“Mathematical Physics brings together the two great disciplines of Mathematics and Physics to the benefit of both, the relationship between them being symbiotic. On the one hand, it uses mathematics as a tool to organize physical ideas of increasing precision and complexity, and on the other it draws on the questions that physicists pose as a source of inspiration to mathematicians. A classical example of this relationship exists in Einstein’s theory of relativity, where differential geometry played an essential role in the formulation of the physical theory while the problems raised by the ensuing physics have in turn boosted the development of differential geometry.” Françoise, J.-P., Naber, G. L., & Tsou Sheung Tsun. (2006). Preface. In Encyclopedia of Mathematical Physics (pp. xiv–xv). Academic Press.

You can learn more about mathematical physics at Professor Michael Stone's course pages for PHYS 508: Mathematical Methods I and PHYS 509:  Mathematical Methods II.