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This edited article about physics originally appeared in Look and Learn issue number 852 published on 13 May 1978.
What is the greatest discovery of the twentieth century?
It is the discovery of the interior of the atom, and of the immense resources of energy within it.
Ernest Rutherford was the first and greatest explorer inside the atom. He was born in New Zealand in 1871, the son of a farmer. In 1895, he obtained a scholarship which enabled him to go to England to do research at Cambridge University.
Rutherford began work at Cambridge by experimenting with the new “wireless waves”, and for a time he held the record for long-distance transmission. But he quickly turned his attention to other matters which he sensed offered far more scope for fundamental discovery.
Near the end of 1895. Rontgen discovered X-rays, and soon after Becquerel discovered the rays from uranium which led to Marie Curie starting her work on radium and radioactivity. Rutherford at once set about following up these developments.
By brilliant experiment and imagination, he discovered what radioactivity was. The atoms of radioactive substances turn into atoms of other elements, and in the process they emit three different kinds of radiation. Rutherford named them alpha, beta, and gamma rays after the first three letters of the Greek alphabet.
Gamma rays are like X-rays, very penetrating electromagnetic waves. Alpha and beta rays are both streams of electrically-charged particles. The beta particles are light and negatively charged electrons. The alpha particles are positively charged and thousands of times more massive than electrons.
Rutherford established that alpha particles were atoms of the gas helium, without their two electrons. In radioactivity, these particles are expelled from the atoms of the radioactive substance at a tremendous velocity, about one-twentieth of the speed of light.
Rutherford used these heavy alpha particles as “bullets” to shoot into matter. He set up an experiment to see what would happen when they were fired at a very thin foil of gold. He used a fluorescent screen which gave a tiny flash of light when an alpha particle bullet hit it. By watching the screen through a microscope in a darkened laboratory, he could count each flash produced.
Most of the alpha particles went through the layers of atoms in the gold without deviating from a straight course, and some were deflected a few degrees. But a very few bounced back from the gold foil.
Rutherford was astonished. He said afterwards: “It was the most incredible thing that ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”
At the time, scientists thought that atoms were made up of electrons, or particles of negative electricity, distributed in a sphere of positive electricity. But such objects would hardly have been able to deflect an alpha particle from its course, let alone turn it back.
The answer came to Rutherford in 1911, after thousands of experiments with his gold foil. The atom was almost entirely empty space! At its centre was an infinitesimally tiny, very dense and heavy core, which about one alpha particle in 10,000 had hit dead on.
To produce a force strong enough to repel such a particle, all the positive electric charges, and nearly all the atomic mass, had to be concentrated in this core, or atomic nucleus.
Rutherford’s atomic bombardment experiments marked the change from one era to another. They revealed the true nature of the atom. If any man might correctly be called the “father of the nuclear age”, it is Rutherford.
At this time, Rutherford was Professor of Physics at Manchester University. He was a big, burly man with a boisterous voice and a resonant laugh. He was a supremely great experimenter: he and the 19th century Englishman, Michael Faraday, are generally regarded as the greatest experimental scientists since Newton’s day.
Eight years after Rutherford had discovered the nuclear structure of the atom, he broke into the nucleus itself. This was another epoch-making experiment. With his favourite alpha particles as projectiles, he split off part of the nucleus of nitrogen. A hydrogen nucleus was knocked out of the nitrogen nucleus, and the nitrogen atom was changed into an oxygen atom, having captured the alpha particle which had hit it.
This was the first man-made “nuclear reaction”, and it released more energy from the collision than the alpha particle had put in. Rutherford had become the first man to fulfil the dream of the alchemists: he had changed one element into another by the work of his own hands.
In 1919, Rutherford was appointed head of the Cavendish Laboratory of Cambridge University. Under his leadership, the Cavendish led the world in research on what is now called nuclear physics. From the 1920s onwards, physics became the story of more and more elaborate apparatus and enormously costly engineering to pursue experiments. Rutherford had worked with incredibly simple equipment.
Rutherford was given a peerage and became Lord Rutherford. He died in 1937 and was buried in Westminster Abbey. He had found the inner citadel of the material universe: a new age had been born.