He was daring, wildly ingenious, passionately curious. He saw a beam of light and imagined riding it; he looked up at the sky and envisioned that space-time was curved. Albert Einstein reinterpreted the inner workings of nature, the very essence of light, time, energy, and gravity. His insights fundamentally changed the way we look at the universe—and made him the most famous scientist of the 20th century.

We know Einstein as a visionary physicist, but he was also a passionate humanitarian and anti-war activist. Born a German Jew, Einstein truly considered himself a citizen of the world. His celebrity status enabled him to speak out—on global issues from pacifism to racism, anti-Semitism to nuclear disarmament. "My life is a simple thing that would interest no one," he once claimed. But in fact, his letters, notebooks and manuscripts tell a dramatically different story. Einstein saw the universe as a puzzle, and he delighted in trying to solve its mysteries. All he needed to contemplate the cosmos was his most valuable scientific tool—his imagination.