The man in the empty sphere

<jats:title>Abstract</jats:title> <jats:p>In December 1912, Ehrenfest gives his inaugural lecture as professor of theoretical physics at Leiden. He uses the occasion to discuss the ether hypothesis and the nature of light, highlighting the work of Lorentz, Einstein and his late friend Walter Ritz. His impressive and enthusiastic lecture inspires the astronomer Willem de Sitter to test Einstein’s special theory of relativity by measuring the light emitted by binary stars.</jats:p>