Georges Sagnac (French pronunciation: [ʒɔʁʒ saɲak]; 14 October 1869 – 26 February 1928) was a French physicist who lent his name to the Sagnac effect, a phenomenon which is at the basis of interferometers and ring laser gyroscopes developed since the 1970s.
While a lab assistant at the Sorbonne, he was one of the first in France to study X-rays, following Wilhelm Conrad Röntgen.
He belonged to a group of friends and scientists that notably included Pierre and Marie Curie, Paul Langevin, Jean Perrin, and the mathematician Émile Borel.
Marie Curie says that she and her husband had traded ideas with Sagnac around the time of the discovery of radioactivity.
The motion of the earth through space had no apparent effect on the speed of the light beam, no matter how the platform was turned.