A few months after Einstein published his first paper on relativity in 1905, he sent in a sort of extended footnote to the same journal. His theory had an odd little consequence. It seemed so strange that he phrased the note's title as a question: “Does Mass Depend on Energy?” To leaf through the next four flimsy pieces of paper and contemplate all that followed is to feel the power of ideas. For better or worse, Einstein had unlocked the secret of the atom. Here was the destruction of Hiroshima and Nagasaki. Here were 40 years of fear and tension as Cold War superpowers pursued their policy of mutual assured destruction, insisting on arsenals so large that even after a first strike they could reduce their adversary to bouncing rubble. Here was the promise of infinitely renewable energy, and the curse of Chernobyl. Here was the first explanation of the Sun's ceaseless light and the starry heavens. Although it is true that chemists had stumbled upon radioactivity before Einstein, and might have developed atomic power without him, Einstein's theory was the torch that led the way. His ideas shaped a century we were lucky to survive.

Einstein concluded his short note by deriving the most famous physics equation of them all,

E = mc2

(pronounced “ee equals em sea squared”): the only equation we will meet in the main text of this book. Here, E stands for energy, m for mass and c for the speed of light.