On March 14, 1951, Albert Einstein was sitting in the back of a car, getting ready to leave his 72nd birthday party at Princeton University. Photographers had been asking Einstein to pose for the camera all evening, and when UPI photographer Arthur Sasse turned his camera on him, the theoretical physicist playfully stuck out his tongue.

The lighthearted photo Sasse captured pleased Einstein so much that he ordered multiple prints to send to friends. In an inscribed copy Einstein gave to news anchor Howard K. Smith, he hinted at what the photo meant to him. “This gesture you will like, because it is aimed at all of humanity,” Einstein wrote (in German). “A civilian can afford to do what no diplomat would dare.” On another print he sent to his friend Johanna Fantova, he wrote, “The outstretched tongue reflects my political views.”

These messages likely reference the political climate that Einstein found himself in during McCarthyism. A few years after the tongue photo, Einstein defended J. Robert Oppenheimer when the United States investigated him as a suspected communist and security risk. The 2023 film Oppenheimer highlighted his blacklisting—but lesser known is the fact that the U.S. government investigated Einstein for “subversive” activities, too.

Science and politics frequently collided in Einstein’s life. In the 1930s, he was one of many Jewish scientists who left Germany after Adolf Hitler seized power. By then, he was already a known and respected scientist famous for his theory of relativity, which the Nazis derided as a “Jewish perversion.”

In the United States, government officials targeted Einstein for his support of civil rights, socialism and nuclear disarmament. “It's about time the American people got wise to Einstein… He ought to be prosecuted,” proclaimed Congressman John Rankin, a member of the House Un-American Activities Committee, in 1945. Though HUAC never charged him with anything, the FBI began a file on Einstein’s allegedly subversive activities in 1932. By the time Einstein died in 1955, the file was 1,427 pages long.

Einstein’s birthday photo, divorced from its political context, struck a chord with fans of the scientist, and became one of the most iconic images of the 20th century. The specific image that became famous was a cropped version of the photo that focused only on Einstein (Sasse’s original photo showed Einstein sitting between his colleague Frank Aydelotte and Aydelotte’s wife, Marie Jeanette).

The birthday photo captures the spirit of the irreverent physicist, who was often skeptical of authority. As he wrote in a 1901 letter, “Unthinking respect for authority is the greatest enemy of truth.”