1) Differential geometry is fairly hard — or at least fiddly with all the tensors and indices.

2) Even when you grasp it, it seems like you just know how to turn the mathematical crank. Without much geometrical or physical intutition.

I did not understand what the Ricci tensor was untill I read “The Meaning of Einstein’s Equation” here http://math.ucr.edu/home/baez/einstein/

]]>http://arxiv.org/abs/gr-qc/0411023 (which refers to 1970 paper

by Stanley Deser)

Feynman’s own work is outlined in his lectures on GR book (which came out in 1995) ]]>