I disagree: you definitely have to be able to understand lots of complicated stuff to get a PhD in astrophysics from a place like Caltech. And even if string th

I disagree: you definitely have to be able to understand lots of complicated stuff to get a PhD in astrophysics from a place like Caltech. And even if string theory isn't physics (i.e., doesn't make testable predictions) it still is a kind of math or at least inspires a lot of math.