Einstein’s general theory of relativity

Albert Einstein (1879–1955)
‘Die Grundlage der allgemeinen Relativitätstheorie’, Annalen der Physik, 1916

With the Great War and continent-wide clashes of arms engulfing Europe, Albert Einstein published papers in Berlin that announced to the world the fundamental field equations of his new theory late in 1915 and then this, ‘The basis of the general theory of relativity’, in 1916. To verify the theory experimentally, Einstein pointed to gravitational effects on Mercury’s orbit, the dispersion of electromagnetic waves in gravity fields, and the predicted deflection of light near a massive body. Anomalies in Mercury’s orbit were precisely explained by the theory but further observational proof seemed unlikely.

P352.c.16.367, p. 768

Einstein is himself accredited with an annus mirabilis, 1905 when he published four fundamental papers including those on his theory of special relativity and the equivalence of mass and energy. Reasons for modifications to the universal laws of Isaac Newton had arisen during the latter half of the 19th century both in theory and experiment. James Clerk Maxwell had expounded the electro-magnetic theory in terms of vibrations propagating waves in a ‘luminiferous æther’; but Albert Michelson and Edward Morley using the Earth as a platform – moving at about 18.5 miles per second – for observing incoming starlight at 186,000mps had not detected the æther or an alteration in the speed of light in any direction. George FitzGerald and Hendrik Lorentz proposed a contraction in space to account for this but it was in 1905 Einstein in his paper formulated his special theory to account for dilation in space and time additionally demonstrating that the æther was undetectable, in essence non-existent. Einstein’s theory was seen as a triumph of modern theoretical physics and experimental tests of the predictions of the theory over succeeding decades have shown the theory to be correct to very high degrees of accuracy.

Einstein was not satisfied simply with explaining the seemingly anomalous experimental results but soon came to realise that the principles of special relativity could be generalised to include gravitational fields. This is far more easily stated than achieved but working from 1907 onwards, by late 1915 Einstein was in a position to publish the field equations that are the cornerstone of gravitational, or general, relativity.

The necessary modification to Newton’s inverse square law of gravitational attraction immediately explained an observation that had long puzzled 19th century astronomers, even to the extent of theorising that there could be an unobserved planet Vulcan very close to the Sun. This observation was that the point at which the planet Mercury was closest to the Sun was not static but moved in space, an effect known as the perihelion precession of Mercury, a movement that was in complete agreement with general relativity.

However, initial attempts by observation of starlight to show that it would lose energy in a gravitational field to become more red in colour as predicted in the high gravitational field close to the Sun were not successful; but that same high gravitational field was by general relativity predicted to bend the course of light, by an amount twice as great as predicted by Newton’s law. How might that gravitation deflection be tested? With the announcement of this fundamental theory to the Prussian Royal Academy of Sciences in Berlin and in the very midst of the devastation of the war engulfing the countries of Europe and Asia, a champion of Einstein’s ideas arose in one of the nations ranged against Germany and her allies – Great Britain

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