I often hear people talk about string theory being the hardest branch of theoretical physics, and require a PhD to even understand the basics. I've even heard of people saying it's getting too hard for humans, much like quantum mechanics is too hard for cats. So to put things into perspective, how hard is it really? Is it any harder than pure math? Is it any harder than other branches of mathematical physics, such as general relativity, fluid dynamics, or statistical mechanics, all of which have unsolved mathematical problems? And is it any harder than understanding the literature on high Tc superconductors which is another great unsolved problem in physics besides quantum gravity?