thought the theory marred by an 'ugly dark spot'. Although he defended it in public, Einstein was never fully convinced by the Entwurf theory, and it was not received well by other physicists. To make matters worse, he went on to 'prove' that suitable, generally covariant, field equations were not attainable since they violated causality, though he later realized that his arguments were flawed.

68 Einstein Die Grundlage der allgemeinen Relativitatstheorie (1916). English translation in

From a letter to Lorentz in August 1913. Quoted from Norton (1984). A mathematical treatment of Einstein's arguments against general covariance is given in Earman and Glymour (1978b); (see also Howard and Norton (1993)).

Einstein and Grossman did not comment on the effect of their theory on the perihelion of Mercury, though Einstein did work it out subsequently in collaboration with an old friend from his student days Michele Besso.1 Their value of 18 ' per century was not published, but was confirmed by Johannes Droste in 1915. In fact, throughout the development of general relativity, Einstein rarely mentioned perihelion shifts, but paid much more attention to light deflection and redshift calculations. This was probably because, since the effects of other matter may well be important, he was not confident that a new theory of gravitation eventually would yield all of the missing 43 '. The same was true of the other competitors in the race for a new theory of gravity. However, immediately after Einstein arrived at the generally covariant field equations for gravity in 1915, he wrote to Sommerfeld and gave three reasons for turning his back on the Entwurf theory, one of which was the fact that it did not give the correct advance for the perihelion of Mercury.72

When Einstein moved to Berlin in 1914, there were two theories of gravitation deserving attention: his and Nordstrom's. The latter was much simpler but did not satisfy Einstein's principle of equivalence. Moreover, since c is constant in the Nordstrom approach, it predicts no deflection for light passing close to the Sun and, hence, there was the possibility of deciding between the alternatives through observation.

On astronomical matters Einstein usually turned for help to his friend, Erwin

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