Among some MIT professors, we find Alan Rickman has a PhD in Astrophysics, and he is a professor of Humanities, without even having a degree on it! Jeffrey A. Hoffman too, and he is a professor of Aerospace engineering. Curious, isn't? Then it is Brian May, the guitarist of Queen (and I think he is famous in the Astrophysicist world just because of that). I don't know if Stephen Hawking did his PhD in Astrophysics or in Theoretical Physics, but he is an emeritus professor of Mathematics. Do you know any other PhD's in Physics who moved on into such branches like this?