Louis Essen - The Speed of Light

The Speed of Light

During World War II, Essen worked on radar and developed a number of instruments, including the cavity resonance wavemeter. It was this work that suggested to Essen the possibility of a more precise measurement of the speed of light. In 1946, in collaboration with A.C. Gordon-Smith, he used a microwave cavity, of precisely known dimensions, and exploited his expertise in time-measurement to establish the frequency for a variety of its normal modes. As the wavelength of the modes was known from the geometry of the cavity and from electromagnetic theory, knowledge of the associated frequencies enabled a calculation of the speed of light. Their result, 299,792±3 km/s, was substantially greater than the prevailing sequence of optical measurements that had begun around the start of the 20th century and Essen had to withstand some fierce criticism and disbelief. Even NPL director Sir Charles Galton Darwin, while supporting the work, observed that Essen would get the correct result once he had perfected the technique. Moreover, W.W. Hansen at Stanford University had used a similar technique and obtained a measurement which was more consistent with the conventional (optical) wisdom. However, a combination of Essen's stubbornness, his iconoclasm and his belief in his own skill at measurement (and a little help with calculations from Alan Turing) inspired him to refine his apparatus and to repeat his measurement in 1950, establishing a result of 299,792.5±1 km/s, . This was the value adopted by the 12th General Assembly of the Radio-Scientific Union in 1957. Most subsequent measurements have been consistent with this value. In 1983, the 17th Conférence Générale des Poids et Mesures adopted the standard value, 299,792.458 km/s for the speed of light.