The two meet as students in Göttingen. In 1912, they move to the Dutch town of Leiden, where Paul becomes a professor, and their house becomes a lively salon for famous scientists. Then comes World War I and in the turbulent times that follow, the couple lead the way with progressive ideas about vegetarianism, pacifism, and gender equality.

In 1926, Tatyana leaves for Moscow, disappointed. As a female scientist, she was kept in the wings. Without Tatyana, Paul becomes unhinged, and despairs the Nazism of Hitler’s Germany. He writes farewell letters and kills himself, after first killing their youngest son Wassik, a boy with Down’s syndrome.

Paul Ehrenfest did not make any major discoveries, but the role he played was perhaps just as important. He connected ideas, brought scientists together, and posed critical questions to colleagues such as Einstein, Bohr, Schrödinger. This is a social history of Europe, seen through the eyes of outsiders: a Jewish boy and a Russian Orthodox girl, whose lives were lived in a fascinating circle.