The Odd Couple: Quasars and Black Holes | Institute for Advanced Study
https://www.ias.edu/ideas/2015/tremaine-quasars
Black holes are among the strangest predictions of Einstein’s general theory of relativity: regions of spacetime in which gravity is so strong
that nothing—not even light—can escape. More precisely, a black hole is a singularity in spacetime surrounded by an event horizon, a surface
that acts as a perfect one-way membrane: matter and radiation can enter the event horizon, but, once inside, can never escape. Remarkably,
an isolated, uncharged black hole is completely characterized by only two parameters: its mass, and its spin or angular momentum.