If there are superadvanced civilizations out there in the nearby universe, they’re hiding themselves pretty well. So concludes an astronomer in the Netherlands who looked at a sample of galaxies that shine unusually brightly at midinfrared wavelengths—a sign that they may harbor a so-called Kardashev type III civilization, one that has the technology to harvest energy from stars across an entire galaxy.
Russian astronomer Nikolai Kardashev proposed in the 1960s grading civilizations by the energy they used: the output of their home planet, their home star, or their home galaxy. A type III, galaxy-wide civilization could hypothetically surround all stars in energy-harvesting “Dyson spheres” (artist's representation above) but these would nevertheless leak a lot of waste heat in the midinfrared. A U.S. team last year drew up a list of several hundred bright midinfrared candidates from 100,000 local galaxies.
But the new study, to be published this week in Astronomy & Astrophysics, compared the midinfrared output from 93 of those galaxies with their emission at radio wavelengths. Most of these measurements followed a rule called the midinfrared radio correlation, which applies to almost all galaxies.
So, the study concludes, the midinfrared brightness of most of the sample galaxies probably comes from natural processes, such as dust clouds heated by regions of active star formation.