andytoh said:
A mathematical physicist is capable of earning two different PhDs, one in math and one in physics.
I'm not sure you'll find that is true. At least no more true than the next quote:
On the other hand, a pure mathematician cannot earn a PhD in physics and a pure physicist cannot earn a PhD in mathematics (unless and take the time to study the other subject from scratch).
is true. Not least because your favoured area of string theory is very likely to be studied by a pure mathematician. In fact I know very few applied people, or those with a physical background who do string theory, and many pure mathematicians.
I would also like to point out that plenty of pure mathematicians have a more than average working knowledge of relativity and quantum mechanics - they are after all undergraduate course. I even know some people who did these subjects for personal interest during Part III and are very pure mathematicians. Just as there are plenty of applied people who have a working knowledge of representation theory, say. The pure mathematics of string theory (that wouldn't be assumed from, say, relativity) and are essectial to the beginner can be summed up as:
essential: Riemann surfaces
useful: category theory, algebraic topology/geometry
all but category theory can be, and is, learned to reasonable level as an undergraduate course. Category theory might be taught, but is often overlooked. To be honest, outside of Baez's n-categorical viewpoint, I'm not sure how useful this really is. The student may end up knowing some highly specialized stuff (in some sense) like chern classes, and poincare duality, or McKay correspondence but it is by no means assured. At the last conference I attended dealing with such matters I don't think I saw a PhD student there, and only a handful of people on post-docs like me. And, no, I'm not a mathematical physicist.
I think it very misleading to imply that a recently qualified Maths Phys PhD is somehow a demigod of both subjects, and knows enough to do research in either. They don't. They certainly ought to know enough to do research in Maths Phys, but that is the only thing nearing a certainty you can say. You start learning the real stuff after the PhD, apparently. If I were you, I'd check out some PhD theses from maths phys students before saying what it is they have to know, and what it is they do at this stage.
This is why I admire mathematical physicists like Hawking and Penrose.
They are certainly admirable names. But an average mathematical physicist is no more or less able than an average mathematician/physicist. They just happen to work at a very fluid boundary.