Einstein’s work on thermodynamics was not done. In that same year, 1905, he also made a historic extension to the first law—that energy is always conserved. This is reflected in the equation for which he is most famous: E = mc2. The E in the equation represents energy, the m represents mass, and the c2 is a large but unvarying number—the speed of light multiplied by itself, or squared. This equation states that, though energy cannot be created or destroyed, it can sometimes take the unlikely form of solid matter.
