Hendrik Antoon Lorentz was widely regarded as the world's leading theoretical physicist at the end of the nineteenth century and the beginning of the twentieth. His earliest work dealt with optical phenomena, but by 1892 he had begun to develop one of the concepts for which he is most famous, the electron theory. Based on the presumed existence of tiny charged particles in matter, this theory explained a number of well-known physical phenomena. In 1902 Lorentz received the Nobel Prize in physics for his work on the interaction of radiation and matter; he shared the award with Pieter Zeeman. In the first decade of the twentieth century, Lorentz worked on the effect of motion on the properties of a particle, foreshadowing some of the fundamental concepts that were later to become part of Albert Einstein's theory of relativity.
Hendrik Antoon Lorentz was born in Arnhem in the Netherlands, on July 18, 1853. His father was Gerrit Frederik Lorentz, owner of a nursery; his mother, Geertruida van Ginkel Lorentz, died when Lorentz was four years old, and five years later his father was married for a second time, to Luberta Hupkes. Lorentz attended primary and secondary schools in Arnhem, and while he showed a special interest in the sciences, he did well in all subjects. In 1870, Lorentz entered the University of Leiden, where he studied physics and mathematics. He became especially interested in the lectures of Frederick Kaiser, a professor of astronomy, and Pieter Leonhard Rijke, the university's only professor of physics. Lorentz passed his candidate's examination (approximately equivalent to a bachelor's degree) in November 1871, only 18 months after entering the university.
Lorentz returned to Arnhem to prepare for his doctoral examination. Over the next two years he supported himself by teaching in the local high school; he passed the examination summa cum laude in 1873. He then began his doctoral research on the theory of reflection and refraction, an aspect of electromagnetic theory that James Clerk Maxwell had left unsolved in his earlier studies of the subject. Lorentz was awarded his Ph.D. in 1875 for his thesis, "The Theory of Reflection and Refraction of Light."
Lorentz remained in Arnhem for four years after receiving his doctorate, continuing to teach high school and trying to decide the future direction of his career. He was uncertain as to whether he should focus on mathematics or physics. That choice was made even more difficult in 1877 when he was offered a chair in mathematics at the University of Utrecht and a chair in theoretical physics at the University of Leiden. The position at Leiden was the first in theoretical physics created in the Netherlands and one of the first in the world. Lorentz chose physics, not only establishing his own future but that of the field of theoretical physics over the next half century.
During his first few years at Leiden, Lorentz concentrated on the study of optical phenomena. One of his accomplishments during this period was the discovery of a formula relating the density of a substance to its index of refraction. That formula is now known as the Lorentz-Lorenz equation in honor of his work and that of Danish physicist Ludwig Lorenz, who made the same discovery almost simultaneously.
Develops Theory of Electrons and Lorentz Transformations
In the early 1890s, Lorentz began work on one of his most important theoretical contributions, commonly known as his theory of electrons. Electrons had not yet been discovered, so the term is somewhat misleading; Lorentz posited the existence of tiny, charged particles to explain a number of physical phenomena, and these particles had properties very similar to those of the electron that was discovered by Joseph John Thomson in 1897. Lorentz argued, for example, that electromagnetic radiation is produced by the vibration of the tiny charged particles, and this has turned out to be true of electrons.
An important consequence of Lorentz's electron theory was the prediction that spectral lines would be split if the source of light from which they came was placed within a strong magnetic field. Spectral lines are an optical phenomenon, and they occur when the light from a gas flame is split into different colors. Lorentz hypothesized that the presence of the magnetic field would alter the motion of the charged particles that produced the spectral lines, causing them to shift positions. Lorentz's theory was confirmed by a series of experiments conducted in 1896 by Lorentz's younger colleague at Leiden, Pieter Zeeman . For their discovery and explanation of the splitting of spectral lines, Lorentz and Zeeman were jointly awarded the 1902 Nobel Prize in physics.
In the last decade of the nineteenth century, Lorentz began to do more work in electrodynamics. One of the foundations of electrodynamic theory at the time was the posited existence of a substance called aether (or ether). The existence of aether was thought necessary to explain a number of physical phenomena, such as light waves, and it was believed to fill all space. In 1887, Albert Michelson and Edward W. Morley performed a classic experiment that failed to find any evidence for the existence of aether. In his own studies of electromagnetic phenomena, Lorentz had found it necessary to postulate the existence of an aether, so he was troubled by the results of the Michelson-Morley experiment.
Lorentz decided that one way to explain the Michelson-Morley results was to assume that objects traveling through the aether (in which he continued to believe) underwent a shortening in length in the direction in which they were moving. He derived a formula that predicted an amount of shortening that would exactly account for the failure of Michelson and Morley to find any evidence of the motion of light through the aether. At almost the same time, Irish physicist George Francis FitzGerald (1851-1901) postulated a formula identical to that of Lorentz's. According to the Lorentz-FitzGerald transformation, the length of a body contracts by a factor of the square root of 1 - v2 /c2 as it moves through the aether (v equals velocity and c is the speed of light). Lorentz also calculated that the mass of a particle would increase by the same factor. The Lorentz-FitzGerald transformation was later found by Albert Einstein to be a necessary consequence of the theory of relativity .
During his long tenure at Leiden, Lorentz had an enormous influence on the development of modern physics. That influence resulted not only from his own research and writing, but also from the students he trained and the professional organizations with which he became involved. In 1912, however, he resigned from his post at Leiden to become director of the Teyler's Stichting Museum in Haarlem, Netherlands. In his new job at Haarlem, Lorentz had his own, well-equipped laboratory, a facility that he had been promised for decades--though never received--at Leiden. During his 11-year tenure in Haarlem, Lorentz continued to teach at Leiden once a week and in 1919 became involved in the Dutch effort to reclaim land from the Zuider Zee. He was also very interested in science education in the Netherlands and served on the government board of education from 1919 until 1926.
Lorentz was held in high esteem by his professional colleagues and was offered a number of important positions in professional organizations. From 1911 to 1927, for example, he was chair of the International Solvay Congress in Physics. He also served as president of the physics section of the Royal Netherlands Academy of Sciences and Letters from 1909 to 1921. In 1923 he was elected as one of the seven members of the International Commission on Intellectual Cooperation of the League of Nations, becoming president of the group two years later. Among his many awards and honors were the 1908 Rumford Medal and the 1918 Copley Medal from the Royal Society. Lorentz was married in 1881 to Aletta Catherina Kaiser, a niece of his astronomy professor at Leiden. The couple had three children. Lorentz was still active in physics when he died in Haarlem on February 4, 1928.
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