‘IF YOU can’t explain your physics to a barmaid,” Ernest Rutherford suggested, “it is probably not very good physics”. And, in truth, Rutherford’s physics passed his own somewhat sexist test with flying colours, its insights deceptively simple and yet, like so much great science, gloriously elegant. And well worth celebrating.
One hundred years ago this month, at the Manchester Literary and Philosophical Society Rutherford, a 39-year-old professor, head of physics at the city’s university and already a Nobel winner for his part in the discovery of the alpha particle, opened a new chapter in the history of science with a remarkable presentation on the shape of the atom, a model now so central to popular understanding it is hard to appreciate its novelty then.
The New Zealander’s new model, a miniature planetary system of tiny, negatively charged electrons revolving around a heavy, positively charged nucleus, laid the basis for the development of nuclear physics in the way that the discovery of the elegant helix shape of DNA would spawn the modern science of genetics.
At the time Rutherford was struggling to reconcile his mentor JJ Thomson’s description of the atom, the “plum pudding” model, with experiments by two colleagues, Hans Geiger and Ernest Marsden. Thomson had suggested the atom consisted of a lump of “positive electricity” in which negatively charged electrons were embedded like plums in a pudding. But the experiments, involving firing particles at a fine gold foil, were difficult to reconcile with this view – most of the particles simply passed through the foil, some were deflected, and a small number, about one in 800, bounced back.
“It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you,” Rutherford would tell his barmaid. But the results suggested, he argued, that the atom consists of a tiny central core containing virtually all the atomic mass, “like a fly in a cathedral”. Most of the bombarding particles simply passed through the empty space but if a particle collided head on with the nucleus it would recoil straight back like a ball bouncing off a wall.
Later, Niels Bohr would explain how electrons kept up their orbits, James Chadwick would discover the neutron, and in 1932, following Rutherford’s suggestion that the atom could be artificially split James Cockcroft and Dubliner Ernest Walton did so in the first particle accelerator. They would be among 11 students and colleagues of Rutherford’s eventually to receive Nobel prizes.