https://physics.stackexchange.com/q/735441/276316 https://physics.stackexchange.com/q/736708/276316 https://physics.stackexchange.com/q/3436/276316 https://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_fast.html https://math.ucr.edu/home/baez/physics/index.html