A black hole is a region of spacetime where gravity is so intense that nothing—not even light—can escape. According to Albert Einstein's theory of general relativity, a sufficiently compact mass can deform spacetime to form a black hole. At its boundary is the event horizon, a point of no return. Once matter or energy crosses the event horizon, it is destined to fall toward the center. At the heart of a non-rotating black hole lies a gravitational singularity, a one-dimensional point where spacetime curvature and density become infinite, and the laws of physics as we know them break down.
The concept of an object so massive that light could not escape was first proposed by English clergyman and natural philosopher John Michell in 1783. The theoretical basis for black holes was established by Albert Einstein's theory of general relativity in 1915. Shortly after, in 1916, Karl Schwarzschild found the first exact solution to Einstein's field equations that characterized a black hole. However, the term 'black hole' was not coined until 1967 by physicist John Archibald Wheeler. The first physical black hole candidate, Cygnus X-1, was identified in 1971. In 2019, the Event Horizon Telescope collaboration captured the first-ever direct image of a black hole and its shadow at the center of the galaxy Messier 87.
Black holes are a major focus of modern astrophysics and cosmology. They are believed to be the final stage in the evolution of massive stars. Supermassive black holes, millions to billions of times the mass of our Sun, are found at the center of most large galaxies, including our own Milky Way (Sagittarius A*), where they play a crucial role in galaxy formation and evolution. The study of black holes pushes the boundaries of theoretical physics, providing a testing ground for general relativity in extreme conditions. Phenomena like gravitational waves, first detected in 2015 from merging black holes, have opened a new window into observing the universe. Furthermore, black holes are central to unresolved questions in physics, such as the information paradox, which explores the conflict between quantum mechanics and general relativity.