Light is a Messenger: the Life and Science of William Lawrence Bragg

  • Graeme K. Hunter
Oxford University Press: 2004. 320 pp. £35, $59.50 019852921X | ISBN: 0-198-52921-X
Leading light: Lawrence Bragg , like his father, was resident professor at the Royal Institution. Credit: HULTON-DEUTSCH/CORBIS

As a child I was enthralled by William Henry Bragg's Concerning the Nature of Things, which answers simply the question recently put to me by my granddaughter: “What are atoms and molecules?”. Both the author and his son William Lawrence Bragg were in their time resident professor at the Royal Institution of Great Britain in London, and both had an unusually well developed ability to communicate with school-children. Their joint Nobel prize, awarded in 1915, was for showing how X-ray diffraction could be used to determine the structure of crystalline substances. It is no coincidence that the title of William Bragg's book is a translation of De rerum natura, in which Lucretius set out his atomic theory of matter. However, Lucretius would have to wait nearly 2,000 years for the Braggs to show that he was right.

Lawrence was born in 1890 in Adelaide, Australia, where his father was a professor. He was a gifted pupil and became a very young member of the sixth form at St Peter's College. However, ignored by his older classmates, he was driven to finding solitary occupations, such as collecting and cataloguing sea-shells. At the age of 16 he proceeded to Adelaide University, where he took a degree in mathematics with first-class honours in 1908.

His father accepted an appointment as a professor at Leeds University, and in 1909 the family came to England. Lawrence entered Trinity College, Cambridge, taking first-class honours in the natural-science Tripos in 1912, and started his research under J. J. Thomson at the Cavendish Laboratory. His father had awakened his interest in Max von Laue's work on the diffraction of X-rays by crystals. Lawrence's studies of von Laue's diffraction patterns led him to postulate that zinc sulphide was based on a face-centred-cubic lattice, an amazing piece of insight. It was during this period that he formulated Bragg's law. Intuitively much simpler than the von Laue equations, it allows an estimate, by inspecting simple crystals, of how strong a particular X-ray reflection would be.

Lawrence started to work with his father in the summer of 1913. Although the older Bragg was still principally interested in X-ray spectra, his X-ray spectrometer also provided a powerful tool for crystal analysis. After showing its power by analysing the structure of diamond, William continued to establish the relations between X-ray spectra and the K and L absorption edges, and Lawrence concentrated on interpreting crystal structures. It was the publication of their results in abridged form in 1915 that earned the two Braggs the Nobel prize for physics in 1915. At just 25 years of age, Lawrence was the youngest ever Nobel laureate.

During the First World War, Lawrence served as a technical adviser on sound ranging in France, where he made a number of friends, including R. W. James. Lawrence was appointed Langworthy professor of physics at Manchester University in 1919, and in 1921 he married Alice Hopkinson. He was neither a skilled a lecturer nor a good administrator, however, and relied on James to keep the department running. But the remarkable science continued, with structures of the silicates and the optical theory of the diffraction of X-rays. The lab was abuzz with famous visitors. His father was then at the Royal Institution in London, presiding over Bill Astbury's unruly genius. Together with Kathleen Lonsdale and John Desmond Bernal, they were working out how to do X-ray structure analysis of complex organic molecules. Between them, the Braggs had it sewn up.

After a year as director of the National Physical Laboratory in 1937–38, Lawrence became Cavendish professor of physics at Cambridge University (1938–53), finding Rutherford a hard act to follow, as he had at Manchester. Lawrence's avuncular style of lecturing was not to the liking of the students, and his crystallography did not please the nuclear physicists. Realizing that Cambridge did not have the resources to become an accelerator lab, he encouraged the study of radio astronomy and protein crystallography, which led to a plethora of Nobel prizes. His support for Max Perutz and his hopeless attempts to solve the Patterson function of haemoglobin was initially difficult to fathom, and later entailed tolerating Francis Crick's penetrating voice. Lawrence's memorable Edwardian epithet concerning Crick was that he was given to “doing someone else's crossword”. All was forgiven when Crick and Jim Watson figured out the structure of DNA, however — not because Lawrence had any interest in biology, but because they beat his rival Linus Pauling. Nevertheless, Lawrence contributed a lot to Perutz's subsequent success with protein structure. On a bizarre level, he was interested in crystal dislocations and, much to the amazement of his colleagues and first-year undergraduates, was able to simulate their motion with rafts of bubbles.

On retirement from the Cavendish, Lawrence became resident professor at the Royal Institution. There he built up a powerful group, led by David Phillips, that solved the first structure of an enzyme. In his lifetime, Lawrence saw X-ray crystallography grow from the seed he helped germinate to a method of solving the structures of the largest macromolecules.

The subtitle of Graeme Hunter's book refers to Lawrence's “life and science”. The ‘life’ section is full of anecdotes and makes fascinating reading. Hunter captures the lonely schoolboy and tells of Alice Bragg — who some of us remember as a rather formidable justice of the peace — as a lively young flapper. He brings out Lawrence as an artist. Moreover, although Lawrence tried to avoid confrontation, his appointment to succeed Edward Andrade at the Royal Institution was accompanied by bad feelings and tension, which is fairly portrayed and analysed by Hunter.

The science is more of a problem. Most of it seems fairly accurate, but one or two sections reminded me of the description of the farm in Stella Gibbons' Cold Comfort Farm, in which the detailed geometrical descriptions resist synthesis. Hunter, who is not a crystallographer, must be commended for his brave attempt to put the science where it really belongs. However, his lack of a real understanding of diffraction theory shows up in numerous mini-howlers.

For example: “nλ=2dsinθ... this was the famous Bragg equation. However, there was nothing really novel about it ... for a line grating, 2esinθ=nλ.” Apart from the θ being different, Hunter misses the point first made by von Laue that diffraction from a three-dimensional lattice is subject to constraints not pertinent for a one-dimensional grating. The Bragg law imposes two conditions: specular reflection and the Bragg equation.

Hunter's lack of comprehension leads to an even bigger howler in Figure 0.2 in the introduction, which is supposed to help the lay reader. Hunter's putative Bragg reflections do not satisfy the Bragg equation, and moreover show that high-order reflections come out at low diffraction angles, and low-order reflections come out at high angles of diffraction; this is exactly the wrong way round. The strange thing is that a bit later, in Figure 2.7, he gets it right. The book, which could easily have been rescued by rigorous professional editing, is already in need of a second edition.