Thursday, October 9, 2014

Karl Schwarzschild and the Event Horizon

Karl Schwarzschild
(1873 – 1916)
On October 9, 1873, German physicist and astronomer Karl Schwarzschild was born. He provided the first exact solution to the Einstein field equations of general relativity, for the limited case of a single spherical non-rotating mass, which he accomplished in 1915, the same year that Albert Einstein first introduced general relativity. The Schwarzschild solution leads to a derivation of the Schwarzschild radius, which is the size of the event horizon of a non-rotating black hole.

Karl Schwarzschild was born in Frankfurt (Main), Germany and it is assumed that his family was well situated. It has been delivered that Schwarzschild was kind of like a child prodigy, who published his first paper on celestial mechanics and constructed his own telescope before his 17th birthday. He began his career at the University of Strasbourg in 1891 where he received a decent education in practical astronomy. In Munich, Schwarzschild earned his doctorate in 1896 with a thesis on Henri Poincaré’s theories. [1,2]

Schwarzschild was then appointed as an assistant at the Von Kuffner Observatory in Ottakring, a suburb of Vienna, where he stayed until 1899. During his time in Vienna, Schwarzschild worked on ways to determine the apparent brightness of stars using photographic plates. While lecturing in Munich as Privatdozent, the scientist noticed a significant difference between the measured photographic magnitudes and the tabulated visual magnitudes. He concluded that the effect was due to the different colors of the stars and the range of magnitude change as measured by his photographic methods was greater than the range of change in visual magnitude. He further concluded that this effect is caused by the changes in surface temperature of the variable star through its cycle. [3]

In the 1910s, Schwarzschild was appointed professor at the prestigious University of Göttingen, where he met and worked with important contemporary scientists including David Hilbert and Hermann Minkowski. Also, Schwarzschild became director of the Göttingen observatory. The scientist was appointed director of the Astrophysical Observatory in Potsdam, the most prestigious post available for an astronomer in Germany at that time and joined the German army with the outbreak of World War II. Still, he somehow managed to write several papers in 1915, two on relativity theory and one on quantum theory. Schwarzschild is now credited with producing the first exact solutions to Albert Einstein’s field equations as well as the Schwarzschild metric. [2]

In Schwarzschild's solution, a radius for a given mass is identified, also known as the Schwarzschild radius. If the mass could be compressed to fit within that radius, no known force or degeneracy pressure could stop it from continuing to collapse into a gravitational singularity or black hole. This means that where the radius of the body is less than its Schwarzschild radius, everything falls inevitably into the central body. When the mass density of this central body exceeds a certain limit, it triggers a gravitational collapse to a Schwarzschild black hole, known as a non-charged, non-rotating black hole. However, the possibility of black holes were in general not accepted until the second half of the 20th Century, and Schwarzschild himself did not believe in the physical reality of black holes, believing his theoretical solution to be physically meaningless. [2]

At yovisto, you may be interested in a short video lecture on Black Holes.



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