Einstein himself apparently had no special plans for what he knew could be a momentous day. He was home in Berlin. He wrote a letter admitting a “blunder” in an ongoing debate with Theodor Kaluza, a German mathematician with a new notion of space-time that required five dimensions. He betrayed no jitters about the fact that, on that day in May, two scientific expeditions were finally putting his theory of general relativity to the test. In Sobral, Brazil, and on Príncipe Island, off the western coast of Africa, two teams were viewing a total solar eclipse; in measuring the deflection of starlight by the sun’s gravitational field, they proved Einstein right. Einstein first received word of their preliminary results in September, and wrote his mother with the “happy news.”