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Services to theoretical physics, especially the discovery of the law of the photoelectric effect
Light shining on metal can knock out electrons, but only if the light's frequency is high enough. In 1905 Einstein explained this by proposing that light comes in packets of energy, later called photons. This helped found quantum theory and is the principle behind solar cells.
Physicist born in Ulm, Germany, who published his theories of relativity in 1905 and 1915 and later moved to the United States.