Black holes and the speed of light

[cudomix]

Gravity is attraction between two bodies with mass. Gravity weakens with distance according to inverse square law. Speed needed to escape gravitational pull of an object is referred to as "escape velocity".

Event horizon of a black hole is a boundary where escape velocity is greater than the speed of light (nothing in our universe can go faster than speed of light, so it's commonly referred to as point of no escape, once inside event horizon of a black hole, you cannot get out because every path will lead to center of black hole). Schwarzschild radius: imagine that all mass of an object is located inside a sphere, and escape velocity from the surface of a sphere is equal to speed of light. Schwarzschild radius would be distance from the center of a sphere (radius of Sun is around 3 km, Earth's around a centimeter, in layman's terms, event horizon of a black hole with mass equal to Earth would be a two centimeters wide). Light orbiting black hole is called photon sphere.