
Albert Einstein, one of the 20th century's most influential thinkers, poses for a portrait in 1946. The late physicist would have been 139 this week.

A young Einstein poses with his sister in this undated photo. Einstein was born in Germany but later moved to the United States and became a U.S. citizen.

Einstein, seen here in 1905, developed the general theory of relativity, which fundamentally changed our understanding of the universe. He also won the Nobel Prize in physics in 1921.

Einstein, second from right, attends an international physics conference in Brussels, Belgium, in 1911.

The earliest surviving manuscript of Einstein's theory of relativity sold for $1.2 million in 1987. It features the mass-energy equivalence formula (E=mc²).

Einstein speaks to inventor Thomas Edison during a radio broadcast in 1920. It was heard by 50 million people.

Einstein stands with his wife, Elsa, as they visit the White House in 1921. Standing in front of Einstein is U.S. President Warren G. Harding.

Einstein and his wife visit Egypt circa 1921.

The Einsteins sit at their home near Berlin circa 1930. They moved to the United States a couple years later during the rise of the Nazis.

Einstein and fellow physicist Leo Szilard drafted a letter to U.S. President Franklin D. Roosevelt in 1939, urging him to research atomic bombs before the Germans could build one first. By 1942, the United States had approved the top-secret Manhattan Project to build a nuclear reactor and assemble an atomic bomb.

Einstein attends a charity concert at a Berlin synagogue in 1930.

Einstein relaxes in Palm Springs, California, in 1932.

Einstein, left, plays violin aboard a German passenger ship in 1933.

Einstein leans against the mast of a sailboat in Saranac Lake, New York, in 1936.

In 1950, tiny metal electrodes are attached to Einstein's head to pick up impulses from his brain and to magnify and record them for study.

Einstein sticks out his tongue for photographers covering his 72nd birthday in 1951. He died April 18, 1955, at the age of 76. In 1999, Time magazine named him the Person of the Century. "He was unfathomably profound -- the genius among geniuses who discovered, merely by thinking about it, that the universe was not as it seemed," wrote Frederic Golden.