### Today 89 years ago: relativity

In November 1915, Einstein presented a series of lectures before the Prussian Academy of Sciences in which he described his theory of general relativity. The final lecture climaxed with his introduction of an equation that replaced Newton's law of gravity. This theory considered all observers to be equivalent, not only those moving at a uniform speed. In general relativity, gravity is no longer a force (as it was in Newton's law of gravity) but is a consequence of the curvature of space-time. The theory provided the foundation for the study of cosmology and gave scientists the tools for understanding many features of the universe that were not discovered until well after Einstein's death. General relativity becomes a method of perceiving all of physics.

The theory was derived with mathematical reasoning and rational analysis, not with experimentation or observation, leading scientists to skepticism. But his equations enabled predictions and tests to be made, and when it was tested by Arthur Eddington by measuring during a solar eclipse how much the light emanating from a star passing close to the sun was bent by the sun's gravity, the predictions from the theory were confirmed.

In the early 1920s, Einstein was the lead figure in a famous weekly physics colloquium at the University of Berlin. On March 30, 1921, Einstein went to New York to give a lecture on his new theory of relativity. In the same year, he was finally awarded the Nobel Prize for his work. Though he is now most famous for his work on relativity, it was for his earlier work on the photoelectric effect that he was given the Prize: in 1921 his work on Relativity was still too disputed to merit a Nobel Prize, so the Nobel committee decided that his earlier, less-contested theory would be a better political move.

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