Light and general relativity
In 1906, the patent office promoted Einstein to Technical Examiner Second Class, but he was not giving up on academia. In 1908, he became a
privatdozent at the
University of Bern. In 1910, he wrote a paper on
critical opalescence that described the cumulative effect of light scattered by individual molecules in the atmosphere, i.e.
why the sky is blue.
During 1909, Einstein published "Über die Entwicklung unserer Anschauungen über das Wesen und die Konstitution der Strahlung" ("
The Development of Our Views on the Composition and Essence of Radiation"), on the
quantization of light. In this and in an earlier 1909 paper, Einstein showed that
Max Planck's energy quanta must have well-defined
momenta and act in some respects as independent,
point-like particles. This paper introduced the
photon concept (although the term itself was introduced by
Gilbert N. Lewis in 1926) and inspired the notion of
wave–particle duality in
quantum mechanics.
In 1911, Einstein became an
associate professor at the
University of Zurich. However, shortly afterward, he accepted a full professorship at the
Charles University of Prague. While in
Prague, Einstein published a paper about the effects of gravity on light, specifically the
gravitational redshift and the gravitational deflection of light. The paper appealed to astronomers to find ways of detecting the deflection during a
solar eclipse. German astronomer
Erwin Freundlich publicized Einstein's challenge to scientists around the world.
In 1912, Einstein returned to Switzerland to accept a professorship at his
alma mater, the
ETH. There he met mathematician
Marcel Grossmann who introduced him to
Riemannian geometry, and at the recommendation of Italian mathematician
Tullio Levi-Civita, Einstein began exploring the usefulness of
general covariance (essentially the use of
tensors) for his gravitational theory. Although for a while Einstein thought that there were problems with that approach, he later returned to it and by late 1915 had published his
general theory of relativity in the form that is still used today . This theory explains gravitation as distortion of the structure of
spacetime by matter, affecting the
inertial motion of other matter.
After many relocations, Mileva established a permanent home with the children in Zurich in 1914, just before the start of
World War I. Einstein continued on alone to
Berlin, where he became a member of the
Prussian Academy of Sciences. As part of the arrangements for his new position, he also became a professor at the
University of Berlin, although with a special clause freeing him from most teaching obligations. From 1914 to 1932 he was also director of the
Kaiser Wilhelm Institute for physics.
During World War I, the speeches and writings of
Central Powers scientists were available only to Central Powers academics, for
national security reasons. Some of Einstein's work did reach the United Kingdom and the United States through the efforts of the Austrian
Paul Ehrenfest and physicists in the Netherlands, especially 1902 Nobel Prize-winner
Hendrik Lorentz and
Willem de Sitter of the
Leiden University. After the war ended, Einstein maintained his relationship with the Leiden University, accepting a contract as an
Extraordinary Professor; he travelled to Holland regularly to lecture there between 1920 and 1930.
In 1917, Einstein published an article in
Physikalische Zeitschrift that proposed the possibility of
stimulated emission, the physical process that makes possible the
maser and the
laser . He also published a paper introducing a new notion, a
cosmological constant, into the general theory of relativity in an attempt to model the behavior of the entire universe .
1917 was the year astronomers began taking Einstein up on his 1911 challenge from Prague. The
Mount Wilson Observatory in California, U.S., published a solar
spectroscopic analysis that showed no gravitational redshift. In 1918, the
Lick Observatory, also in California, announced that they too had disproven Einstein's prediction, although their findings were not published
However, in May 1919, a team led by British astronomer
Arthur Stanley Eddington claimed to have confirmed Einstein's prediction of
gravitational deflection of starlight by the Sun while photographing a solar eclipse in
Sobral northern
Brazil and
Principe. On
November 7, 1919, leading British newspaper
The Times printed a banner headline that read: "Revolution in Science – New Theory of the Universe – Newtonian Ideas Overthrown". In an interview Nobel laureate
Max Born praised general relativity as the "greatest feat of human thinking about nature"; fellow laureate
Paul Dirac was quoted saying it was "probably the greatest scientific discovery ever made".
In their excitement, the world media made Albert Einstein world-famous. Ironically, later examination of the photographs taken on the Eddington expedition showed that the experimental uncertainty was of about the same magnitude as the effect Eddington claimed to have demonstrated, and in 1962 a British expedition concluded that the method used was inherently unreliable. The deflection of light during a solar eclipse has, however, been more accurately measured (and confirmed) by later observations.
There was some resentment toward the newcomer Einstein's fame in the scientific community, notably among German physicists, who later started the
Deutsche Physik (German Physics) movement.
Having lived apart for five years, Einstein and Mileva divorced on
February 14, 1919. On
June 2 of that year, Einstein married
Elsa Löwenthal, who had nursed him through an illness. Elsa was Albert's
first cousin (maternally) and his
second cousin (paternally). Together the Einsteins raised Margot and Ilse, Elsa's daughters from her first marriage.