When Einstein proposed his theories of Relativity, it took decades for various forms of experimental validation to be possible. Early on, his proof essentially was a mathematical one, solving on paper contradictions with existing classical theories. It was also an extraordinarily different theory, using radically different, novel, and very complex concepts of how the world works compared to classical mechanics accepted for centuries prior.

Based on the principle of "extraordinary claims require extraordinary evidence", was skepticism of Einstein's Theory of Relativity strong in the scientific community following his publication? Did fellow scientists demand experimental proof before accepting it? Or did an essentially mathematical proof gain acceptance despite lack of practical evidence backing it up at the time?

For comparison, existing theories beyond the Standard Model (e.g. String Theory, M-Theory, Loop Quantum Gravity) are often criticized for lack of experimental proof. Despite the known shortcomings of the Standard Model, many consider experimental proof a necessity of any theory claiming to supercede it, with a purely mathematical resolution of contradictions not being considered enough.