Albert Einstein is known for his contributions to the theory of relativity and quantum mechanics. His 1905 paper introduced the special theory of relativity, which revolutionized the understanding of space, time, and energy. He later developed the general theory of relativity in 1915, explaining gravity as the curvature of space-time caused by mass. Additionally, Einstein made significant contributions to quantum mechanics, especially regarding the photoelectric effect, which provided key insights into the dual nature of light. His groundbreaking ideas laid the foundation for modern physics and significantly impacted technology, cosmology, and our understanding of the universe.