This is perhaps the most famous equations of all time. Einstein’s emblematic E=mc2 is an icon in its
own right. It has graced the covers of magazines like Time, it’s been the subject of documentaries and biographies, the title of theatrical plays, pop songs and music album covers. At its core, however, it’s like other equations: it emerged from a dissatisfaction with the way things were explained; its first appearance was different from the form in which we know it today; it transformed the way we look at the world; and it had unexpected consequences.

Theoretical physics was new when Albert Einstein (1879-1955) was a young man, and it was dominated by two fields: one was over 200 years old, founded by Isaac Newton; the other was revolutionized by James Clerk Maxwell in the 1850s-1860s. Classical Newtonian physics assumed that light moves at an unfixed speed, whereas Maxwell’s theory describes light as an electromagnetic wave traveling at a constant speed. Einstein hated this contradiction. He saw it as a sign that something was missing. Einstein focused on a key element of speed: time. Through a thought experiment, Einstein realized that the flow of time is different for an observer who is moving, versus one who is standing still. He imagined traveling at the speed of light and looking back at the clock tower in the center of Bern, Switzerland, where he was living at the time. To observers near the tower, time would appear to pass normally. But to the light-speed traveler, the clock would appear to stand still: the light it reflects wouldn’t be able to catch up. The faster one moves through space, the slower one moves through time. Special relativity was born. Einstein published his new theory in 1905, and followed soon thereafter with another short paper entitled “Ist die Trägheit eines Körpers von seinem Energiegehalt abhängig?” [“Does the Inertia of a Body depend upon its Energy-Content?”]. The salt of his argument can be summarized in the following sentence: “The mass of a body is a measure of its energy content.” This is the first appearance of the idea that would later become E=mc2. The concept transformed some of the most fundamental notions of how the Universe is assembled. It put together two things long through to be utterly different: mass and energy. Energy can become matter, and matter can become energy. Even the tiniest bit of mass embodies enormous potential energy that gets released through nuclear reactions. This equation describes the engine that illuminates the stars.