Which famous physicist developed E = mc squared?

Einstein, in 1905, in a three-page afterthought to a longer paper - the last of four he published that year, any one of which would have made a career. The others introduced special relativity, explained Brownian motion as proof that atoms are real, and described the photoelectric effect, which is what he was actually awarded the Nobel Prize for.

What the equation says is that mass and energy are the same thing in different accounting units, and c squared is simply the exchange rate. Because the speed of light is enormous and squaring it more so, a very small amount of mass corresponds to a very large amount of energy. The Sun converts about four million tonnes of itself into sunlight every second and will keep doing it for billions of years.

It is not primarily a bomb equation, though that is its reputation. It equally explains why the Sun shines, why a nucleus weighs slightly less than its parts, and why a hot object is fractionally heavier than a cold one. Einstein's connection to the bomb was a letter he signed in 1939 urging Roosevelt to investigate before Germany did - he took no part in building it, was denied clearance as a security risk, and later said that if he had known the Germans would fail he would have done nothing.