http://www.nytimes.com/2015/11/24/science/a-century-ago-einsteins-theory-of-relativity-changed-everything.html
What a shame that Dennis Overbye's story about the General Theory of Relativity didn't mention the mathematical foundation for Einstein's work. The General Theory could not have been developed without Tensor Calculus, which makes possible modeling of complex motion in uneven space. Tenor calculus is so difficult even Einstein needed a tutor, his friend mathematician Marcel Grossman, and "Between the years of 1915 to 1919, Einstein held a correspondence with the Italian mathematician Tullio Levi-Civita – who in 1900 published perhaps the most important work on tensor calculus to this very day - who desired to help him [Einstein] fix some mathematical errors he had found in Einstein’s work."
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A few years back I was cleaning out some old things my father-in-law had left behind at our house. These included some old engineering workbooks that he worked with out on our back porch to pass the time in his retired years. Hey Julie! Says I, look at this. All that time you thought your dad was doing crossword puzzles in the backyard, he was working on tensor calculus problems for fun. What's tensor calculus, says my loving wife. Says I, math so hard to understand that even Einstein needed a tutor to figure it out so he could invent the Theory of Relativity. Smart guy your dad.
