The famous formula E=mc^2 really says that mass and energy are the same thing, but measured in different units. So the separate laws of conservation- one for mass and the other for energy- are now merged into one law.
Einstein's theory of special relativity (1905) shows that matter (as mass) and energy can be converted into each other according to the famous equation E = mc2, where E is energy, m is mass, and c is the speed of light.
Hope It helps you.
