3.3 General Relativity
Einstein’s first theory of relativity in 1905 was special because it is applied only for inertial frames and did not take gravity into account, so he naturally started thinking about generalizing it to include non-inertial frames and incorporate gravity. The starting point was in 1907, with a thought experiment of an observer in free fall, from which he deduced, after eight years of research, the equivalence between the gravitational force and the pseudo-force experienced by an observer in an accelerated frame of reference. He presented his work to the Prussian Academy of Science in November 1915, which later became known as the Einstein Field Equations that describe how the geometry of space and time is influenced by the presence of matter and radiation.
It was very clear from the beginning that General Relativity is revolutionary and much more advanced than the classical Newtonian theory of gravity, as Einstein showed soon afterwards how his new theory could explain the anomalous perihelion advance of the planet Mercury. However, when Arthur Eddington (1882-1944) confirmed in his expedition that General Relativity correctly predicted the deflection of starlight by the Sun during the total solar eclipse of May 29, 1919, Einstein became instantly very famous. Nevertheless, for the several decades to follow, the new theory remained a curiosity among most physicists, since its applications are mostly related to large scale cosmology and extreme gravitational fields, while they reduce to the classical Newton’s law of gravitation when the gravitational field is weak and the velocities are much less than the speed of light.