In 2024, we are celebrating the 50th anniversary of Hawking's groundbreaking formula, a landmark in scientific theory that uncovers the remarkable nature of black holes. When Hawking passed away in March 2018 at the age of 76, his wish was honored, and the formula was inscribed on his tombstone in Westminster Abbey. He donated his office and personal belongings to the nation instead of paying inheritance tax. While sorting through Hawking's possessions, my colleagues at the Science Museum in London discovered the profound impact of the formula, which appeared in his papers, written bets, keepsakes, and even a silver beaker presented to him by the producers of the 2015 Hollywood movie "The Theory of Everything."
The idea behind black holes, which are the focus of this notable equation, was contemplated by theorists long before any tangible proof was found. In 1783, John Michell, a parson in Thornhill near Leeds, speculated about 'dark' stars using Sir Isaac Newton's principles. Newton regarded gravity as a force and light as particle-based. Michell suggested that light particles from a star would slow down due to the star's gravity, akin to how a bullet decelerates when fired into the sky from Earth. If the star’s gravity was intense enough, it would pull the light back to the surface. Although Michell's concept hinted at black holes, he was incorrect in crucial aspects. Albert Einstein’s 1915 theory of general relativity, which posits that the speed of light is constant, redefined gravity not as a force but as a distortion of spacetime, combining space and time. Earth, for example, bends the Universe in this manner, causing satellites to orbit along these curves, which we interpret as gravity.
Soon after Einstein released his theory, Karl Schwarzschild, a German artillery officer and physicist, used Einstein’s equations to propose that a mass could warp spacetime so severely that it would become invisible. However, his conclusions did not gain much recognition. In 1939, American theoretical physicist J. Robert Oppenheimer, who would later become famous for his role in developing the atomic bomb, alongside Hartland Snyder, showed how a spherical dust cloud could collapse into a region from which light could not escape. Their work did not immediately persuade their contemporaries, though astronomers eventually found evidence of extremely dense objects.