconi and Morrison in 1959 [3] that we should search nearby sun-like stars for direct radio communications from extraterrestrial civilisations stimulated discussion on the topic and several other authors were quick to propose alternative strategies.

Extraterrestrial probes

In a paper published in May 1960, Bracewell argued that technologically advanced civilisations are more likely to be further away and would instead send probes to find and communicate with other intelligent life - we should search for these in our solar system [4].

Megastructures surrounding other stars

Freeman Dyson took another perspective in a paper published in Science in June 1960 [1]. He argued that a sufficiently advanced civilisation will expand beyond the resource limits of its home planet by disassembling other planets in its solar system and constructing a shell around its host star to form an artificial biosphere. This would give a huge surface area for habitation and for machinery that can harness a larger proportion of the star’s energy than is possible from a planet. Dyson apparently said he was inspired by the 1937 science fiction novel Star Maker by Olaf Stapledon, which I have also added to my reading list.

Dyson described a hypothetical artificial biosphere created using the mass of Jupiter: a 2-3 m thick shell located around the Sun at twice the distance to Earth. In the criticism of Dyson’s idea, John Maddox argues that this shell could neither be rigid enough to be stable nor supported by radiation pressure from the Sun [2]. Poul Anderson raised similar objections but stopped short of saying the spheres are impossible. Dyson responded to this with the clarification that indeed a solid sphere would be “mechanically impossible” and that he had instead envisaged a swarm of objects in orbit around the star [2].

Dyson estimated that the energy required to construct this shell would be equivalent to about 800 years of radiation from our sun. This project would therefore take thousands of years, given that any civilisation would also require energy for its continued growth on the planet. Anderson argued that no civilisation would have the patience for such an expensive and lengthy task and that any government that could enforce it would be more likely to implement population control instead [2]. Dyson counters that he believes there would be a gradual emergence of an artificial biosphere via a “proliferation of city-states” with no central enforcer required.

Searching for dark stars

Dyson proposed searching the skies for unusually dark stars - i.e. stars that are emitting in the far infrared. This, he argued, is because these artificial biospheres would absorb all of the host star’s radiation and re-emitt it at low frequency (i.e. as heat, similar to how the Sun’s radiation is re-emitted by Earth). He hypothesised that the surface temperature of these dark stars would appear to be 200-300 K. Radiation of this wavelength can pass through Earth’s atmosphere so would be detectable by ground-based telescopes.

Whole sky scan

Ideally, Dyson suggested we should search the entire sky for objects down to 5th or 6th magnitudes. This was not technically feasible at the time of his paper.

Far-IR anomalies

Dyson argued that visible stars may also present evidence of these artificial biospheres if the civilisation that created them were not able to enclose the entire star or if they enclose only one star in a muti-star system. He suggested looking at known binaries with invisible companions for unusually high infrared emissions.


Despite the criticims presented by other scientists (presumably because these were not as widely read), the idea of extraterrestrial civilisations constructing artificial biospheres in the form of a shell around their host stars became very popular and these hypothetical constructions were dubbed “Dyson spheres”. Dyson himself stated soon after his original paper that solid spheres would be impossible and reformulated his idea as a swarm. Variations on this idea have since been proposed and work has been done to search for them and I will be reading (and writing) a lot more about it.


  1. 1. Dyson FJ: Search for Artificial Stellar Sources of Infrared Radiation. Science 1960, 131:1667–1668.
  2. 2. Maddox J, Anderson P, Sloane EA, Dyson FJ: Artificial Biosphere. Science, New Series 1960, 132:250–253.
  3. 3. Cocconi G, Morrison P: Searching for Interstellar Communications. Nature 1959, 184:844–846.
  4. 4. Bracewell RN: Communications from Superior Galactic Communities. Nature 1960, 186:670–671.