What should we call an expert majoring in mathematical physics? We call one majoring in math a mathematician, one majoring in algebra an algebraist, one majoring in geometry a geometrist, but mathematical physics is a branch of (applied) mathematics, so "mathematical physicist" might be a little weird.

Is there a big difference between theoretical physicist and mathematical physicist? Or at least between the meanings in which these two phrases are used?

Hm, it seems that Wikipedia basically answers my question there is a section Mathematical vs. theoretical physics in the WP article on Mathematical physics.

@MartinSleziak emm... Mathematical physics is a branch of mathematics, while when you say "mathematical physicist", you're referring to a specific kind of physicist, so I'm not sure what should we call an expert on mathematical physics. (I have to admit it's a tricky question)