The point of no return
Inside every black hole lies a gravitational singularity, a one-dimensional point predicted by Albert Einstein's theory of general relativity. According to the equations, this point contains all the black hole's mass crushed into an infinitely small space, resulting in infinite density and an infinite curvature of spacetime. This isn't just a theoretical curiosity; the Penrose-Hawking singularity theorems, developed in the 1960s and 70s, showed that singularities are an unavoidable consequence of gravitational collapse under the known laws of general relativity. Roger Penrose's work on this was so foundational it earned him a share of the 2020 Nobel Prize in Physics.
The boundary defining a black hole is the event horizon, a spherical surface with a radius known as the Schwarzschild radius, first calculated by Karl Schwarzschild in 1916. This is not a physical surface, and is the point where the escape velocity equals the speed of light, approximately 299,792,458 meters per second. Once any object crosses this boundary, it cannot escape and is destined to reach the singularity. For an object with the mass of the Sun, the Schwarzschild radius is about 3 kilometers; for Earth, it's a mere 9 millimeters. Anything that falls past the event horizon is torn apart by extreme tidal forces in a process sometimes called "spaghettification" before reaching the center.