E = mc²

It may be the most famous equation: E = mc².

It came to the public with a 1905 publication by Albert Einstein. Even people with no real interest or ties to science tend to know the equation. The vast majority of people who know of and can tell you the equation have no idea what it means.

In 1905, Einstein finally completed his doctoral thesis on “A new determination of molecular dimensions.” But he also published four groundbreaking papers that banner year in the German journal Annalen der Physik (Annals of Physics).

One paper  (“On a heuristic point of view concerning the production and transformation of light”) proved that light could behave as a particle as well as a wave, and gave rise to quantum theory.

In another paper, he used Brownian motion (the irregular movement of small but visible particles suspended in a liquid or gas) to prove empirically that atoms exist.

The third paper, “On the electrodynamics of moving bodies,” presented his theory of Special Relativity, which deals with the way the speed of light affects the measurement of time and space.

And on September 27, 1905, he submitted a paper that asked, “Does an object’s inertia depend on its energy content?” This is where he used the equation E = mc².

It translates to mean that energy (E) equals mass (m) times the speed of light squared (c²) and that this reveals that matter and energy are deeply connected.

The equation came from his work on Special Relativity. While working through his calculations, he had an insight that surprised him. If an object emits energy, the object’s mass must decrease by a proportionate amount. Einstein actually wrote about it to a friend, “This thought is amusing and infectious, but I cannot possibly know whether the good Lord does not laugh at it and has led me up the garden path.”

Brian Greene explains it in this simpler way: “When you drive your car, E = mc² is at work. As the engine burns gasoline to produce energy in the form of motion, it does so by converting some of the gasoline’s mass into energy, in accord with Einstein’s formula.” To get the energy to move the car you have to lose some gasoline.

Einstein’s original thoughts and certainly not his intention was the discovery that a small amount of mass can be converted into a large amount of energy. This was what would eventually lead other scientists to the development of nuclear energy, and the atomic bomb.

The atomic bombs that destroyed Hiroshima and Nagasaki used less than an ounce of matter into explosive energy.