Lawrence Bragg – seeing the light

The question as to whether light was a wave or was made up of particles had been debated fiercely since the 1600s, boggling the boffins. The matter had plagued physicists since the French scientist, Gassendi, and Isaac Newtons’ particle or “corpuscular” theory conflicted with Dutchman, Christian Huygen’s, observation of the wave-like properties of light. Newton “settled” the question at the time, saying that the particle theory must be correct because light travelled in straight lines and could travel through a vacuum. Huygens, however, noted that light diffracted, that is, it spread out from a point, and that it displayed interference patterns – discrete impressions of light and shadow formed by the intersection of nodes and antinodes. These were distinctly wave-like properties. 

Lawrence Bragg was a physicist who was born in Adelaide in 1890 and by the time he was twenty-five he had come up with the evidence that made everyone happy – it was both. It was pretty obvious really – light was made of particles that travelled in waves. However, this raised as many questions as it answered and gave physicists something to really make them happy – a conundrum to keep them intrigued for decades in trying to find an explanation for this new dilemma.

Bragg’s grandfather, Charles Todd, had been sent out to South Australia in 1855 as an astronomer and to set up a telegraph line across the continent. In Britain, Todd had been at Cambridge Observatory and had gained experience with the telegraph at the galvanic department at the Royal Observatory in Greenwich. As Post-Master General in Adelaide, in charge of the Electric Telegraph Department, he built the famous “overland telegraph” from Adelaide to Darwin, cutting communication time between Australia and the world from months to minutes. As Astronomical and Meteorological Observer, he was a pioneer of meteorology and also helped introduce electricity to Australia, giving light shows to demonstrate the new wonder.

In Australia, Todd’s daughter, Gwendoline, married William Henry Bragg, a British-born mathematician, who, fresh from Cambridge at the age of twenty-three, had emigrated to South Australia in 1885, to head up the new Mathematics and Experimental Physics Department at the University of Adelaide. Their son, William Lawrence, was born in Adelaide in 1890. In 1895, on his way from New Zealand to a couple of Nobel Prizes in Physics and the model of the atom that was to bear his name, Ernest Rutherford met up with William Bragg in Adelaide. Inspired by his father and Rutherford and after tinkering with the bits and pieces Dad brought home from work, Lawrence entered the University of Adelaide in 1905 at the age of fifteen, graduating with his degree three years later. Moving to Cambridge, his father and another atomic physics pioneer, J. J. Thomson, were the mentors for his doctoral degree and within a short time, Lawrence became director of the Cavendish Laboratory.

Working as a team at Leeds University, father and son, Henry and Lawrence Bragg developed x-ray crystallography, making observations impacting on the long-standing question of the dual nature of light. Young’s double slit experiment, which demonstrated a distinctive interference pattern. had helped verify Huygens’ wave theory. Using the x-ray crystallography methods they developed, the Braggs observed diffraction, which was also consistent with Huygen’s wave theory.

However, while Lawrence Bragg observed that x-rays passing through sodium chloride (salt) crystals could be diffracted, which is a property of waves, they could also be ionised, that is, turned into positively or negatively charged particles, something waves can’t do, but particles can. This was physical evidence that x-rays, and therefore electromagnetic radiation, could act as both waves and particles…

In 1915, at twenty-five, Lawrence became the youngest person ever to be awarded the Nobel Prize, when he and his father were jointly awarded the Prize for Physics for “their services in the analysis of crystal structure by means of X-rays”. He was the first Australian-born scientist to win the Prize for Physics and his record as the youngest person to ever be awarded any Nobel Prize remained unbroken until seventeen-year-old, Malala Yousafzai, was awarded the Peace Prize in 2014. He is still the youngest to be awarded the prize for Physics.

With wave-particle duality confirmed, it raised more questions that it answered and the problem remains one of the core mysteries tantalising physicists today. It was the Braggs’ experimental observations and his association with Rutherford that led Niels Bohr to his revolutionary ideas of quantum physics and Albert Einstein to make his declaration that he refused to believe that all interactions in the universe were due to chance, come to be famously quoted as “God does not play dice with the universe.”

Perhaps, like Oppenhemier overseeing the Manhattan project, Lawrence Bragg’s brilliance as a scientist was as much as an administrator as a physicist. It was while under his instructions at the Cavendish that Watson and Crick made their revolutionary discovery of the structure of DNA and it was Bragg who announced their discovery at the Solvay Conference in Brussells in 1953. It was he who nominated them for the Nobel Prize in 1962, along with Wilkins, a researcher at Kings College London, who established the structure of DNA using the x-ray crystallography techniques that the Braggs had developed.

At that time, Watson and Crick at the Cavendish laboratory and Wilkins and Rosalind Franklin at Kings College, were in a race with Linus Pauling of Caltech in the USA to establish whether DNA had a double helix structure or consisted of a triple helix model as proposed by Pauling. The story goes that Bragg put Watson and Crick in a lab alone together and instructed them not to come out until they had figured it out first.

Pauling was somewhat disadvantaged, in that, thanks to Kings initially denying him a look at their material and then the US State Department denying him a passport to visit Britain for being a pacifist, he did not have access to the x-ray photographs that the scientists at Kings and Cavendish did, which had inspired the double helix idea. He didn’t get to the double helix first, but when the Nobel Prize was awarded to Watson, Crick and Wilkins in 1962 for that discovery, just to show that Sweden has a sense of humour, Pauling was awarded the Nobel Peace Prize…