In 1784 John Michell suggested that there might exist dark stars, whose gravitation was so strong that light could not escape from them and they would therefore be dark. He noted that a body whose density is not less than the Sun’s and whose diameter is more than 500 times than the Sun’s, would have an escape velocity greater than the speed of light.
In 1916 Karl Schwarzschild (1873-1916) worked out the first exact solutions to the relativity field equations. The calculations showed that for any mass (approximating to a star) there is a critical radius, now called the Schwarzschild radius, and if this mass is squeezed inside the critical radius it will cease to radiate energy, i.e. it would become a black hole. For the Sun, the Schwarzschild radius is 2.9 km; for Earth, it is 0.88 cm. Theoretical studies show that a black hole is defined by three properties – its mass, its charge and its rotation (angular momentum). An uncharged, non-rotating black hole is known as a Schwarzschild black hole.
A singularity is a place where the laws of physics as we know them break down and certain physical quantities reach infinite values. In 1965 Roger Penrose (1931- ) was able to prove that anything that falls into a non-rotating black hole must be crushed into the singularity at the centre of the black hole. With Stephen William Hawking (1942-2018) he worked on gravitational singularities theorems in the framework of general relativity, and the theoretical prediction that black holes emit radiation, often called the Hawking radiation.
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