Lawrence Bragg

Australian-born British physicist

• Born: March 31, 1890
• Birthplace: Adelaide, South Australia, Australia
• Died: July 1, 1971
• Place of death: Ipswich, Suffolk, England

Bragg used X-ray diffraction to determine the arrangement of atoms in many crystals and helped establish the field of X-ray crystallography. With his father, also a physicist, he won the Nobel Prize in Physics in 1915.

Early Life

Lawrence Bragg was the son of William Henry Bragg, a professor of mathematics and physics at the University of South Australia, and Gwendoline Todd, whose father was postmaster general and government astronomer. A second son, Robert Charles, was born a year later, and a daughter, Gwendolen Mary, in 1907.

Bragg started school at five years of age, but there was a setback when, through a tricycle accident, he shattered his left elbow. This incident involved Bragg in his first experience with X rays: The X-ray photograph of his injured arm was the first medical use of X rays in Australia. At eleven, he was sent to St. Peter’s College, the leading Church of England school in South Australia, and when he was only fifteen, he entered Adelaide University. During this time, significant events were happening in his father’s life. Marie Curie had done experiments that implied that all alpha particles emitted by radium traveled about the same distance into the surrounding air. William Henry Bragg believed that this could not be correct. He believed that the number of alpha particles penetrating a given distance into the air did not change much with distance until a certain critical value was passed, and then there was a rapid fall. He tested his ideas and found that they were confirmed. This work led to William Henry Bragg’s election as a fellow of the Royal Society.

Meanwhile, his son was having a successful career as an undergraduate at Adelaide University. He concentrated on mathematics, with subsidiary courses in chemistry, and he was graduated at the age of eighteen with first-class honors. Within a short time of Lawrence’s graduation, his father accepted the chair of physics at Leeds University, and the family left Australia in January, 1909. It would be more than fifty years before Lawrence would see Australia again.

Following in his father’s footsteps, Lawrence went to Trinity College, Cambridge University, and began studying mathematics. After a year, however, at the suggestion of his father, he transferred to physics. The move was a wise one, because Lawrence had a strong visual (rather than abstract) grasp of things, and he liked the way simple equations could be used to gain insight into natural phenomena.

Pictures of Lawrence from this period show him already balding (a trait he inherited from his father), with the mustache he would wear throughout his life (again, like his father). Throughout his undergraduate years, Lawrence remained in close touch with his family at Leeds. His mother had trouble adjusting to England, and his father was engaged in a controversy with Charles Barkla over the nature of X rays (William Henry thought they were particles; Barkla, waves). After obtaining first-class honors in the natural science examination, the young Bragg decided to do research under Sir Joseph John Thomson.

Life’s Work

During his summer vacation in 1912, Lawrence heard from his father about the work of Max von Laue in Germany. Laue had the idea that a crystal irradiated with X rays might give diffraction effects. He had had this tested, and indeed, X rays were scattered by the crystals in discrete directions, giving on a photographic plate a pattern of spots that depended on the orientation of the crystal and on its symmetry.

Lawrence and his father were deeply interested in this new phenomenon, and William Henry hoped to explain it in terms of particles. On his return to Cambridge, Lawrence continued to think about diffraction, and he had the idea that the pattern of spots could be interpreted as reflections of the X rays from the planes of atoms in the crystal. Laue pictures, then, could be used to get information about the structures of crystals. He developed an equation that described the angles at which X rays would be most effectively diffracted by a crystal. This was the start of the X-ray analysis of crystals.

The young Bragg’s paper, which appeared in January, 1913, was called “The Diffraction of Short Electromagnetic Waves by a Crystal,” because he was still unwilling to abandon his father’s idea that X rays were particles. He believed that they might possibly be particles accompanied by waves. His father, prompted by Lawrence’s results, constructed an X-ray spectrometer and found that each metal used in the X-ray tube as a source of radiation gave a characteristic X-ray spectrum of definite wavelengths. William Henry at first used the spectrometer to pursue his interest in the corpuscular nature of X rays, but it soon became evident that the device was a far more powerful way of analyzing crystals than the Laue photograph. This is when father and son joined forces and founded the new science of X-ray crystallography. According to Lawrence, during the summer of 1913 in Leeds, the X-ray spectrometer opened up a new world: He saw it as being like discovering a goldfield with nuggets lying around waiting to be picked up. He used the new instrument to determine the crystal structures of diamond, zinc blende, fluorspar, iron pyrites, and calcite.

The outbreak of World War I in August, 1914, brought this period of his life to an end. Lawrence was commissioned as a second lieutenant in the Leicestershire Royal Horse Artillery. He was later transferred to the map section of general headquarters, where he acted as technical adviser on sound ranging, a method of locating enemy guns from the sound of their firing. His personal life was deeply affected in the autumn of 1915 by the death of his brother Bob at Gallipoli, where the young physicist Henry Moseley was also killed. News of a more pleasant kind, the awarding of the 1915 Nobel Prize in Physics to him and his father, came when he was setting up the first sound-ranging station near the front line in Belgium.

Early in 1919, after the end of the war, Bragg returned to Cambridge to take up his duties at Trinity College, but he learned that Manchester University had created a special chair of crystallography for him, and he accepted the Langworthy Professorship of Physics in succession to Ernest Rutherford. Bragg’s early days in Manchester were not easy, for he took over at a difficult time. During the war, most of the staff had been away on war work, and now the teaching had to be organized (a task for which Bragg had no previous experience).

Bragg continued his work in crystallography at Manchester, and he began to reflect on the new ideas about atomic structure and chemical bonding that had emerged in the war years. He developed the idea that atoms had characteristic sizes in crystals, and he assigned sizes to certain ions (charged atoms or groups of atoms). Unfortunately, in his scheme, the negative ions were too small and the positive ions too large, but his concept of ionic size proved to be a valuable one.

During his early years at Manchester, Bragg analyzed crystal structures quantitatively. He saw the value of making experimental measurements of the absolute intensities of the X-ray reflections and relating these to the effective number of electrons contributing to each reflection. For his accomplishments in X-ray crystallography, he was made a fellow of the Royal Society in 1921. Among the letters of congratulation he received was one from Alice Hopkinson, a woman whom he had known earlier. She was then in her final year of history studies at Cambridge University. They were married in December, 1921. It was a happy marriage, resulting in two boys and two girls.

From 1925, Bragg concentrated on an intensive program of research into the structures of silicate minerals. The first of these structures he described were the olivines, and then he turned to the analysis of beryl. The analysis of other structures revealed that the properties of many silicates were determined by the tetrahedral grouping of a central silicon atom surrounded by four oxygen atoms, but Bragg was reluctant to recognize the importance of these tetrahedral units as complex ions and preferred to consider the individual atoms separately. In contrast, Linus Pauling proposed a method of describing the structures of the silicates in terms of these tetrahedral units, and he went on to propose a set of principles governing the structures of ionic crystals.

In 1930, a crisis occurred in Bragg’s life. His mother had died in 1929, and he was working too hard. Furthermore, his relationship with his father, always guarded, became more distant. Bragg recovered his equilibrium by taking a leave in Arnold Sommerfeld’s laboratory in Munich during the spring of 1931. Back in Manchester at Easter, Bragg changed his style of working, involving himself less closely in the day-to-day experiments and becoming more involved in those general scientific affairs that took him outside the university.

In 1937, Bragg became director of the National Physical Laboratory, but after Rutherford’s premature death in October of that year, he was invited to Cambridge as Cavendish professor of experimental physics. Again, his task was not easy, for he was a crystallographer succeeding a nuclear physicist; Bragg’s interests and management style differed from those of Rutherford. He did, however, make some important appointments: For example, he hired Max Perutz, who introduced him to the problem that would occupy him for the rest of his career protein-structure analysis.

In September, 1939, World War II began, and Bragg’s desire was to make what contribution he could to the war effort. His greatest achievement in this period was his invention of the bubble model of metal structure. He played little part in the war research at the Cavendish Laboratory, but he did contribute to the sound-ranging section of the army and to the development of sonar for the navy. He also traveled to Canada and Sweden in connection with his war service.

At the end of the war, his X-ray Analysis Group played a critical role in organizing crystallographic research internationally; for example, a new journal, Acta Crystallographica, was founded. Bragg also changed the nature of work being done at the Cavendish Laboratory. He believed that it could no longer be dominated by one research group under one strong scientific personality. He decentralized the work of the laboratory, thus breaking away from the Rutherford tradition. Among the groups that he formed, however, Bragg took the greatest personal interest in the work of Perutz, who was engaged in obtaining structural information directly from the diffraction patterns of hemoglobin crystals. Hemoglobin was a protein, and Bragg was attracted by the helical idea for the structure of protein molecules. He, John Kendrew, and Max Perutz tried various forms of the helical chain to show how proteins were structured, but they allowed free rotation about the single bonds in the chain and thus failed to find any convincing structure. Pauling, whose chemical insight showed him that some bonds had restricted rotation, came up with the idea of the alpha helix, which turned out to be a great success.

Bragg played no direct part in the study of the structure of deoxyribonucleic acid (DNA) at the Cavendish. He did insist, however, that Alexander Todd check the chemistry underlying the double-helix model of James D. Watson and Francis Crick. The successful solution of the DNA structural problem brought Bragg great happiness, since this important result came out of the Cavendish and not out of Pauling’s laboratory in the United States.

In 1954, Bragg moved to the Royal Institution in London. He had actually been playing an increasingly important part in the affairs of the Royal Institution during his time at Cambridge, and he took this position to revive the fortunes of an institution that had been experiencing much friction and discord. For the third time, then, Bragg accepted an appointment at a difficult time for the institution involved. His aim was to avoid further public scandal, though the immediate problem was financial. He turned to industry and business for support by instituting a new category of corporate subscribers. He himself gave many of the public lectures to large audiences. He also built up the protein-research program. His years at the Royal Institution turned out to be happy ones, despite advancing age and illness. In the tradition of Humphry Davy and Michael Faraday, he popularized science, making it palatable for the nonscientist. Through his lectures and a television series, he became an admired public figure.

Bragg retired from the Royal Institution in 1966. After his formal retirement, he continued to live in London and to lecture at the Royal Institution. He also published a book on the history of crystallography and maintained an active interest in crystallography and scientific popularization. He died in a hospital near his home on July 1, 1971.

Significance

An insight into the three-dimensional relationships underlying natural phenomena formed the core of Bragg’s scientific work. This insight helped him to revolutionize mineralogy, metallurgy, chemistry, and molecular biology. His revolution in crystallography coincided with another revolution in quantum physics, but for Bragg, quantum mechanics was of only peripheral interest. Throughout his life, he remained a classical physicist in the tradition of those who thought in terms of tangible rather than mathematical models.

Bragg was important for the development of twentieth century science in several ways. First, he united elements from various fields, for example, X rays and crystallography in X-ray crystallography. He also had the insight and an unusual amount of commitment to stimulate and direct the development of the field of X-ray crystallography. Finally, he was a great scientific organizer. He radiated enthusiasm, and enthusiasm is contagious. His ideas spread throughout his field, throughout physics, and, by means of his many students and followers, throughout the world.

Bibliography

Bragg, William Lawrence. The Development of X-Ray Analysis. Edited by D. C. Phillips and H. Lipson. London: G. Bell and Sons, 1975. Bragg’s last book, which was barely complete when he died, displays his vigorous mind and many examples of his ability to summarize complex problems simply.

Bragg, William Lawrence, and William Henry Bragg. X-Rays and Crystal Structure. London: G. Bell and Sons, 1915. The book that was largely responsible for the recognition of the new crystallography. In the preface Henry Bragg emphasizes that his son was responsible for the main idea behind X-ray crystallography. Intended for scientists, this book gives a good picture of the early accomplishments in the field.

Caroe, G. M. William Henry Bragg, 1862-1942: Man and Scientist. New York: Cambridge University Press, 1978. This biography of Lawrence Bragg’s father was written by his sister for general readers. It is dedicated to Lawrence Bragg’s memory, for he planned his book with her. Besides much information on the family, the book contains material on the work that Lawrence and his father did together.

Crowther, J. G. The Cavendish Laboratory, 1874-1974. London: Macmillan, 1974. This book, written in honor of the Cavendish centennial, contains a section on the history of the laboratory under Bragg’s leadership.

Hunter, Graeme K. Light Is a Messenger: The Life and Science of William Lawrence Bragg. New York: Oxford University Press, 2004. This first biography of Bragg recounts the events of his life and explains his scientific research, describing how his work influenced not only physics but also chemistry and the emerging science of molecular biology.

Lipson, H., et al. “Dedicated to Sir Lawrence Bragg on His Eightieth Birthday.” Acta Crystallographica A26 (March 31, 1970): 171-188. This special issue contains, in addition to two articles of reminiscences by Bragg, homages and memoirs by several of his students and colleagues.

Phillips, David. “William Lawrence Bragg, 31 March 1890 1 July 1971.” In Biographical Memoirs of Fellows of the Royal Society 25 (1975): 75-143. An extensive biographical article on Bragg. Gives a good presentation of the life and work of the scientist, a complete bibliography of his papers and books, and good general references. It was written with the cooperation of the Bragg family, and the author made use of material in the Bragg Archives of the Royal Institution.