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. In the interval 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.[19]

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 a well-defined momentum and act in some respects as independent, point-like particles. This paper is the introduction of the "photon" concept (although the term itself was introduced by Gilbert N. Lewis in 1926). Even more importantly, Einstein showed that light must be simultaneously a wave and a particle.

In 1911, Einstein became an associate professor at the Universität Zürich, however shortly afterward he accepted a full professorship at the Univerzita Karlova in Prague, Czechloslovakia. While in Prague, Einstein published a paper challenging astronomers to test a prediction of his nascent theory of relativity: that gravity affected even light, producing a "redshift" that should be measurable during a solar eclipse. German astronomer Erwin Freundlich publicized Einstein's challenge to scientists around the world.[20]

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. It was at this time that Einstein began to refer to time as the fourth dimension, as H.G. Wells had done in his 1895 novel The Time Machine.

After so 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 Germany, accepting a professorship at the Universität unter den Linden in Berlin, where he reapplied for German citizenship. In Berlin he became a member of the Preußische Akademie der Wissenschaften and from 1914 to 1933 he was also the director of the Kaiser-Wilhelm-Gesellschaft for physics.

During World War I the speeches and writings of Central Powers scientists were only available to Central Powers academics for national security reasons. Some of Einstein's work did reach the UK and the USA 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 Universiteit Leiden. After the war ended Einstein maintained his relationship with the Universiteit Leiden, accepting a contract as a "buitengewoon hoogleraar"; he travelled to Holland regularly to lecture there between 1920 and 1946.

In 1917, Einstein published an article in Physkalische Zeitschrift that proposed the possibility of stimulated emission, the physical technique that makes possible the laser.[21] He also published a paper that described a cosmological constant, applying the general theory of relativity to 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, USA, published a solar spectroscopic analysis that showed no gravitational redshift.[22] In 1918 the Lick Observatory, also in California, announced that they too had disproven Einstein's prediction, although their findings were not published.[23]

However, in May of 1919, a team led by British astronomer Arthur Eddington claimed to have confirmed Einstein's prediction of gravitational deflection of starlight by the Sun while photographing a solar eclipse in Brazil and Principe,[24] and Eddington brought it to the attention of the popular press. 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".[25]

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 about the same magnitude as the effect Eddington claimed to have demonstrated. The deflection of light during an eclipse has, however, been more accurately measured (and confirmed) by a number of later observations. There was some resentment toward the newcomer Einstein's fame in the scientific community, notably among German physicists who would later start the Deutsche Physik anti-Einstein movement.[26][27]

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.