New paper argues we should look for smaller, hotter Dyson spheres

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Artist’s impression of the Dyson Sphere, a proposed alien megastructure that is the target of SETI surveys. Credit: Breakthrough Listen/Daniel Futselaar

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Artist’s impression of the Dyson Sphere, a proposed alien megastructure that is the target of SETI surveys. Credit: Breakthrough Listen/Daniel Futselaar

In 1960, renowned physicist Freeman Dyson published his seminal paper “The Search for Artificial Stellar Sources of Infrared Radiation”, in which he proposed that extraterrestrial civilizations might be so advanced that they could build megastructures large enough to surround their parent star. Can build.

They also indicated that these “Dyson spheres”, as they are known, could be detected based on the “waste heat” emitted at mid-infrared wavelengths. To date, infrared signatures are considered a viable technical signature in the search for extraterrestrial intelligence (SETI).

So far, attempts to detect Dyson spheres (and their variations) by their “waste heat” signatures have come up empty, leading some scientists to recommend changes in search parameters. In a new paper posted on the preprint server arXivJason T. Wright, professor of astronomy and astrophysics at the Center for Exoplanets and Habitable Worlds and the Penn State Center for Extraterrestrial Intelligence (PSTI), suggests that SETI researchers refine the search by looking for signs of activity. In other words, he recommends looking for Dyson spheres based on what they might be used for, rather than just their heat signature.

Key to Wright’s study is the Landsberg limit, a concept in thermodynamics that represents the theoretical efficiency limit for harvesting solar radiation. This is significant because Dyson’s original proposal was largely based on the idea that all life exploits free energy gradients such as photosynthetic life forms which depend on it to produce oxygen gas and organic nutrients. He further argued that technologically advanced life could use and harness greater amounts of this energy. However, there is an absolute limit to this capacity: the total energy emitted by a star (visible light, infrared, ultraviolet, etc.).

Because energy must be conserved, Freeman Dyson argued that some of this energy must be expelled from the Dyson structure as waste heat. Taking advantage of advances in infrared astronomy, a growing field in Dyson’s time, astronomers could theoretically measure the energy used by an advanced civilization by looking for this heat. To date, only three all-sky mid-infrared studies have been conducted, including the Infrared Astronomical Satellite (IRAS), the Wide-field Infrared Survey Explorer (WISE), and AKARI.

“Traditionally, we look for infrared emission from stars to see if they have orbital material hotter than the starlight,” Wright told Universe Today via email. “If it’s not the kind of star that typically has material orbiting it, we can look more closely to see if the material looks like dust or something else.” However, all discoveries made to date have been hampered to some extent by the fact that there is no underlying theory of what waste heat will look like because the properties of the materials in the Dyson sphere are unknown.

Several theoretical models have been proposed by astrophysicists (including Wright himself) for how their thermal signatures might look, but these are quite simple and based on a number of assumptions. These include the spherical symmetry of the shell and its orbital distance from the star, while failing to predict the specific temperature, radiative interactions, or optical depth of the material. This leads to another important concept considered by Wright, which concerns the purpose of the Dyson structure (what “function” does it do?), from which predictions can be made about its physical properties.

Dyson acknowledged that capturing a star’s energy was only one possible motivation for building such a megastructure. For example, several SETI researchers have proposed that the Dyson structure could be used as a stellar engine that could move stars (a Shkadov thruster) or as a giant supercomputer (a Matryoshka brain). In. Like its name, the Matryoshka brain has a nested structure, where the inner layer directly absorbs sunlight and the outer layers exploit waste heat from the inner layer to optimize computational efficiency.

Additionally, Wright addressed the engineering challenges of building such a structure. While Dyson focused on the laws of physics as the sole basis for the megastructure’s existence, Wright also considered the engineering practicalities involved. From this, he ventured that civilization could be induced to gradually build up sections of a sphere around a star to gradually increase its habitable volume. With all this in mind, Wright applied the thermodynamics of radiation to Dyson spheres as calculating machines and what the observable results would be.

They concluded that there is little or no advantage in building nesting shells and that optimal use of mass would lead to smaller, warmer Dyson spheres. Furthermore, he indicated that there would be observable differences between “complete” Dyson spheres (completely assembled around a star) and those still in progress. As Wright explained:

“Contrary to the expectations of some authors that Dyson spheres would be extremely large and cold to maximize their efficiency, I think that for a given mass budget, the optimal configuration is actually for much smaller, hotter spheres than most Catch but not all the light which survives. [W]We can expand our search parameters to temperatures above 300K (slightly hotter than Earth) because closer to the star, where things are hotter, the task of starlight extraction is more efficient.”

These findings may help inform future searches for Dyson structures, which are unfortunately limited at this time. A notable exception is the Astrophysics Ph.D. Is the work of. Student Mathias Suazo (Uppsala University) and his colleagues in Project Hephaistos. Suazo presented his work as part of the second annual Penn State SETI Symposium in June, where he described how project scientists worked with ESA’s Gaia Observatory, the Two Micron All Sky Survey (2MASS), and NASA’s Wide-field Infrared Survey Combined data from Explorer. (WISE) to limit the search for thermal signatures that may indicate the presence of megastructures.

The combined data revealed approximately 5 million potential candidates in a volume ~1,000 light-years in diameter. After creating a “best fit” model based on temperature and brightness profiles that eliminated possible natural sources, Suazo and his team narrowed down the list to 20 viable candidates. These sources will likely be subject to follow-up observations by next generation telescopes in the near future. In the meantime, the search continues, and although it has not yielded any definitive evidence of a megastructure, the possibility remains.

As Dyson famously said when addressing possible motivations for such engineering. “My rule is, there is nothing so big or so crazy that not one in a million technological societies would feel motivated to do it, provided it was physically possible.” If only a handful of advanced civilizations commit to mega-engineering projects in our galaxy, sooner or later we will run them out.

more information:
Jason T. Wright, Application of radiation thermodynamics to Dyson spheres as work extractors and computational engines, and their observational consequences, arXiv (2023). DOI: 10.48550/arxiv.2309.06564

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