Einstein used his own theory to argue that black holes were impossible
In 1939, Albert Einstein published a calculation meant to show that black holes could not exist. He assumed pressure or spin would always halt a collapsing star before it shrank too far. He overlooked the possibility that an implosion could simply overpower both. That same year, Robert Oppenheimer and Hartland Snyder described exactly such a collapse.
The notion of a star too heavy for light to escape is surprisingly old. In 1784 the English clergyman John Michell calculated that a star as dense as the Sun but 500 times wider would trap its own light, and suggested such dark bodies might betray themselves by tugging on visible neighbours. Pierre-Simon Laplace floated a similar idea independently. Both imagined giant stars rather than the ultra-dense objects we recognise today.
Einstein's general relativity, completed in 1915, turned speculation into mathematics. Within months, Karl Schwarzschild found a solution for a non-spinning sphere that became singular at a certain radius, though nobody then grasped what that meant. Sceptics abounded: Arthur Eddington called the idea a flaw in an immature theory. Oppenheimer and Snyder's model showed that, seen from far away, a collapsing star appears to slow down and freeze at that critical radius. In 1958 David Finkelstein identified it as an event horizon, a one-way membrane that lets things in but lets no influence out.
The mid-1960s to mid-1970s became a golden age for the subject. Roy Kerr found the solution for a spinning black hole in 1963, and work by several groups established what John Wheeler summed up as black holes having no hair: a settled black hole is fully described by just its mass, spin and electric charge. Cygnus X-1, an X-ray source studied between 1971 and 1974, became the first widely accepted example.
Black holes usually form when massive stars collapse at the end of their lives, and supermassive versions sit at the centres of most galaxies. Ours, Sagittarius A*, holds about 4.3 million solar masses. Although invisible themselves, they reveal their presence through glowing disks of infalling gas, through the orbits of nearby stars and through gravitational waves released when two merge. Theory predicts they slowly leak Hawking radiation, but even the smallest known ones currently gain more from the cosmic microwave background than they lose.
Source: Black hole
