Tiger Yu-Yang Hsaio(NTHU); Tomotsugu Goto(NTHU)
The search for extraterrestrial intelligence (SETI) has been conducted for nearly 60 years. A Dyson Sphere, a spherical structure that surrounds a star and transports its radiative energy outward as an energy source for an advanced civilization, is one of the main targets of SETI. In this study, we discuss whether building a Dyson Sphere around a black hole is effective. We consider five energy sources: (i) the cosmic microwave background, (ii) the Hawking radiation, (iii) an accretion disk, (iv) a corona, and (v) relativistic jets. To develop future civilizations (for example, a Type II civilization), 4*10^26 W is expected to be needed. In cases (i) and (ii), the collectable energy would be ~10^10 Wand 10^-28 W, respectively, for a stellar-mass black hole. These are too small to maintain the civilization. Among (iii) to (v), the largest luminosity can be collected from an accretion disk, reaching 10^5 solar luminosity, enough to maintain a Type II civilization. Moreover, if a Dyson Sphere collects not only the electromagnetic radiation but also other types of energy (e.g., kinetic energy) from the jets in a black hole system, the total collected energy would be approximately 5 times larger. Considering the emission from a Dyson Sphere, our results show that the Dyson Sphere around a stellar-mass black hole in the Milky Way 10kpc is detectable in the ultraviolet, optical, near-infrared, and mid-infrared wavelengths via the waste heat radiation using current telescopes such as the Hubble Space Telescope.