Black Holes

The concept of a black hole is a direct solution to the Einstein Field Equations. Shortly after Einstein published his theory, Karl Schwarzschild found the first exact solution for the gravitational field of a non-rotating, spherically symmetric mass. This solution featured a characteristic radius, now known as the Schwarzschild radius, at which the equations exhibit a coordinate singularity. This radius defines the event horizon, the point of no return. Any object crossing it is inevitably pulled towards the central singularity, a point of theoretically infinite density where the known laws of physics cease to apply. The ‘no-hair theorem’ posits that a stable black hole is completely characterized by only three properties: its mass, angular momentum (spin), and electric charge.

For a long time, black holes were considered a mathematical curiosity. However, astronomical evidence has since confirmed their existence. Stellar-mass black holes form when very massive stars exhaust their nuclear fuel and collapse. Supermassive black holes, with masses millions to billions of times that of our Sun, are found at the centers of most large galaxies, including our own Milky Way (Sagittarius A*). While black holes are invisible, their presence is inferred through their interaction with other matter, such as the orbits of nearby stars or the intense X-ray radiation from matter heated in an accretion disk before falling in.