Louis Essen
FRS O.B.E.
The Most Excellent Order of the British Empire is a British order of chivalry, rewarding contributions to the arts and sciences, work with charitable and welfare organisations,
and public service outside the civil service. It was established o ...
(6 September 1908 – 24 August 1997) was an English
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 caus ...
whose most notable achievements were in the
precise measurement of
time
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, to ...
and the determination of the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
. He was a critic of
Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
'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 ...
, particularly as it related to
time dilation
In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them ( special relativistic "kinetic" time dilation) or to a difference in gravitational ...
.
Early work
Born in
Nottingham
Nottingham ( , East Midlands English, locally ) is a city status in the United Kingdom, city and Unitary authorities of England, unitary authority area in Nottinghamshire, East Midlands, England. It is located north-west of London, south-east ...
, Essen earned his degree in
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
from the
University of London
The University of London (UoL; abbreviated as Lond or more rarely Londin in post-nominals) is a federal public research university located in London, England, United Kingdom. The university was established by royal charter in 1836 as a degree ...
in 1928, having studied at
University College Nottingham
, mottoeng = A city is built on wisdom
, established = 1798 – teacher training college1881 – University College Nottingham1948 – university status
, type = Public
, chancellor ...
. He started work at the
National Physical Laboratory (NPL) the following year, under
D. W. Dye, investigating the potential of
tuning fork
A tuning fork is an acoustic resonator in the form of a two-pronged fork with the prongs (tines) formed from a U-shaped bar of elastic metal (usually steel). It resonates at a specific constant pitch when set vibrating by striking it against ...
s and
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses a piezoelectric crystal as a frequency-selective element. The oscillator frequency is often used to keep track of time, as in quartz wristwatches, to provide a stable cloc ...
s 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
An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysical, oceanography and volcanology are examples of disciplines for which observatories have been constructed. His ...
throughout the world.
The speed of light
During
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, Essen worked on
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
and developed a number of instruments, including the
cavity resonance wavemeter An absorption wavemeter is a simple electronic instrument used to measure the frequency of radio waves. It is an older method of measuring frequency, widely used from the birth of radio in the early 20th century until the 1970s, when the developme ...
. 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
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
for a variety of its
normal mode
A normal mode of a dynamical system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation. The free motion described by the normal modes takes place at fixed frequencies. ...
s. As the
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of the modes was known from the geometry of the cavity and from
electromagnetic theory
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
, knowledge of the associated frequencies enabled a calculation of the speed of light. Their result, 299,792±3 km/s, was substantially greater than that from 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
Sir Charles Galton Darwin (19 December 1887 – 31 December 1962) was an English physicist who served as director of the National Physical Laboratory (NPL) during the Second World War. He was a son of the mathematician George Howard Darwin an ...
, 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
Stanford University, officially Leland Stanford Junior University, is a private research university in Stanford, California. The campus occupies , among the largest in the United States, and enrolls over 17,000 students. Stanford is consider ...
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
Alan Mathison Turing (; 23 June 1912 – 7 June 1954) was an English mathematician, computer scientist, logician, cryptanalyst, philosopher, and theoretical biologist. Turing was highly influential in the development of theoretical com ...
) 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
The International Union of Radio Science (abbreviated ''URSI'', after its French name, french: link=no, Union radio-scientifique internationale) is one of 26 international scientific unions affiliated to the International Council for Science ( ...
in 1957. Most subsequent measurements have been consistent with this value. In 1983, the 17th
Conférence Générale des Poids et Mesures
The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established i ...
adopted the standard value, 299,792.458 km/s for the speed of light.
Atomic clocks
![Atomic Clock-Louis Essen](https://upload.wikimedia.org/wikipedia/commons/5/55/Atomic_Clock-Louis_Essen.jpg)
Essen earned his
PhD PHD or PhD may refer to:
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(1941) and
Doctor of Science
Doctor of Science ( la, links=no, Scientiae Doctor), usually abbreviated Sc.D., D.Sc., S.D., or D.S., is an academic research degree awarded in a number of countries throughout the world. In some countries, "Doctor of Science" is the degree used f ...
(1948) from the University of London before becoming interested in the possibility of using the frequency of
atomic spectra
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wav ...
to improve time measurement. The feasibility of measuring time using
caesium
Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
as an atomic reference had been demonstrated by the US
National Bureau of Standards
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...
. In 1955, he developed, in collaboration with Jack Parry, the first practical
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
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 Standard may refer to:
Symbols
* Colours, standards and guidons, kinds of military signs
* Standard (emblem), a type of a large symbol or emblem used for identification
Norms, conventions or requirements
* Standard (metrology), an object th ...
. The
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
had already been proposed as such but Essen was convinced that caesium would prove more stable. However, the
International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
meeting in Rome in 1952 had adopted the
ephemeris time scale, on a proposal by
Gerald Clemence defining the time unit in terms of the Earth's motion round the sun. The ephemeris second, based on a fraction of the tropical year derived from
Simon Newcomb
Simon Newcomb (March 12, 1835 – July 11, 1909) was a Canadian–American astronomer, applied mathematician, and autodidactic polymath. He served as Professor of Mathematics in the United States Navy and at Johns Hopkins University. Born in Nov ...
's expression for the mean solar motion, became a standard in 1960, but in 1967, at the 13th Conférence Générale des Poids et Mesures, the
second
The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds ...
was redefined in terms of a value for the ephemeris second that had been precisely measured by Essen in collaboration with
William Markowitz
William Markowitz (February 8, 1907 in Melč, Austrian Silesia – October 10, 1998 in Pompano Beach, Florida) was an American astronomer, principally known for his work on the standardization of time.
His mother was visiting Melč near Ví ...
of the
United States Naval Observatory
United States Naval Observatory (USNO) is a scientific and military facility that produces geopositioning, navigation and timekeeping data for the United States Navy and the United States Department of Defense. Established in 1830 as the Depo ...
in terms of the frequency of a chosen line from the spectrum of caesium.
Later life
Essen spent all his working life at the National Physical Laboratory.
In 1971 he published ''The Special Theory of Relativity: A Critical Analysis'', questioning
Special relativity
In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates:
# The laws o ...
, which apparently was not appreciated by his employers.
Essen said in 1978:
He retired in 1972 and died in
Great Bookham
Great Bookham is a village in Surrey, England, one of six semi-urban spring line settlements between the towns of Leatherhead and Guildford. With the narrow strip parish of Little Bookham, it forms part of the Saxon settlement of ''Bocham'' ("the ...
,
Surrey
Surrey () is a ceremonial and non-metropolitan county in South East England, bordering Greater London to the south west. Surrey has a large rural area, and several significant urban areas which form part of the Greater London Built-up Area. ...
in 1997.
Awards and honours
*A.S. Popov Gold Medal from the
USSR Academy of Sciences
The Academy of Sciences of the Soviet Union was the highest scientific institution of the Soviet Union from 1925 to 1991, uniting the country's leading scientists, subordinated directly to the Council of Ministers of the Soviet Union (until 1946 ...
(1959)
*
O.B.E.
The Most Excellent Order of the British Empire is a British order of chivalry, rewarding contributions to the arts and sciences, work with charitable and welfare organisations,
and public service outside the civil service. It was established o ...
(1959)
*
Fellow of the Royal Society
Fellowship of the Royal Society (FRS, ForMemRS and HonFRS) is an award granted by the judges of the Royal Society of London to individuals who have made a "substantial contribution to the improvement of natural science, natural knowledge, incl ...
(1960)
*Rabi Award of the
(1987)
References
External links
*
Sciencemuseum: Louis Essen and atomic clocks
{{DEFAULTSORT:Essen, Louis
English physicists
Alumni of the University of London
Alumni of the University of Nottingham
Fellows of the Royal Society
People educated at Nottingham High Pavement Grammar School
People from Nottingham
1908 births
1997 deaths
Scientists of the National Physical Laboratory (United Kingdom)
20th-century British inventors
Relativity critics