What was Albert Einstein's second theory?

Answer 1

In 1905 Albert Einstein published four papers dealing with physics and general properties of molecules. His second theory, which was contained in one of these published papers, was the theory of Special Relativity. This would later become his General Theory of Relativity.

Answer 2

Albert Einstein's second theory, after his theory of special relativity, was the theory of general relativity. This theory revolutionized our understanding of gravity by describing it as a curvature of spacetime caused by mass and energy. General relativity has been confirmed through various experiments and observations, and it forms the foundation of our current understanding of the universe's large-scale structure.