General relativity didn’t merely deal with abstruse situations—it was also written in abstruse mathematics. The theory is very mathematically complex, far more so than quantum mechanics. Einstein famously had to enlist the help of a mathematician friend, Marcel Grossman, just to learn the differential geometry necessary to formulate and understand his own theory. This combination of unfamiliar subject matter and obscure mathematics made it difficult to be sure of what the theory was saying and led many physicists to be suspicious of its conclusions.
