Karl Schwarzschild

Karl Schwarzschild (; 9 October 1873 – 11 May 1916) was a German physicist and astronomer.
Schwarzschild 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 Einstein first introduced general relativity. The Schwarzschild solution, which makes use of Schwarzschild coordinates and the Schwarzschild metric, leads to a derivation of the Schwarzschild radius, which is the size of the event horizon of a non-rotating black hole.
Schwarzschild accomplished this while serving in the German army during World War I. He died the following year from the autoimmune disease pemphigus, which he developed while at the Russian front. Various forms of the disease particularly affect people of Ashkenazi Jewish origin.
Asteroid 837 Schwarzschilda is named in his honour, as is the large crater "Schwarzschild", on the far side of the Moon.
Life.
Karl Schwarzschild was born on the 9th of October 1873 in Frankfurt on Main to Jewish parents. His father was active in the business community of the city, and the family had ancestors in the city dating back to the sixteenth century. The family owned two fabric stores in Frankfurt. One of his brothers is the painter Alfred Schwarzschild.  Karl attended a Jewish primary school until 11 years of age and then the Lessing-Gymnasium (as in secondary school). He received an all-encompassing education, including subjects like Latin, Ancient Greek, music and art, but developed a special interest in astronomy early on. In fact he was something of a child prodigy, having two papers on binary orbits (celestial mechanics) published before he was sixteen.
After Graduation in 1890 he attended the University of Strasbourg to study astrology.After 2 years he transferred to the Ludwig Maximilian University of Munich where he obtained his doctorate in 1896 for a work on Henri Poincaré's theories.
From 1897, he worked as assistant at the Kuffner Observatory in Vienna. His work here was dedicated towards photometry of star clusters and laid the foundations for a formula linking the intensity of the star light, exposure time, and the resulting contrast on a photographic plate. An integral part of that theory is the Schwarzschild exponent (astrophotography). In 1899 he returned to Munich to complete his Habilitation.
From 1901 until 1909 he was a professor at the prestigious institute at Göttingen, where he had the opportunity to work with some significant figures, including David Hilbert and Hermann Minkowski. Schwarzschild became the director of the observatory in Göttingen. He married Else Rosenbach, a great granddaughter of Friedrich Wöhler and daughter of a professor of surgery at Göttingen, in 1909. Later that year they moved to Potsdam, where he took up the post of director of the Astrophysical Observatory. This was then the most prestigious post available for an astronomer in Germany.
From 1912, Schwarzschild was a member of the Prussian Academy of Sciences.
At the outbreak of World War I in 1914 he joined the German army, despite being over 40 years old. Yet he volunteered for service. He served on both the western and eastern fronts specifically helping with ballistic calculations.Thereby he rose to the rank of lieutenant in the artillery.
While serving on the front in Russia in 1915, he began to suffer from a rare and painful autoimmune skin disease called pemphigus. Nevertheless, he managed to write three outstanding papers, two on the theory of relativity and one on quantum theory. His papers on relativity produced the first exact solutions to the Einstein field equations, and a minor modification of these results gives the well-known solution that now bears his name — the Schwarzschild metric.
In March 1916 Schwarzschild was cleared from service due to his sickness and returned to Göttingen. Two months later, on May 11, 1916, Schwarzschild's struggle with pemphigus probably eventually led to his death at the age of 42.
He rests in his family grave at the Stadtfriedhof Göttingen.
With his wife Else he had three children: Agathe Thornton (1910-2006) who emigrated to Great Britain in 1933. In 1946 she moved New Zealand where she became a Classics professor at the University of Otago in Dunedin; Martin who went on to become a professor of astronomy at Princeton University; and Alfred (1914-1944) who took his own life due to the persecution of Jews in the Holocaust.
Work.
Thousands of dissertations, articles, and books have since been devoted to the study of Schwarzschild's solutions to the Einstein field equations. However, although Schwarzschild's best known work lies in the area of general relativity, his research interests were extremely broad, including work in celestial mechanics, observational stellar photometry, quantum mechanics, instrumental astronomy, stellar structure, stellar statistics, Halley's comet, and spectroscopy.
Some of his particular achievements include measurements of variable stars, using photography, and the improvement of optical systems, through the perturbative investigation of geometrical aberrations.
Physics of photography.
While at Vienna in 1897, Schwarzschild developed a formula, now known as the Schwarzschild law, to calculate the optical density of photographic material. It involved an exponent now known as the Schwarzschild exponent, which is the formula_1 in the formula:
formula_2
(where formula_3 is optical density of exposed photographic emulsion, a function of formula_4, the intensity of the source being observed, and formula_5, the exposure time, with formula_1 a constant). This formula was important for enabling more accurate photographic measurements of the intensities of faint astronomical sources.
Electrodynamics.
According to Wolfgang Pauli (Theory of relativity), Schwarzschild is the first to introduce the correct Lagrangian formalism of the electromagnetic field as
formula_7
where formula_8 are the electric and magnetic field, formula_9 is the vector potential and formula_10 is the electric potential.
He also introduced a field free variational formulation of electrodynamics (also known as "action at distance" or "direct interparticle action") based only on the world line of particles as 
formula_11
where formula_12 are the world lines of the particle, formula_13 the (vectorial) arc element along the world line. Two points on two world lines contribute to the Lagrangian (are coupled) only if they are a zero Minkowskian distance (connected by a light ray), hence the term formula_14. The idea was further developed by Tetrode 
and Fokker in the 1920s and Wheeler and Feynman in the 1940s and constitutes an alternative/equivalent formulation of electrodynamics.
Relativity.
Einstein himself was pleasantly surprised to learn that the field equations admitted exact solutions, because of their "prima facie" complexity, and because he himself had only produced an approximate solution. Einstein's approximate solution was given in his famous 1915 article on the advance of the perihelion of Mercury. There, Einstein used rectangular coordinates to approximate the gravitational field around a spherically symmetric, non-rotating, non-charged mass. Schwarzschild, in contrast, chose a more elegant "polar-like" coordinate system and was able to produce an exact solution which he first set down in a letter to Einstein of 22 December 1915, written while Schwarzschild was serving in the war stationed on the Russian front. Schwarzschild concluded the letter by writing: "As you see, the war treated me kindly enough, in spite of the heavy gunfire, to allow me to get away from it all and take this walk in the land of your ideas." In 1916, Einstein wrote to Schwarzschild on this result:
Schwarzschild's second paper, which gives what is now known as the "Inner Schwarzschild solution" (in German: "innere Schwarzschild-Lösung"), is valid within a sphere of homogeneous and isotropic distributed molecules within a shell of radius r=R. It is applicable to solids; incompressible fluids; the sun and stars viewed as a quasi-isotropic heated gas; and any homogeneous and isotropic distributed gas.
Schwarzschild's first (spherically symmetric) solution "does not" contain a coordinate singularity on a surface that is now named after him. In Schwarzschild coordinates, this singularity lies on the sphere of points at a particular radius, called the Schwarzschild radius:
where "G" is the gravitational constant, "M" is the mass of the central body, and "c" is the speed of light in a vacuum. In cases where the radius of the central body is less than the Schwarzschild radius, formula_16 represents the radius within which all massive bodies, and even photons, must inevitably fall into the central body (ignoring quantum tunnelling effects near the boundary). When the mass density of this central body exceeds a particular limit, it triggers a gravitational collapse which, if it occurs with spherical symmetry, produces what is known as a Schwarzschild black hole. This occurs, for example, when the mass of a neutron star exceeds the Tolman–Oppenheimer–Volkoff limit (about three solar masses).
Cultural references.
Karl Schwarzschild appears as a character in the science fiction short story "Schwarzschild Radius" (1987) by Connie Willis.
Schwarzschild's Cat is a comic on XKCD.com comparing the size and cuteness of cats.
Works.
The scientific estate of Karl Schwarzschild is stored in a special collection of the Lower Saxony National- and University Library of Göttingen.