According to Levin, how can black holes be investigated?

• A. By observing their effect on surroundings
• B. By direct imaging the horizon
• C. By measuring Hawking radiation only
• D. By building a miniature black hole in the lab

Answer: (A)

Black holes reveal themselves through their gravitational influence on nearby matter, not by shining of their own light. Observing how they tug on surrounding stars and gas lets us infer the black hole’s mass and presence. For example, tracking the orbital motions of stars around an invisible, compact object tells us there is a black hole there. Gas in accretion disks gets violently heated as it spirals inward, producing X-rays we can detect and study to learn about the environment near the hole. Light from background sources can also be bent and distorted by the hole’s gravity, a telltale sign known as gravitational lensing. These indirect effects together provide reliable and practical means to investigate black holes.

Direct imaging of the horizon is extremely demanding and not routinely possible for most black holes, Hawking radiation from astrophysical black holes is far too weak to be a practical observational tool, and building miniature black holes in a lab isn’t something we can do with current technology. So the most informative approach is to study how black holes affect their surroundings.