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For centuries, humans have gazed up and wondered if we are alone in the universe. In 1960, physicist Freeman Dyson changed the conversation by suggesting some cool celestial tech that would allow us to more easily detect signs of alien civilisations.
Dyson suggested that if an alien society’s energy needs outstripped the supply of its planet, it could build a megastructure known as a ‘Dyson sphere’ around its host star to harness the power on a massive scale. Not technically a sphere, this structure would be composed of a fleet of orbiting or stationary satellites able to transform solar energy into usable energy.
This would make it easier for us to find such civilisations, Dyson argued, as the process could create waste heat and therefore abnormal infrared signals.
Now, in a new paper published in the Monthly Notices of the Royal Astronomical Society, scientists say these spheres could be even more ambitious – aliens could build them around black holes, too.
The research team, led by astronomer Tiger Yu-Yang Hsiao of National Tsing Hua University in Taiwan, delved into the physics behind some excitingly high-concept questions: How would a Dyson sphere around a black hole work? How much energy could it gather, and for what type of alien society? Could we detect such a structure from Earth?
In particular, the team looked at hypothetical technologically advanced alien civilisations (Type II or III, according to the Kardashev scale).
‘They need a more powerful energy source than their own sun,’ the researchers write in the study.
Of course, nothing escapes the monstrous gravitational pull of a black hole, but the team considered energy-intense processes beyond the event horizon – where a super-hot disc of matter swirls around the black hole like water around a cosmic drain.
By looking at models of various-sized black holes (from a little more than our sun’s mass right up to the mass of the supermassive monster at the heart of our galaxy), they found that a sphere of satellites could effectively gobble up energy from many of these processes.
‘Our results suggest that for a stellar-mass black hole… the accretion disc could provide hundreds of times more luminosity than a main sequence star,’ they write.
For an even bigger black hole of 20 solar masses, they report it could provide the same amount of power as Dyson spheres around 100,000 normal stars – and the number soars to one million for a supermassive black hole.
And that’s just the power harnessed from the accretion disc.
‘Moreover, if a Dyson sphere collects not only the electromagnetic radiation but also other types of energy (e.g., kinetic energy) from the [relativistic] jets, the total collected energy would be approximately five times larger,’ the researchers write.
But would these Dyson spheres be detectable by earthly technology?
Hsiao and the team calculate that if this tech existed around a stellar-mass black hole within our galaxy, we could spot its “waste” heat at ultraviolet, optical and infrared wavelengths. It could be detectable by current telescopes such as Hubble, or large surveys such as the Sloan Digital Sky Survey or the Wide-field Infrared Survey Explorer.
But the team cautions that since black holes emit a lot of radiation, signals from a Dyson sphere would risk being lost in the noise.
They suggest observations could be confirmed via the radial velocity method, currently used to detect exoplanets by spotting the minute gravitational wobble of their stars.
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Lauren Fuge is a science journalist at Cosmos. She holds a BSc in physics from the University of Adelaide and a BA in English and creative writing from Flinders University.