Georges Sagnac (14 October 1869 – 26 February 1928) was a French

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...

who lent his name to the

Sagnac effect The Sagnac effect, also called Sagnac interference, named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferomet ...

, a phenomenon which is at the basis of interferometers and ring laser gyroscopes developed since the 1970s.

Life and work

Sagnac was born at

Périgueux Périgueux (, ; oc, Peireguers or ) is a commune in the Dordogne department, in the administrative region of Nouvelle-Aquitaine, southwestern France. Périgueux is the prefecture of Dordogne, and the capital city of Périgord. It is al ...

and entered the

École Normale Supérieure École may refer to: * an elementary school in the French educational stages normally followed by secondary education establishments (collège and lycée) * École (river), a tributary of the Seine flowing in région Île-de-France * École, S ...

in 1889. While a lab assistant at the

Sorbonne Sorbonne may refer to: * Sorbonne (building), historic building in Paris, which housed the University of Paris and is now shared among multiple universities. *the University of Paris (c. 1150 – 1970) *one of its components or linked institution, ...

, he was one of the first in France to study

X-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nbs ...

, following

Wilhelm Conrad Röntgen Wilhelm may refer to: People and fictional characters * William Charles John Pitcher, costume designer known professionally as "Wilhelm" * Wilhelm (name), a list of people and fictional characters with the given name or surname Other uses * Mount ...

. He belonged to a group of friends and scientists that notably included

Pierre Pierre is a masculine given name. It is a French form of the name Peter. Pierre originally meant "rock" or "stone" in French (derived from the Greek word πέτρος (''petros'') meaning "stone, rock", via Latin "petra"). It is a translation ...

and

Marie Marie may refer to: People Name * Marie (given name) * Marie (Japanese given name) * Marie (murder victim), girl who was killed in Florida after being pushed in front of a moving vehicle in 1973 * Marie (died 1759), an enslaved Cree person in Tr ...

Curie,

Paul Langevin Paul Langevin (; ; 23 January 1872 – 19 December 1946) was a French physicist who developed Langevin dynamics and the Langevin equation. He was one of the founders of the ''Comité de vigilance des intellectuels antifascistes'', an an ...

, Jean Perrin, and the mathematician

Émile Borel Félix Édouard Justin Émile Borel (; 7 January 1871 – 3 February 1956) was a French mathematician and politician. As a mathematician, he was known for his founding work in the areas of measure theory and probability. Biography Borel was ...

. Marie Curie says that she and her husband had traded ideas with Sagnac around the time of the discovery of radioactivity. Sagnac died at Meudon-Bellevue.

Sagnac effect

In 1913, Georges Sagnac showed that if a beam of light is split and sent in two opposite directions around a closed path on a revolving platform with mirrors on its perimeter, and then the beams are recombined, they will exhibit interference effects. From this result Sagnac concluded that light propagates at a speed independent of the speed of the source. 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. The effect had been observed earlier (by Harress in 1911), but Sagnac was the first to correctly identify the cause. This

(in vacuum) had been theoretically predicted by Max von Laue in 1911. He showed that such an effect is consistent with stationary ether theories (such as the

Lorentz ether theory What is now often called Lorentz ether theory (LET) has its roots in Hendrik Lorentz's "theory of electrons", which was the final point in the development of the classical aether theories at the end of the 19th and at the beginning of the 20th cen ...

) as well as with Einstein's

theory of relativity The theory of relativity usually encompasses two interrelated theories by Albert Einstein: special relativity and general relativity, proposed and published in 1905 and 1915, respectively. Special relativity applies to all physical phenomena in ...

. It is generally taken to be inconsistent with a complete ether drag; and also inconsistent with emission theories of light, according to which the speed of light depends on the speed of the source. Sagnac was a staunch opponent of the theory of relativity, despite the Sagnac effect being consistent with it.Zappe, Hans. (2010). ''Fundamentals of Micro-Optics''. Cambridge University Press. p. 143.

Life and work

Sagnac effect

See also

References

Further reading