How did Marcel Grossmann help Albert Einstein develop general relativity?

In 1912, the mathematician Marcel Grossmann played a pivotal role in helping Albert Einstein develop the general theory of relativity by introducing him to the tensor analysis developed by Bernhard Riemann, Tullio Levi-Civita, and Gregorio Ricci-Curbastro.1

While Einstein had established the physical principles of his theory—such as the equivalence of gravity and acceleration—he struggled for years to find the appropriate mathematical framework to express these ideas across different coordinate systems. Grossmann’s introduction to Riemannian geometry provided the necessary tools to describe the curvature of space-time, which Einstein ultimately identified as the source of gravitational forces. This collaboration allowed Einstein to move beyond the limitations of Euclidean geometry and successfully complete his work on general relativity in 1915.12