Lawrence Bragg might have been the youngest ever Nobel Prize laureate, but being part of a father-and-son team meant it was years before he received true recognition for his seminal work in X-ray crystallography.

Receiving a Nobel Prize at the tender age of 25 can be a mixed blessing, especially if your fellow recipient happens to be not just an established and renowned scientist but also your father. While receiving the prize bestows a great deal of recognition, and becoming the youngest laureate perhaps more so, doubts will inevitably linger over how your efforts compare to those of your father, and suspicions will be rife as to whether nepotism played a part in your success.

For William Lawrence Bragg, the Nobel Prize in Physics he shared with his father William Henry Bragg for their “services in the analysis of crystal structure by means of X-rays” proved to be such a case. The younger Bragg sought to distinguish himself in more than just his first name – he wanted to be referred to as Lawrence to avoid any confusion with his father William.

Not that William ever made any attempt to deny his son’s intellectual ownership of the “Bragg equation”. Quite the reverse, in fact, the elder Bragg always emphasized Lawrence’s credit for formulating the law that helped give birth to a new scientific field, called X-ray crystallography. Thanks to Lawrence’s equation, researchers could now identify the structure of a crystal by passing X-rays through it and examining the pattern of dots that are produced by the reflected rays.

It all began with a letter sent to William by his former student, the Norwegian physicist Lars Vegard, from Würzburg on 26 June 1912. Vegard’s letter contained precise and detailed information on Max von Laue’s recent discovery that X-rays could be diffracted in crystals. The letter reached William while he was holidaying with Lawrence in Cloughton on the coast of Yorkshire in northern England, and the details of von Laue’s experiment were intensively discussed between father and son.

The Bragg family had returned to England three years before, in 1909, when William had been appointed Cavendish Professor of Physics at Leeds University. That same year, Lawrence entered Trinity College, Cambridge, to study physics. England was a totally new experience for Lawrence. He had been born and brought up in Australia where William, a native Englishman from Cumbria, had joined the Department of Physics at the University of Adelaide in 1885. William had successfully developed the department from two students in the physical laboratory to over 100. So expectations were high when Bragg began to work in Leeds, as captured in a rhyme the students would sing:

Here’s to Professor Bragg
Who sailed in from down under
To make this College wag
Its physics tail in wonder.

William had left his beloved Australia with mixed emotions. In Adelaide, he had excelled as a brilliant teacher, combining classical physics with the exciting new developments in the field. Yet it wasn’t until 1904 that William started his own research programme, inspired by the recent discoveries of Radium and radioactivity by Pierre and Marie Curie. William began to make important findings concerning how radiation removes electrons from an atom, known as ionization, and he enjoyed fruitful correspondence about his results with one of the main pioneers in the field, Ernest Rutherford.

In recognition of his research, William was elected Fellow of the Royal Society in 1907. There was one drawback: If William wanted to further develop his research career, he knew he had to resign from the post in Adelaide that he had held for almost a quarter of a century and return to Britain. “My life and my work in this country have been so singularly happy”, he wrote to the Council of the University, “The reasons which prompt me to seek a change relate only to the nature of my work.”

The work that William describes in that letter had begun to change direction. He had become increasingly engaged in the debate about the nature of the mysterious X-rays, which had remained unsolved since their discovery by Wilhelm Röntgen. William became a firm proponent of the theory that X-rays are made up from particles, and he was trying to prove this experimentally as well as defending his view in intense discussions in the pages of the journal Nature with Charles Barkla and others, who believed X-rays are made up from waves.

So Vegard’s letter describing von Laue’s experiment would have come as a huge blow, as it appeared to thoroughly refute William’s opinion. When beaming X-rays through a crystal, Vegard reported, von Laue “gets a number of very sharp, regularly arranged ray-bundles surrounding the primary beam”. In other words, von Laue saw a pattern of spots, similar to the way visible light diffracts through holes, or gratings, and his explanation was that the space within the regular structure of a crystal was a similar size to the wavelength of X-rays, which are the conditions under which diffraction occurs. von Laue’s results could only mean one thing: X-rays are waves.

A bolt from the blue

Lawrence’s immediate reaction was to defend his father’s position, but he grew more convinced that von Laue’s explanation was correct. However, von Laue was unable to explain the phenomenon in greater detail. Finding out how exactly the diffraction spots were created was “my golden opportunity”, as Lawrence later recalled.

In his first year of postgraduate study in Cambridge, Lawrence abandoned all earlier projects and concentrated fully on the diffraction question. Having just been examined on light, crystals and waves for his degree, Lawrence’s mind was fresh with all the relevant theories, and the words of Vegard’s letter still lingered in his head. One day, like a bolt from the blue, he felt these “unrelated bits of knowledge click together to suggest something new”, as Lawrence later wrote: “I can remember the exact spot on the Backs [the riverside in Cambridge] where the idea suddenly leapt into my mind that Laue’s spots were due to the reflection of X-ray pulses by sheets of atoms in the crystal.”

This idea was simple, but ingenious. After von Laue’s discoveries, scientists knew that crystals are like a three-dimensional grid or lattice structure of regularly repeating atoms. But Lawrence visualized this lattice structure as being constructed from a series of sheets of atoms, laid one on top of another, with each sheet behaving like a mirror. So the intricate pattern of dots that were produced by passing X-rays through a crystal is caused by a complex series of interactions in which some X-rays are reflecting off the first sheet of atoms encountered, some from the second, some from the third, and so on. From this intellectual breakthrough Lawrence could formulate his “Bragg equation”, which connects the wavelength of X-rays (λ), the distance between successive sheets of atoms in a crystal (d) and the angle at which the X-rays strike these sheets (θ). Or as the equation states: nλ = 2d sinθ where n is a whole number.

On 11 November 1912 William Lawrence presented his findings to the Cambridge Philosophical Society. His father was proud of his son’s achievement and wrote in his annual Christmas letter to an Australian friend: “Billy is coaching and demonstrating in Cambridge, and has just brought off rather a fine bit of work in explaining the new X-ray and crystal experiment.” In a letter to Nature, which continued to host much of the debate about the nature of X-rays, William referred readers to a paper in which “my son has given a theory which makes it possible to calculate the positions of the spots for all dispositions of the crystal and photographic plate”.

For the next two years, father and son joined forces in an extraordinarily productive collaboration. Based on Lawrence’s reflection idea, William built the first X-ray spectrometer, designed to examine the reflections of X-rays from crystals. Together, they examined a series of crystal structures, like common salt, or explored the X-ray spectra emitted by different elements. “It was a wonderful time”, Lawrence remembered. “Like discovering a new goldfield where nuggets could be picked up on the ground, with thrilling new results every week”.

Identifying the structure of diamond was arguably their greatest success, and they summarized their newly gained knowledge in the Proceedings of the Royal Society under the title “The reflection of X-rays by crystals”. The year after von Laue received the Nobel Prize for Physics for his breakthrough discovery, both Braggs were awarded the same prize “for their services in the analysis of crystal structure by means of X-rays.”

Despite his achievements and recognition, Lawrence did not seem to be taken seriously by the scientific community. Shortly after his Law was published the second of the prestigious Solvay Conferences on “The structure of matter” took place in Brussels in October 1913. William was invited, but not Lawrence. Instead, Lawrence received a postcard, signed by famous Solvay participants like Marie Curie, Albert Einstein, von Laue, Hendrik Lorentz, Ernest Rutherford and others, congratulating him for “advancing the course of natural science.” When the Braggs were both awarded the prestigious Barnard Medal in spring 1915, Rutherford wrote to William: “It is very early for your boy to be getting these distinctions.”

Lawrence was knighted in 1941, around 20 years after William had received the same honour, and the elder Bragg felt this should have put any matter regarding due recognition to rest. “I am so very glad for his sake”, William wrote in a letter to his sister-in-law, one year before his death. “In spite of all care, people mix us up and are apt to give me a first credit on occasions when he should have it: I think he does not worry about that at all now, and will never anyhow have cause to do so now.”

However, Lawrence still seemed keen to set the record straight about the source of “Bragg’s equation”. Lawrence had succeeded Rutherford as the Cavendish Professor of Experimental Physics in Cambridge in 1937. There he championed the emergence of molecular biology by means of X-ray crystallography, culminating in the Nobel Prizes in 1962 being awarded to four of his Cavendish researchers: John Kendrew, Max Perutz, Francis Crick and James Watson. Lawrence was a “scientific father”, as Max Perutz wrote after Bragg’s death: “Nowadays cynics want us to believe that scientists work only for fame and money,” wrote Perutz, “but Bragg slaved away at hard problems when he was a Nobel Prize laureate of comfortable means.”

Lawrence Bragg has been the only Nobel Prize laureate so far who has celebrated the Golden Jubilee of his award with a Nobel Guest Lecture in Stockholm, which he gave in 1965. Despite receiving one of the few honours that had eluded his father William began his talk with a claim for his scientific credit. “It is sometimes said that my father and I started X-ray analysis together,” he said, “but actually this was not the case.”

Bibliography

Bragg, William Lawrence: The Diffraction of X-rays by Crystals. Nobel Lecture, September 6, 1922.

Ferry, Georgina. Max Perutz and the Secret of Life. Chatto & Windus, London, 2007.

Jenkin, John. William and Lawrence Bragg, Father and Son. The most extraordinary collaboration in science. Oxford University Press, 2007.