<P> On 11 February 2016, the LIGO collaboration announced the first observation of gravitational waves; because these waves were generated from a black hole merger it was the first ever direct detection of a binary black hole merger . On 15 June 2016, a second detection of a gravitational wave event from colliding black holes was announced . </P> <P> The idea of a body so massive that even light could not escape was briefly proposed by astronomical pioneer and English clergyman John Michell in a letter published in November 1784 . Michell's simplistic calculations assumed that such a body might have the same density as the Sun, and concluded that such a body would form when a star's diameter exceeds the Sun's by a factor of 500, and the surface escape velocity exceeds the usual speed of light . Michell correctly noted that such supermassive but non-radiating bodies might be detectable through their gravitational effects on nearby visible bodies . Scholars of the time were initially excited by the proposal that giant but invisible stars might be hiding in plain view, but enthusiasm dampened when the wavelike nature of light became apparent in the early nineteenth century . If light were a wave rather than a "corpuscle", it became unclear what, if any, influence gravity would have on escaping light waves . Modern relativity factually dispels Michell's notion of a light ray shooting directly from the surface of a supermassive star, being slowed down by the star's gravity, stopping, and then free - falling back to the star's surface . </P> <P> In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion . Only a few months later, Karl Schwarzschild found a solution to the Einstein field equations, which describes the gravitational field of a point mass and a spherical mass . A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties . This solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite . The nature of this surface was not quite understood at the time . In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates (see Eddington--Finkelstein coordinates), although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was a non-physical coordinate singularity . Arthur Eddington did however comment on the possibility of a star with mass compressed to the Schwarzschild radius in a 1926 book, noting that Einstein's theory allows us to rule out overly large densities for visible stars like Betelgeuse because "a star of 250 million km radius could not possibly have so high a density as the sun . Firstly, the force of gravitation would be so great that light would be unable to escape from it, the rays falling back to the star like a stone to the earth . Secondly, the red shift of the spectral lines would be so great that the spectrum would be shifted out of existence . Thirdly, the mass would produce so much curvature of the space - time metric that space would close up around the star, leaving us outside (i.e., nowhere)." </P> <P> In 1931, Subrahmanyan Chandrasekhar calculated, using special relativity, that a non-rotating body of electron - degenerate matter above a certain limiting mass (now called the Chandrasekhar limit at 1.4 M) has no stable solutions . His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse . They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star, which is itself stable because of the Pauli exclusion principle . But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately 3 M (the Tolman--Oppenheimer--Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes . </P>

What is the strength of a black hole