Einstien the great scientist of all time Clearly and abudantly showed by his tactics and experience that energy and mass are interconvertible. So to admire him Physics is the mass enegy relation mainly as we know that energy is actually mass and mass is actually energy. What to say is that energy mass relationship is clearly expressed by Einstien theory.
What is that actually given that Light which is actually enegy and gives about the most part of enegy to earth in radiation given by SUN. So light actually given by De-brouglie is the most refined form of mass. Hence mass burnt convert to energy and energy in highly compressed form is mass actually.
Hence Einstien is famous for his basis of physics definition Mass and energy CONVERSION.