Featured Article Summary #191

A black hole is an extremely compact object whose gravity is so strong that even light cannot escape. Einstein’s general relativity says that if enough mass is packed into a small enough space, a black hole will form, with the boundary called the event horizon. When something crosses the event horizon, it becomes trapped, and general relativity also predicts a central singularity where spacetime curvature becomes infinite. Black holes were first considered as theoretical ideas, but later work showed they are a common result of general relativity, and the first widely accepted example was Cygnus X-1. They usually form from the collapse of massive stars and can grow by taking in matter, with supermassive black holes found at the centers of most galaxies. Scientists detect black holes by observing how they affect nearby matter and light, and by measuring gravitational waves from black hole mergers, such as Sagittarius A* in the Milky Way. In the late 1700s, John Michell and Pierre-Simon Laplace independently suggested that extremely massive stars could be so strong that even light would not escape. Michell calculated that a large star could have an escape speed greater than the speed of light and could be found by how its gravity affects nearby objects, while Laplace later proposed that a star might become invisible if it were big enough and provided a mathematical explanation after being asked. In 1905, Einstein showed that the laws of electromagnetism are the same for observers moving at different speeds, and in 1907 he proposed the equivalence principle, linking inertial and gravitational mass. He used it to predict effects of gravity on light, like gravitational redshift and lensing, though the lensing result was smaller than what the later theory would give. By 1915, Einstein developed general relativity, explaining how matter shapes spacetime and how that controls the motion of other matter, laying the foundation for black hole physics. Soon after Einstein introduced general relativity, Karl Schwarzschild and then Johannes Droste found the simplest solution describing a non-rotating, spherically symmetric star. In this solution, at a specific distance from the mass—the Schwarzschild radius—mathematical terms in the equations become infinite, though the meaning of this radius was unclear at the time. Many scientists doubted black holes; for example, Arthur Eddington questioned the idea in 1926, and in 1939 Einstein argued black holes could not form by assuming pressure or rotation could stop collapse, overlooking that further implosion could push past the critical radius. By the 1920s, astronomers found that some white dwarfs were too dense to be explained by normal star cooling and thermal pressure. Ralph Fowler and later Chandrasekhar showed that quantum “degeneracy pressure” from electrons can balance gravity only up to a limit, explaining why white dwarfs have a maximum mass. Work by Oppenheimer and Volkoff extended this idea to neutron stars using general relativity, leading to another mass limit (the TOV limit); later calculations confirmed that the limit stands, so the only way to avoid collapse into a black hole is for a star to dynamically lose enough mass as it cools.