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Louis Essen Biography
Louis Essen (September 6, 1908 - August 24, 1997) was an English physicist whose most notable achievements were in the precise measurement of time and the determination of the speed of light.

Early work
Born in Nottingham, Essen earned his degree in physics from the University of London in 1928, having studied at University College Nottingham. He started work at the National Physical Laboratory (NPL) the following year, under D.W. Dye, investigating the potential of tuning forks and quartz crystal oscillators for precise time measurement. His research led to his development of the quartz ring clock in 1938, the clock soon becoming a standard for time measurement at observatories throughout the world.

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 subsantially 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 stubornness, his iconocalsm 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.

Atomic clocks
Essen earned his Ph.D. (1941) and D.Sc. (1948) from the University of London before becoming interested in the possibility of using the frequency of atomic spectra to improve time measurement. The feasibility of measuring time using caesium as an atomic reference had been demonstrated by the US National Bureau of Standards. In 1955, he developed, in collaboration with Jack Parry, the first practical atomic clock, by integrating the caesium atomic standard with conventional quartz crystal oscillators to allow calibration of existing time-keeping.

Time standards
This work led Essen to champion the caesium spectrum as an international time standard. The ammonia molecule had already been proposed as such but Essen was convinced that caesium would prove more stable. However, the International Astronomical Union meeting in Dublin in 1955 had adopted the ephemeris time, developed by Simon Newcomb in the 19th century in terms of the Earth’s motion round the sun. The ephemeris second became a standard in 1960 but in 1967, at the 13th Conférence Générale des Poids et Mesures, the second was redefined in terms of the spectrum of caesium that had been precisely measured by Essen in collaboration William Markowitz of the United States Naval Observatory.

Later life
Essen spent all his working life at the National Physical Laboratory but caused some embarrassment to his employers in (1971) when he published The Special Theory of Relativity: A Critical Analysis in which he questioned Einstein's theories.

He retired in 1972 and died in Great Bookham, Surrey.

A.S. Popov Gold Medal from the USSR Academy of Sciences - (1959)
O.B.E. - (1959)
Fellow of the Royal Society - (1960)
Rabi Award of the IEEE - (1987)
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article Louis Essen.