Dark Stars (1783)

Einstein's General Relativity (1916)

Schwarzschild's Solution (1916)

Limits of Gravity (1930-1939)

Kerr's Solution (1963)

The Black Hole is Born (1968)

    The concept of black holes was first introduced over two centuries ago. In 1783, Reverend John Mitchell, an amateur British astronomer, proposed that gravity could affect light as well as matter. Mitchell showed that an object with the density of the sun, yet five hundred times larger would exert a gravitational pull so great that  "all light emitted from such a body would be made to return toward it." 

    In 1795, Pierre-Simon Laplace, a French physicist who independently reached the same conclusions, reasoned that: "it is therefore possible that the greatest luminous bodies in the universe are on this account invisible."

    Both Mitchell and Laplace believed that the escape velocity, the speed necessary to escape the star's gravity, for a sufficently large star would be greater than the speed of light. Because light could not escape from the gravitational pull, the dark star would appear invisible to an observer against the night sky. Both theories were based on Newton's theory of gravitation and corpuscular light.

    For many years afterward, the idea of an object with enough gravity to keep itself invisible was discarded by many scientists because in 1799 Thomas Young demonstrated that light acted as a wave. It would take another century before the idea of  black holes would come back into the light. 

Einstein's General Relativity (1916)

    In 1916, Albert Einstein published his ground breaking paper on the General Theory of Relativity. By including gravitation, this theory completed his previous 1905 paper on the Special Theory of Relativity, which set the speed of light as constant. General Relativity theorized that mass curves space and time and that gravity is a result of that curvature. 

    Einstein's General Relativity is mathematically expressed in a set of 10 extremely complex, coupled, nonlinear partial differential field equations. From these field equations one could predict the existence of black holes. 

Schwarzschild's Solution (1916)

    The modern concept of the black hole was introduced in 1916. Briefly following the release of Einstein's General Relativity, Karl Schwarzschild, a German physicist, discovered a mathematical solution to Einstein's field equations that described the gravitational field of a point mass while fighting for the German army in World War I. Schwarzschild died several months later from a rare disease contracted during the war.

    This solution, known as Schwarzschild Geometry, describes the space and time around any spherical mass including the distance from the center of a sphere at which light cannot escape. This distance is known as the Schwarzschild Radius (r_s). If the mass of an object is entirely inside the Schwarzschild Radius, we have a perfectly spherical, non-rotating black hole.

Limits of Gravity (1930-1939)

    For some time, black holes would remain only an incarnation of relativity. How could such an object actually exist in nature? The answer lies in the death of a star.

    In 1930,  Subrahmanyan Chandrasekhar, an Indian physicist, calculated the mass limit  (1.4 solar masses) of a star by which its gravitational collapse would be prevented by the exclusion principle for electrons and become a white dwarf.  A white dwarf with mass greater than this limit is unable to support itself against its own gravitational pull and crunches into a neutron star.

    In 1939, J.R. Oppenheimer, an American scientist, calculated that maximum mass limit of a neutron star is about 3.2 solar masses. Oppenheimer proposed that in a star with mass over this limit, gravity would be unopposed and the star would become a black hole. Oppenheimer would later abandon his work on black holes to help build the atomic bomb during World War 2.

Kerr's Solution (1963-1967)

    In 1963, Roy Kerr, a New Zealand physicist, found the solution to Einstein's field equations that describes space and time around spinning stars. Kerr's solution was later proven to actually describe the space and time around  spinning black holes and all black holes that exist in nature.

    In 1967, Werner Israel, a Canadian Scientist, showed that a non-rotating black hole must  be very simple, its size is based only on its mass. Whereas non-rotating black holes only have mass, rotating black holes should only have mass and rate of rotation as later introduced by Brandon Carter and proven by Steven Hawking. Because a black hole only has these properties regardless of what it formed from, we say that "A black hole has no hair."

The Black Hole is Born (1968)

    In a 1968 lecture to the American Astronomical Society, John Wheeler, an American scientist, coined the term "black hole." The phrase "black hole" conveyed a sense of mystery and darkness that raised interest about the subject. The black hole was born. 

    The idea of black holes had carried on for over two centuries based solely on the power of human imagination. Now physical evidence was needed to support the actual existence of black holes in our Universe. In 1972, with the advance of X-ray technology, that evidence was found in a binary star system, Cygnus X-1.