He Didn't Win the Nobel Prize for Relativity
As with most scientific revolutions, Einstein's breakthrough insights on special relativity in 1905 did not arise out of a vacuum. His genius lay in how he transformed previous work by scientists like Henri Poincaré and Hendrik Lorentz into a new, unified theory, one that removed the friction between Newtonian physics and James Clerk Maxwell's theory of light.
Published in 1916, Einstein's theory of general relativity completed special relativity by bringing gravity and acceleration into the picture through the concept of warped space-time. Unfortunately, it took years to prove one of its key predictions, the lensing effect of gravity. When astronomers finally confirmed the bending of starlight during observations of a 1919 solar eclipse, it launched Einstein into overnight celebrity, but three more years would pass before the Nobel committee retroactively awarded him the 1921 Nobel Prize in physics in 1922.
Einstein received the prize for "the discovery of the law of the photoelectric effect." The photoelectric effect refers to the release of electrically charged particles (ions or electrons) from (or within) a material that absorbs electromagnetic radiation (such as light). Einstein's crucial work in this area resolved perplexing questions regarding the particle-wave duality of light. Nevertheless, Einstein's acceptance speech focused on his work in general relativity, a problem that had occupied him for nearly a decade, and whose importance would not be fully appreciated for decades to come.