A Dyson Sphere around a black hole (2106.15181v2)

Abstract: 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 civilisation, 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 six energy sources: (i) the cosmic microwave background, (ii) the Hawking radiation, (iii) an accretion disk, (iv) Bondi accretion, (v) a corona, and (vi) relativistic jets. To develop future civilisations (for example, a Type II civilisation), $4\times10^{26}\,{\rm W}$($1\,{\rm L_{\odot}}$) is expected to be needed. Among (iii) to (vi), the largest luminosity can be collected from an accretion disk, reaching $10^{5}\,{\rm L_{\odot}}$, enough to maintain a Type II civilisation. Moreover, if a Dyson Sphere collects not only the electromagnetic radiation but also other types of energy (e.g., kinetic energy) from the jets, 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 ($10\,\rm kpc$ away from us) is detectable in the ultraviolet$(\rm 10-400\,{\rm nm)}$, optical$(\rm 400-760\,{\rm nm)}$, near-infrared($\rm 760\,{\rm nm}-5\,{\rm \mu m}$), and mid-infrared($\rm 5-40\,{\rm \mu m}$) wavelengths via the waste heat radiation using current telescopes such as Galaxy Evolution Explorer Ultraviolet Sky Surveys. Performing model fitting to observed spectral energy distributions and measuring the variability of radial velocity may help us to identify these possible artificial structures.

• The paper quantifies energy extraction from black holes by evaluating six sources, emphasizing that accretion disks produce orders of magnitude more energy than typical stars.
• It demonstrates that relativistic jets, with their electromagnetic and kinetic energy, provide a viable alternative energy source for advanced civilizations.
• The study redefines SETI strategies by suggesting that anomalies from black hole waste heat emissions could indicate engineered structures around these exotic objects.

Analysis of "A Dyson Sphere around a Black Hole"

The paper "A Dyson Sphere around a Black Hole" by Hsiao et al. presents a speculative yet quantitatively backed exploration of the concept of constructing a Dyson Sphere not around a star, as famously proposed by Freeman Dyson, but around a black hole. This paper explores the feasibility of various energy sources associated with black holes that could potentially sustain a Type II civilization according to the Kardashev scale.

Summary and Key Findings

The authors explore six potential energy sources from black holes: cosmic microwave background (CMB), Hawking radiation, accretion disks, Bondi accretion, coronal emissions, and relativistic jets. Among these, it is revealed that while the theoretical frameworks of Dyson Spheres have been primarily linked with stars, black holes' accretion disks and jets also present exceptionally high energy outputs that could theoretically support an advanced civilization:

1. Accretion Disks: The paper highlights accretion disks as a prolific energy source. For stellar-mass black holes, the energy harnessed could be several orders of magnitude higher than the luminosity of a typical star. This makes accretion disks a substantial focus for energy extraction in this context.
2. Relativistic Jets: Relativistic jets offer significant energetic potential, with the possibility to harness both their electromagnetic radiation and kinetic energy. This suggests that with the appropriate technology, jets could provide a vast energy resource for sustaining advanced civilizations.
3. Coronal Emissions: The hot corona surrounding active black holes emits radiation that accounts for roughly one-third to half of the disk's luminosity. This could serve as a supplementary energy source beyond the primary accretion disk radiation.
4. Bondi Accretion and Hawking Radiation: These sources were deemed inefficient due to their relatively low energy outputs, particularly Hawking radiation, which is negligible for any feasible structural utilization at the scales required by a Type II civilization.
5. Cosmic Microwave Background: In both the current and early Universe, using the CMB as an energy source is impractical due to the low energy densities available.

Implications and Future Directions

The hypothetical constructs discussed in the paper emphasize a need to broaden the potential targets for SETI initiatives, introducing black holes as entities of interest. This shifts the perspective from traditional stellar observations to include exotic astrophysical phenomena in the search for advanced civilizations.

The concept of a Dyson Sphere around a black hole presents intriguing opportunities for discussions related to the evolution of energy needs and technological capabilities of hypothetical extraterrestrial societies. It suggests that if such civilizations exist, they might exploit high-energy astrophysical phenomena, providing a basis for identifying signatures of such advanced engineering via waste heat emissions detectable in the infrared spectrum.

For future research, advancing theoretical models that integrate black hole dynamics and energy extraction mechanisms will be essential. Additionally, diversifying observational strategies to detect anomalies associated with potential artificial structures around black holes could open a new frontier in astrobiology and astrophysics.

This paper underscores the need for interdisciplinary approaches that synthesize astrophysics, engineering, and theoretical conceptualizations of extraterrestrial life, challenging and expanding the boundaries of current scientific inquiries on cosmic-scale engineering and civilization survival strategies.