Albert Einstein with Paul Ehrenfest and his son at the piano in Leiden.

Digital ImagePaul Ehrenfest was a physicist and professor at the University of Leiden in the Netherlands. He married Tatyana Alexeyevna Afanassjewa, a mathematician. He was known as a profound, yet engaging teacher who popularized and explained many new ideas in physics, particularly in the categories of quantum theory and relativity. His own research and collaborations with his wife served to solidify existing concepts in physics, while his personal devotion to science forged bonds between himself and the likes of physicists Max Planck and Albert Einstein. Ehrenfest succeeded Hendrik Lorentz as professor of theoretical physics at the University of Leiden, where he remained until his death. Ehrenfest suffered from depression. In 1933 he shot his youngest son, Wassik, who had down syndrome, before killing himself