John Michell (1783) and Pierre-Simon Laplace (1796) were the first people to propose the concept of “dark stars” or object which, if compressed into a sufficiently small size, would have an escape velocity which exceeded even the speed of light. Later, the term “frozen star” was used to describe the last phase of a star’s gravitational collapse, when light unable to escape from its surface would make the star appear frozen in time to an observer. In the 20th century, John Wheeler eventually coined the phrase “black hole” as the object would absorbs all the light that hits it while reflecting nothing back.

According to Einstein’s  General Theory of Relativity, any massive object actually distorts the space-time around it, including our Sun, Earth, or even us. A black hole is an extreme case in the sense that at its singularity the curvature of space-time becomes infinite, preventing even light to escape. The boundary beyond which light cannot escape the black hole’s gravity well is known as the event horizon, while its radius is called the Schwarzschild radius. Once particles and light-rays go past the event horizon their light cones “tip over” and point to the singularity, which now represents all future-directed paths with no escape possible.