The Most Advanced Alien Civilizations Might Run Their Computers Near Absolute Zero, Far From Their Star. A Manchester Astronomer Says That Would Leave a 'Slysh Halo' We Could Already Have on Disk.

For six decades, searches for alien megastructures have looked for the same thing: a star whose light is being partly swallowed by machinery and re-emitted as heat, glowing in the mid-infrared at a few hundred degrees above absolute zero. A new paper argues that the most advanced civilizations would not look like that at all, and that astronomers may already have the data needed to find them.

The paper, posted to the arXiv preprint server on Aug. 31 by Michael Garrett of the University of Manchester, Leiden University and the University of Malta, starts from a piece of physics called Landauer's principle. Every irreversible computation, every bit erased, releases a minimum amount of heat, and that minimum scales directly with temperature. A computer running at 10 kelvin is 30 times more efficient than an identical one at room temperature, about 300 kelvin. A civilization whose energy budget is dominated by computation, Garrett argues, would therefore have every incentive to push its hardware as cold as physics allows.

The coldest place in any planetary system is its outer edge, where the ambient temperature approaches the 2.7-kelvin floor set by the cosmic microwave background. Garrett's proposal is that a mature civilization would scatter its computers in a wide, diffuse shell far from its star, close enough to collect starlight for power, far enough to radiate waste heat into deep space at somewhere between 5 and 30 kelvin. He calls the resulting structure a Slysh halo, after the Soviet radio astronomer V. I. Slysh, who argued in the 1980s that searches for Dyson spheres should look for cold ones.

A Slysh halo would be a partial Dyson swarm with a distinctive signature. Its emission would peak in the far infrared and submillimeter, not the mid-infrared where every previous megastructure search has concentrated, and it would be grey and line-free, thermal radiation with none of the spectral features of dust or gas. Because the area needed to radiate a given amount of heat scales as the inverse fourth power of temperature, a cold halo would have to be enormous, which makes it potentially conspicuous despite being faint.

The main confounder is that many stars already have cold halos of natural debris, like the sun's Kuiper belt and Oort cloud, which also glow in the submillimeter. Garrett assembles six observational tests to separate engineered radiators from natural ones, including the shape of the spectrum, the lack of emission lines and the geometry of the source. He argues that M dwarfs, the small, cool stars that make up most of the galaxy, are especially favorable targets, and that a fully built halo would be detectable from about 100 light-years away with existing instruments.

That last point is what makes the paper more than a thought experiment. Archival far-infrared and submillimeter surveys of nearby stars, including the Herschel DEBRIS and DUNES programs and the JCMT's SONS survey, can be reinterpreted to constrain cold waste heat at roughly the 10-to-the-20-watt level, several orders of magnitude below the luminosities that earlier infrared searches were sensitive to. Planck all-sky data, the James Clerk Maxwell Telescope and interferometers like ALMA and NOEMA offer more. Garrett lays out a three-tier search program that begins with data already on disk.

"Even a null result would provide the first temperature-complete assessment of Dysonian technosignatures," he writes. In other words, if nobody finds a Slysh halo, that is the first time anyone will have been able to say that the cold end of the spectrum is empty too.

The idea rests on an assumption, that computation would define an advanced civilization the way it increasingly defines ours, and on physics that is not in dispute. Whether anyone out there has taken the hint is a question the archives may be able to answer sooner than a new telescope could.