I've read that while special relativity was "in the air," so that someone else would have come up with it, general relativity was so theoretical, and so austere, that it might have taken another century for others to piece together all the clues (which came later) and produce that theory. But I'll leave this to those who know more than I do.
"I've read that while special relativity was "in the air," so that someone else would have come up with it, general relativity was so theoretical, and so austere, that it might have taken another century for others to piece together all the clues (which came later) and produce that theory."
You've got it backwards.
General relativity was Einstein's attempt to "unify" Special Relativity with Newtonian mechanics, especially universal gravitation.
In fact, Special Relativity purposefully and obviously ignored gravity -- so there is nothing surprising about Einstein addressing it in a later effort. If he hadn't somebody else would have tried to.
Special Relativity was far more out of left field. But of course it didn't arise fullblown out of nowhere (cf. Maxwell's equations, etc.).
IIRC that Hilbert had independently developed many of the same insights when Einstein published his Theory of Special Relativity. The may have even been some collaboration between the two before SR was published.
In Gravitation by Misner, Thorne, and Wheeler, (IIRC), there is speculation that Riemann might have come up with relativity (he died in 1866 at age 40, two years after Maxwell published his equations). He did, after all, develop the geometry needed by Einstein.