Tonight we will all watch a lunar eclipse, which will last a while.  As we learn some of the basics of general relativity, maybe it is relevant to think about how it’s acceptance gradually came to be, since it’s conclusions are often counter-intuitive.

One of the most important tests of general relativity came in 1919, during a total solar eclipse.  Arthur Eddington traveled to the African Island of Principe to observe the eclipse.  Eddington was an astronomer, mathematician, internationalist, and pacifist.  Immediately after the great war, he was still willing to consider the new theories of a young German physicist, and he put them to the test during the 6 minutes and 51 seconds of total eclipse.  General relativity predicts a bending of light due to the curvature of space-time near a large mass.  The sun, while massive, is not very dense, and measuring the effect of the curvature of space-time near it’s surface requires extreme precision. Observations were made simultaneously in Brazil and West-Africa. Only during an eclipse is starlight visible next to the surface of the sun. They were monitoring a specific star, looking for a deviation from it’s expected position.  Dyson, Eddington and Davidson published their results the following year, and Einstein’s general relativity became the prevailing theory of gravity over the Newtonian, with headlines in newspapers around the world.

Famously, when asked by his assistant what his reaction would have been if general relativity had not been confirmed by Eddington and Dyson in 1919, Einstein replied: “Then I would feel sorry for the dear Lord. The theory is correct anyway.”

Rosenthal-Schneider, Ilse: Reality and Scientific Truth. Detroit: Wayne State University Press, 1980. p 74. See also Calaprice, Alice: The New Quotable Einstein. Princeton: Princeton University Press, 2005. p 227.)

Dyson, F.W.; Eddington, A.S.; Davidson, C.R. (1920). “A Determination of the Deflection of Light by the Sun’s Gravitational Field, from Observations Made at the Solar eclipse of May 29, 1919”. Phil. Trans. Roy. Soc. A 220 (571-581): 291–333. Bibcode:1920RSPTA.220..291D. doi:10.1098/rsta.1920.0009.