Albert Einstein is known for developing the theory of relativity, which revolutionized our understanding of space, time, and gravity. His 1905 paper on special relativity introduced groundbreaking concepts such as the equivalence of mass and energy, encapsulated in the famous equation E=mc². Later, in 1915, he expanded these ideas into the general theory of relativity, explaining how massive objects like planets and stars warp the fabric of spacetime, leading to the phenomenon we perceive as gravity. Einstein's theories have had profound implications in physics and cosmology, influencing everything from GPS technology to our understanding of the universe.