page_title: Louis Essen
section_title: Atomic clocks
hierarchical_section_title: Louis Essen / Atomic clocks
caption_attribution_description: English: Louis Essen and J. V. L. Parry standing next to the world's first caesium atomic clock, developed at the UK National Physical Laboratory in 1955. Deutsch: Louis Essen und J. V. L. Parry zeigen die erste Caesium-Atomuhr (1955)
context_page_description: Louis Essen FRS O.B.E. was an English physicist whose most notable achievements were in the precise measurement of time and the determination of the speed of light. He was a critic of Albert Einstein's theory of relativity, particularly as it related to time dilation.
context_section_description: Essen earned his Ph.D. (1941) and Doctor of Science (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.
caption: Louis Essen (right) and Jack Parry (left) standing next to the world's first caesium-133 atomic clock.
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