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Einstein, Albert

light relativity theory motion

[iyn shtiyn] (1879–1955) German–Swiss–US theoretical physicist: conceived the theory of relativity.

Einstein’s father was an electrical engineer whose business difficulties caused the family to move rather frequently; Einstein was born while they were in Ulm. Despite a delay due to his poor mathematics he entered the Swiss Federal Institute of Technology in Zürich at the age of 17, and on graduating became a Swiss citizen and sought a post in a university, or even in a school. However, he had great difficulty in finding any job and settled for serving in the Swiss Patent Office in Bern. It worked out well; he was a good patent examiner and the job gave him enough leisure for his research. In 1903 he married a fellow physicist, Mileva Maric; their illegitimate daughter, born in 1902, was adopted; two sons followed. This marriage ended in divorce in 1919 and he then married his cousin Elsa, who had two daughters by a previous marriage. It was while at the Patent Office that he produced the three papers published in 1905, each of which represented an enormous achievement, covering Brownian motion, the photoelectric effect and special relativity.

Einstein’s first university post was secured in 1909, when he obtained a junior professorship at the University of Zürich, and a full professorship at Prague (1910) and Zürich (1912) followed. In 1913 he was made Director of the Institute of Physics at the Kaiser Wilhelm Institute in Berlin. The general theory of relativity was completed during the First World War and following its publication (1915) Einstein was awarded the 1921 Nobel Prize for physics for his work of 1905.

He began to undertake many lecture-tours abroad and was in California when Hitler came to power in 1933. He never returned to Germany, resigning his position and taking up a post at the Institute of Advanced Study, Princeton. Einstein put much effort into trying to unify gravitational, electromagnetic and nuclear forces into one set of field equations, but without success. He had some involvement in politics, in that he helped initiate the Allied efforts to make an atomic bomb (the Manhattan project) by warning Roosevelt, the American president, of the possibility that Germany would do so, in a letter in 1939. In 1952 Einstein was offered, and sensibly declined, the presidency of Israel. He was also active in promoting nuclear disarmament after the Second World War. He led a simple life, with sailing and music as his main relaxations.

The first of his papers of 1905 considered the random movement of small suspended particles (Brownian motion, discovered in 1828). The bombardment by surrounding molecules will make a tiny particle in a fluid dart around in an erratic movement, and Einstein’s calculations provided the most direct evidence for the existence of molecules when confirmed experimentally by (1908).

The next paper by Einstein tackled the photoelectric effect by considering the nature of electromagnetic radiation, usually thought of as waves obeying equations. Einstein assumed that light energy could only be transferred in packets, the quanta used by to derive the black body radiation spectrum. Einstein then was able to explain fully the observations of (1902), in which the energy of electrons ejected from a metallic surface depended on the wavelength of light falling on it but not on the intensity. The result became a foundation for quantum theory and clothed Planck’s quanta with a physical interpretation.

Finally, Einstein set out the special theory of relativity (restricted to bodies moving with uniform velocity with respect to one another) in his third paper. Maxwell’s electromagnetic wave theory of light indicated that the velocity of a light wave did not depend on the speed of the source or observer and so contradicted classical mechanics had found a transformation of Maxwell’s equations for a region in uniform motion which left the speed of light unchanged and not altered by the relative velocity of the space and observer (the Lorentz transformation).

Einstein correctly proposed that the speed of light is the same in all frames of reference moving relative to one another and, unknown to him, this had been established by the experiment (1881, 1887). He put forward the principle of relativity, that all physical laws are the same in all frames of reference in uniform motion with respect to one another. When applied it naturally gives rise to the Lorentz–FitzGerald transformation, with classical mechanics obeying this rather than simple addition of velocity between moving frames (the Galilean transformation). A further consequence derived by him was that if the energy of a body changes by an amount E then its mass must change by E/ c 2 where c is the velocity of light.

From 1907 Einstein sought to extend relativity theory to frames of reference which are being accelerated with respect to one another. His guiding principle (the principle of equivalence) stated that gravitational acceleration and that due to motion viewed in an accelerating frame are completely equivalent. From this he predicted that light rays should be bent by gravitational attraction. In 1911 he reached a specific prediction: that starlight just grazing the Sun should be deflected by 1.7” of arc. During a total eclipse of the Sun in 1919 measured this in observations made at Principé in West Africa, finding 1.61” of arc. This dramatic confirmation immediately made Einstein famous world-wide and made it clear that he had moved the foundation of physics.

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