Albert Einstein

Einstein's path to general relativity began with a thought he later called the happiest of his life: a person falling freely feels no gravity at all, so the pull of gravity and the acceleration of a reference frame are, locally, indistinguishable. He reached this equivalence principle around 1907, eight years before he published the finished theory, and spent the intervening years working out its mathematical consequences with the help of his friend, the mathematician Marcel Grossmann, who introduced him to the differential geometry needed to describe curved, four-dimensional spacetime.

The theory Einstein published in November 1915 replaced Newton's picture of gravity as a force acting instantly across empty space with a geometric one: mass and energy curve the fabric of spacetime itself, and objects moving through that curved spacetime follow paths that look, from a distance, like the effect of a gravitational pull. A planet orbits the sun not because the sun reaches out and tugs on it but because the sun's mass bends the spacetime the planet is traveling through, and the planet is simply going the straightest available path. The field equations Einstein derived describe precisely how mass and energy determine that curvature, and how that curvature in turn determines how mass and energy move.

The theory made a specific, checkable prediction that Newton's gravity could not: light itself, though massless, should be bent by the sun's gravity by a calculable amount, twice what a naive Newtonian estimate would give. Confirming that prediction, four years later, turned Einstein from a respected physicist into a household name, and general relativity has since passed every experimental test put to it, from the precise orbit of Mercury to the detection of gravitational waves a century after the theory was published.

Einstein left Germany for good in 1933, the year Hitler took power, and settled at the Institute for Advanced Study in Princeton, where he spent the rest of his life. He had been a committed pacifist for decades, but the rise of Nazism led him to a harder position: in 1939, alarmed that German scientists might build an atomic weapon using the nuclear fission just discovered by Otto Hahn and Fritz Strassmann, he signed a letter, largely drafted by the physicist Leo Szilard, urging President Franklin Roosevelt to fund research into a bomb before Germany could build one first. The letter helped set in motion what became the Manhattan Project, though Einstein himself, denied a security clearance because of his earlier pacifist and left-leaning associations, took no part in the weapon's design. He spent much of his later life arguing for nuclear disarmament and international control of atomic weapons, work that culminated in the Russell-Einstein Manifesto, signed days before his death in 1955, warning of the risk nuclear war posed to humanity as a whole.

Einstein was equally outspoken on civil rights in his adopted country. He joined the NAACP, corresponded and appeared publicly with the singer and activist Paul Robeson and the scholar W. E. B. Du Bois, and in a 1946 speech at Lincoln University, a historically Black college, called racism America's worst disease. In 1952 the newly founded state of Israel offered him its presidency, a largely ceremonial post, he declined, saying he lacked the aptitude and experience for public office. Across all of it, Einstein treated his scientific authority as a platform for moral argument rather than something to keep separate from public life, an approach that drew both admiration and, especially during the McCarthy era, suspicion from an FBI file that ran to more than 1,400 pages.