After Einstein published his two groundbreaking papers in 1905 (on the photoelectric effect and special relativity), he continued to make significant contributions to theoretical physics throughout his career. While it is true that he did not produce a major new theory on the same scale as general relativity, his subsequent work was far from insignificant. Here are some notable contributions he made after his early papers:
General Relativity: Einstein's most famous work after his 1905 papers was the development of the theory of general relativity. He spent nearly a decade refining this theory, which he published in 1915. General relativity revolutionized our understanding of gravity and provided a new framework for describing the dynamics of space, time, and matter on a large scale.
Gravitational Waves: Einstein's general theory of relativity predicted the existence of gravitational waves—ripples in the fabric of spacetime caused by the motion of massive objects. However, it took several decades for the experimental confirmation of gravitational waves, which occurred in 2015. The detection of gravitational waves validated Einstein's predictions and opened up an entirely new field of observational astronomy.
Quantum Mechanics: While Einstein is often associated with the development of the theory of relativity, he also made significant contributions to the foundations of quantum mechanics. He engaged in debates with other physicists, most notably with Niels Bohr, regarding the completeness and interpretation of quantum theory. Einstein's arguments, such as the famous "EPR paradox," stimulated further research and philosophical discussions in quantum mechanics.
Unified Field Theory: Throughout his life, Einstein sought to unify the fundamental forces of nature into a single framework—a theory often referred to as a unified field theory or theory of everything. Although he was not able to fully realize this ambition, his work on unified field theory inspired subsequent generations of physicists and paved the way for ongoing research in this area.
It is important to recognize that groundbreaking contributions to science, such as those made by Einstein early in his career, are exceptional and rare. Einstein's subsequent work, while perhaps not as revolutionary as his early papers, still had a profound impact on physics and set the stage for future developments in the field.