Ernest Rutherford


Ernest Rutherford was the son of James Rutherford, a small business man, and Martha, who had been a schoolteacher before her marriage. His parents, both British born, had emigrated to New Zealand, where Rutherford was born on a farm in the province of Nelson.

His earliest physics research dates back to his college days, when he was already aware of current developments from reading the Proceedings of the Royal Society. Most notable among his earliest work, published in New Zealand, is a report on his detection with a magnetometer of radio waves travelling a distance through space, work that predates Marconi's announcement in 1896 of wireless transmission of telegraph signals.

At the beginning of October 1895, Rutherford arrived in Cambridge on a research fellowship to work in the Cavendish. After having settled down at the Cavendish, Rutherford began by continuing his experiments with radio waves. It was a marvelous time for an aspiring experimentalist to start his career. Three months after his arrival, Rontgen published his discovery of X-rays. Another three months later came Becquerel's first paper on radioactivity. A year thereafter, Thomson's announcement of the electron.

In 1896, Rutherford and Thomson published a joint paper on ionization induced by X-rays. In 1897 Rutherford turned to radioactivity, and later, in 1898 he announced the discovery of two distinct kinds of radioactive rays, alpha-rays and beta-rays. In 1898 he moved to Canada to become professor of physics at McGill University in Montreal. Young people traveled there to work with him. Among them Soddy, who collaborated with Rutherford in formulating the radiactive transformation theory, according to which radioactive bodies contain unstable atoms of which a fixed fraction decays per unit time. Among them also Hahn, who later would be one of the discoverers of nuclear fission.

In 1908, at the age of 37, Rutherford was awarded the Nobel Prize for chemistry "for his investigations of radioactive elements and the chemistry of radioactive substances". Meanwhile, he had left Montreal to take up a professorship at the University of Manchester in 1907. There, he achieved an unique feat : to make the greatest discovery of his career after having won a Nobel Prize.

Atomic Nucleus

In 1908, one of Rutherford's research students in Manchester, Geiger, had published a paper on the scattering of alpha-particles by thin foils of gold and aluminum, in which he reported that some of the alpha-particles were deflected by quite an appreciable angle. Then, early in 1909, Rutherford suggested to Marsden (a twenty year old New Zealand born undergraduate) that he pursue this matter further. In Marsden's words

One day Rutherford came into the room where Geiger and I were counting alpha particles. He turned to me and said, "See if you can get some effect of alpha-particles directly reflected from a metal surface." I do not think he expected any such result, but to my surprise, I was able to observe the effect looked for.

I remember well reporting the result to Rutherford a week after, when I met him on the stairs.

In May 1909, Geiger and Marsden submitted a paper in which they reported that about 1 in 8000 alpha-particles were deflected by more than 90 degree. Rutherford has told that this finding was the most incredible result in his life. An alpha-particle, weighing about 8000 times more than an electron, traveling at a speed of something like 10,000 miles per seconds, hits a bunch of electron and -- since we are still in the Thomson period -- some nondescript smeared out jelly of positive charge. That under those circumstances an alpha-particel would rebound at a large angle is as incredible as that a loaded Mack truck would veer back upon hitting a Volkswagen.

A year and a half later, in December 1910, Rutherford knew the answer to this puzzle. He wrote to a colleague, "I think I can devise an atom much superior to J.J.'s" At that time, Rutherford had neither much expertise in nor much taste for theoretical physics. It is perhaps more interesting to note that no other theorist had responded to this challenge, nor that this seminal experiment had been repeated anywhere else.

Rutherford's atom consists of a number Z of electron, each with charge -e, and a small sized central body with charge Ze in which practically all the mass of the atom is concentrated. This body is many thousands of times heavier than the electron. Not until October 1912 did Rutherford refer to it as the nucleus ("The atom must contain a highly charged nucleus"). He further assumed that the role of the Z electrons in alpha-particle scattering can be neglected, which is actually a good approximation. So that this process is entirely due to the electrostatic Coulomb interaction between the alpha-particle and the nucleus. The theoretical exercise is then to calculate the path of an alpha particle as it first approaches, then moves away from the nucleus. Rutherford produced a formula that describes the dependence of the orbits on the angle of scattering, on the alpha-particle velocity, and on Z. In particular, the probability for alpha-particle scattering is proportional to Z^2. At that time, the data were not yet sufficient to verify these predictions in all detail, but it did not take long before it was found that his answers worked well.

On 7 March 1911, Rutherford presented his results publicly for the first time. An eyewitness has recalled : "I remember well the occasion on which the idea was first put forward. It was a meeting of the Manchester Literary and Philosophical Society to which all workers in the laboratory were invited. Rutherford's account of his theory, backed by Geiger with some new experimental evidence, created a profound impression." The definitive paper on the subject appeared in the May 1911 issue of Philosophical Magazine. Here Rutherford records a first decent estimate of the radius of a nucleus : about a hundred thousand times smaller than that of an atom. Thus if one imagines an atom blown up to the size of a football field, then the nucleus would be the size of a marble placed at the kick off point. Nuclei are not only heavy. They are also exceedingly small, even by atomic standards. Matter consists largely of emptiness.

Manchester in 1912 was the world's foremost center of experimental studies in radioactivity. The discovery of the atomic nucleus is in fact a by-product of such explorations. But at that time, the Rutherford model was still not taken seriously. There was no mention of it in any place. Rutherford's own reticence must have been an important contributing factor. He remained silent about his model during a major international conference (the first Solvay conference) held in the fall of 1911. In his 670 page book, completed a year later, only three pages are devoted to alpha-particle scattering.

Bohr later wrote :

This effect -- the large angle scattering of alpha-particles -- though to all appearances insignificant, was disturbing to Rutherford, as he felt it difficult to reconcile it with the general idea of atomic structure then favoured by the physicsists.