E=MC²

In 1905, Albert Einstein came up with the mathematical scientific conclusion of E=MC². (TV: The Lie of the Land) It was part of the theory of relativity, and stated that energy equals mass times the speed of light squared. (TV: Four to Doomsday) According to the Third Doctor, this meant that "when you lose mass, the energy has to go somewhere." (TV: The Dæmons)

During the Monk invasion of Earth, the Monks launched propaganda claiming to have helped Einstein come up with this conclusion. (TV: The Lie of the Land)

Behind the scenes
The famous conclusion, E=MC², only holds for a body at rest. The more general conclusion derived by Einstein was, in fact, E2=(MC2)2+(PC)2, where E represents the energy of the body, M represents its mass, C is the speed of light, and P is the body's momentum.

In the real world, though mass-energy equivalence was not a core principle in, as Four to Doomsday supposes, it was a natural consequence of that theory, which Einstein formalised with his 1905 derivation.