Essen, Louis

▪ 1998

      British physicist (b. Sept. 6, 1908, Nottingham, Nottinghamshire, Eng.—d. Aug. 24, 1997, Great Bookham, Surrey, Eng.), built the cesium-beam atomic clock, a device that ultimately changed the way time is measured. Atomic clocks measure the passage of time by using a device that counts the extremely regular waves of electromagnetic radiation emitted from atoms. This new method of timekeeping generated a more accurate time scale than was previously possible and eventually, in 1967, replaced the astronomical methods used until then to determine the international time standard. Essen developed the clock in 1955 with his collaborator J.V.L. Parry at the National Physical Laboratory in Teddington, Middlesex, Eng. Essen began working at the laboratory in 1929, not long after receiving his B.Sc. in physics (1928) from the University of London, where he also earned a Ph.D. (1941) and D.Sc. (1948). There he began his career-long focus on the physics of frequency generation and measurement. In 1938 he developed the Essen quartz ring clock, a highly stable timepiece that allowed measurement of the seasonal variations in the Earth's rate of rotation. During World War II Essen devised a number of instruments to measure radio waves, such as the cavity resonance wavemeter, and after the war he used these devices to measure the velocity of light. In 1946 his measurements, carried out with A.C. Gordon-Smith, showed that light travels 16 km (10 mi) per second faster than the value accepted at the time. Essen's expertise in measuring time and velocity led him to question some aspects of Einstein's theories, criticisms he detailed in his paper The Special Theory of Relativity: A Critical Analysis. Essen received numerous honours, including the A.S. Popov Gold Medal from the U.S.S.R. Academy of Sciences in 1959, the same year he was made an O.B.E. In 1960 he was elected a fellow of the Royal Society of London.

* * *

▪ British physicist

born Sept. 6, 1908, Nottingham, Nottinghamshire, Eng.

died Aug. 24, 1997, Great Bookham, Surrey

      English physicist who invented the quartz crystal ring clock and the first practical atomic clock. These devices were capable of measuring time more accurately than any previous clocks.

      Essen studied physics at Nottingham University College, where he earned a University of London physics degree (1928), Ph.D. (1941), and D.Sc. (1948). In 1929 he began work on frequency and time standards at England's National Physical Laboratory at Teddington in Middlesex, making studies of tuning forks and quartz oscillators. His investigations culminated in the quartz ring clock (1938), which used the electrically induced vibrations of a quartz crystal to measure time. Essen's clock entered into wide use as a time standard in observatories and was the first device accurate enough to measure the minute variations in the Earth's speed of rotation; prior to Essen's work, scientists had thought that the speed was constant.

      During World War II Essen invented several radio-wave measuring devices, and in 1946 he and A.C. Gordon-Smith used one such device, a cavity resonance wavemeter, to measure the speed of light with unprecedented accuracy. The figure they obtained, 299,792 ± 3 kilometres per second, was 16 km/sec greater than the most accurate value achieved to that time. In 1950 they used an improved cavity resonator to obtain a value of 299,792.5 ± 1 km/sec for light's velocity, a figure differing by less than two metres per second from the more accurate laser-based value officially adopted in 1975.

      By 1950 Essen had become interested in the possibility of using the frequency of atomic spectral lines to keep time with extraordinary accuracy. The clock that he and his colleague J.V.L. Parry had developed by 1955 was regulated by the natural resonance frequency of cesium atoms. It was accurate to within one part in 10 billion and was the first atomic clock to meet the required standards of accuracy for such devices. By 1957 they had developed an improved version of the clock that was accurate to within one part in one trillion. The extremely accurate value obtained by Essen and Parry for the frequency of the cesium atom in 1958 provided a new standard for measuring time, called atomic time, and was eventually (1967) used to redefine the standard SI unit of time, the second (International System of Units), in terms of atomic frequencies.

      Essen became deputy chief scientific officer at the National Physical Laboratory in 1960 and was elected a fellow of the Royal Society that same year. He angered both the Royal Society and the British government in the early 1970s when he published criticisms of Einstein's special theory of relativity. Essen retired in 1972.

* * *