Louis Essen
Encyclopedia
Louis Essen FRS O.B.E.
(6 September 1908 – 24 August 1997) 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 critical of Albert Einstein
's theory of relativity
, particularly as it related to time dilation
.
, 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 fork
s and quartz
crystal oscillator
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
throughout the world.
, 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 mode
s. 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.
(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.
. 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 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
'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
was redefined in terms of a value for the ephemeris second that had been precisely measured by Essen in collaboration with William Markowitz
of the United States Naval Observatory
in terms of the frequency of a chosen line from the spectrum of caesium.
In 1971 he published The Special Theory of Relativity: A Critical Analysis in which he questioned Einstein
's theory, which apparently was not appreciated by his employers.
As Essen later stated (1978) :
No one has attempted to refute my arguments, but I was warned that if I persisted I was likely to spoil my career prospects.
He retired in 1972 and died in Great Bookham
, Surrey
.
Order of the British Empire
The Most Excellent Order of the British Empire is an order of chivalry established on 4 June 1917 by George V of the United Kingdom. The Order comprises five classes in civil and military divisions...
(6 September 1908 – 24 August 1997) was an English
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
whose most notable achievements were in the precise
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...
measurement of time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
and the determination of the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
. He was critical of Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
's theory of relativity
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....
, particularly as it related to time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...
.
Early work
Born in NottinghamNottingham
Nottingham is a city and unitary authority in the East Midlands of England. It is located in the ceremonial county of Nottinghamshire and represents one of eight members of the English Core Cities Group...
, Essen earned his degree in physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
from the University of London
University of London
-20th century:Shortly after 6 Burlington Gardens was vacated, the University went through a period of rapid expansion. Bedford College, Royal Holloway and the London School of Economics all joined in 1900, Regent's Park College, which had affiliated in 1841 became an official divinity school of the...
in 1928, having studied at University College Nottingham
University of Nottingham
The University of Nottingham is a public research university based in Nottingham, United Kingdom, with further campuses in Ningbo, China and Kuala Lumpur, Malaysia...
. He started work at the National Physical Laboratory
National Physical Laboratory, UK
The National Physical Laboratory is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington, London, England. It is the largest applied physics organisation in the UK.-Description:...
(NPL) the following year, under D. W. Dye, investigating the potential of tuning fork
Tuning fork
A tuning fork is an acoustic resonator in the form of a two-pronged fork with the prongs formed from a U-shaped bar of elastic metal . It resonates at a specific constant pitch when set vibrating by striking it against a surface or with an object, and emits a pure musical tone after waiting a...
s and quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
crystal oscillator
Crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...
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
Observatory
An observatory is a location used for observing terrestrial or celestial events. Astronomy, climatology/meteorology, geology, oceanography and volcanology are examples of disciplines for which observatories have been constructed...
throughout the world.
The speed of light
During World War IIWorld War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, Essen worked on radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
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
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
for a variety of its normal mode
Normal mode
A normal mode of an oscillating 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 frequencies of the normal modes of a system are known as its natural frequencies or resonant frequencies...
s. As the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
, 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
Charles Galton Darwin
Sir Charles Galton Darwin, KBE, MC, FRS was an English physicist, the grandson of Charles Darwin. He served as director of the National Physical Laboratory during the Second World War.-Early life:...
, 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
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...
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 Turing
Alan Mathison Turing, OBE, FRS , was an English mathematician, logician, cryptanalyst, and computer scientist. He was highly influential in the development of computer science, providing a formalisation of the concepts of "algorithm" and "computation" with the Turing machine, which played a...
) inspired him to refine his apparatus and to repeat his measurement in 1950, establishing a result of 299,792.5±1 km/s
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
, . 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
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
for the speed of light.
Atomic clocks
Essen earned his Ph.D.Ph.D.
A Ph.D. is a Doctor of Philosophy, an academic degree.Ph.D. may also refer to:* Ph.D. , a 1980s British group*Piled Higher and Deeper, a web comic strip*PhD: Phantasy Degree, a Korean comic series* PhD Docbook renderer, an XML renderer...
(1941) and Doctor of Science
Doctor of Science
Doctor of Science , usually abbreviated Sc.D., D.Sc., S.D. or Dr.Sc., is an academic research degree awarded in a number of countries throughout the world. In some countries Doctor of Science is the name used for the standard doctorate in the sciences, elsewhere the Sc.D...
(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
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
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
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...
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 standardStandardization
Standardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....
. The ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
had already been proposed as such but Essen was convinced that caesium would prove more stable. However, the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
meeting in Rome in 1952 had adopted the ephemeris time
Ephemeris time
The term ephemeris time can in principle refer to time in connection with any astronomical ephemeris. In practice it has been used more specifically to refer to:...
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
Simon Newcomb was a Canadian-American astronomer and mathematician. Though he had little conventional schooling, he made important contributions to timekeeping as well as writing on economics and statistics and authoring a science fiction novel.-Early life:Simon Newcomb was born in the town of...
'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
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
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
William Markowitz was an American astronomer, principally known for his work on the standardization of time....
of the United States Naval Observatory
United States Naval Observatory
The United States Naval Observatory is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation, and Timing for the U.S. Navy and the U.S. Department of Defense...
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 in which he questioned Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
's theory, which apparently was not appreciated by his employers.
As Essen later stated (1978) :
No one has attempted to refute my arguments, but I was warned that if I persisted I was likely to spoil my career prospects.
He retired in 1972 and died in Great Bookham
Great Bookham
-Today:The village has a high street, located in Great Bookham, which is, as its name suggests, the larger of the two villages. It has two butchers, a family run fishmongers and two traditional greengrocers...
, Surrey
Surrey
Surrey is a county in the South East of England and is one of the Home Counties. The county borders Greater London, Kent, East Sussex, West Sussex, Hampshire and Berkshire. The historic county town is Guildford. Surrey County Council sits at Kingston upon Thames, although this has been part of...
.
Awards and honours
- A.S. Popov Gold Medal from the USSR Academy of Sciences (1959)
- O.B.E.Order of the British EmpireThe Most Excellent Order of the British Empire is an order of chivalry established on 4 June 1917 by George V of the United Kingdom. The Order comprises five classes in civil and military divisions...
(1959) - Fellow of the Royal Society (1960)
- Rabi Award of the IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society (1987)