Did the nuclear age begin in 1942, when Chicago Pile-1, a reactor built in a squash court, went "critical" by achieving self-sustaining chain reaction?
Or was it on 16 July 1945 in the Jemez mountains in New Mexico, when "The Gadget", the first atomic bomb, was successfully tested and Robert Oppenheimer quoted the Bhagavad Gita? Maybe it was June 1954, when the Russian Obninsk nuclear station first generated electricity for the grid.
In reality, it was during a meeting of the Manchester Literary and Philosophical Society that the nuclear age was announced, on Tuesday, 7 March 1911, by Professor Ernest Rutherford, the 39-year-old head of physics at Manchester University. Rutherford was born in 1871, in Spring Grove, New Zealand. Descended from Scottish emigrants, it was from this scattered rural community on the north coast of the South Island that Rutherford's aptitude for science and maths led in 1895 to a coveted place at Cambridge. There, under the direction of JJ Thomson, Rutherford established a reputation as a fine experimentalist with a study of X-rays.
Though surrounded at Cambridge by all the excitement generated by Thomson's discovery of the electron in 1897, Rutherford opted to investigate radioactivity and soon found that there were two distinct types of radiation emitted from uranium, which he called alpha and beta, before a third was discovered, called gamma rays.
Aged just 27, in 1898, he was appointed professor of physics at McGill University in Montreal, Canada. Among his successes over the next nine years the most important was the discovery, with his collaborator Frederick Soddy, that radioactivity was the transformation of one element into another due to the emission of an alpha or beta particle.
Rutherford regarded "all science as either physics or stamp collecting" but saw the funny side when he received the 1908 Nobel prize for chemistry for this seminal work. By then he was in Manchester.
"Youthful, energetic, boisterous, he suggested anything but the scientist," was how Chaim Weizmann, then a chemist but later the first president of Israel, remembered Rutherford in Manchester.
"He talked readily and vigorously on any subject under the sun, often without knowing anything about it.
"Going down to the refectory for lunch, I would hear the loud, friendly voice rolling up the corridor."
At the time Rutherford was busy using the alpha particle to probe and unlock the secrets of the atom. But what exactly is an alpha particle? It was a question that Rutherford and his German colleague Hans Geiger answered. It was a helium ion; that is, a helium atom that had been stripped of its two electrons. Rutherford had noticed, while still in Montreal, that some alpha particles passing through thin sheets of metal were slightly deflected, causing fuzziness on a photographic plate. It was something he asked Geiger to investigate.
As instructed by Rutherford he fired beams of alpha particles at some gold foil and by the tiny flashes of light when they struck a zinc sulphide screen discovered that a few "were deflected through quite an appreciable angle". Soon afterwards Rutherford assigned a research project to a promising undergraduate called Ernest Marsden: "Why not let him see if any alpha particles can be scattered through a large angle?"
Marsden found some alpha particles bouncing straight back after hitting the gold foil and Rutherford was shocked: "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."
Marsden and Geiger made comparative measurements using different metals and they discovered exactly they same large angle scattering. In June 1909 they published their extraordinary results, but with Rutherford unable to offer any kind of explanation they attracted little interest.
After decades of intense arguments, by 1910 the reality of atoms was established beyond reasonable doubt. The most widely-accepted atomic model was Thomson's so-called "plum pudding". Its ingredients consisted of a ball of diffuse "positive electricity" in which negatively charged electrons were embedded like plums in a pudding. But Rutherford knew that the atom of his old mentor couldn't explain alpha particle scattering. The probability that the accumulated effect of a number of tiny ricochets off electrons in Thomson's atom resulted in even one alpha particle being scattered backwards was almost zero. By December 1910, Rutherford believed that given the mass and energy of an alpha particle the large deflections must be the result of a single collision with an atom. It led him "to devise an atom superior to J.J's" he said at time.
Rutherford's atom consisted of a tiny central core containing virtually all the atomic mass, which he later called the nucleus, but it occupied only a minute volume "like a fly in a cathedral".
Most alpha particles would pass straight through Rutherford's atom in any "collision", since they were too far from the tiny nucleus at its heart to suffer any deflection. But if an alpha particle approached the nucleus head-on, the repulsive force between the two would cause it to recoil straight back like a ball bouncing off a brick wall. Rutherford said that such direct hits were "like trying to shoot a gnat in the Albert Hall at night". Rutherford's model allowed him to make definite predictions using a simple formula he had derived about the fraction of scattered alpha particles to be found at any angle of deflection.
Experimental checks performed by Geiger and Marsden confirmed the predictions, but few physicists beyond Manchester gave any serious attention to the nuclear atom.
Although Rutherford did not explicitly suggest a planetary model of the atom, there were those who knew that's exactly what it was. For most that settled the matter, Rutherford's atom was fatally flawed.
A model of the atom with electrons moving around the nucleus, like planets orbiting the sun, would collapse. Any object moving in a circle undergoes acceleration, if it happens to be a charged particle, like an electron, as it accelerates it continuously losses energy in the form of radiation.
An electron in orbit around the nucleus would spiral into it. Rutherford's atom was unstable and the existence of the material world was compelling evidence against it.
Enter Niels Bohr.
Arriving in Manchester in March 1912 to learn about radioactivity, it wasn't before long the 27-year-old Dane began thinking about how to prevent Rutherford's nuclear atom from collapsing. His solution employed the quantum – the idea that energy comes in packets. Bohr argued that electrons inside an atom could only move in certain orbits in which they did not radiate energy and therefore couldn't spiral into the nucleus. Bohr said that each orbit had a certain energy associated with it, so all the allowed orbits were in effect a series of energy levels, like the rungs of a ladder. For an electron to move between levels, the famous quantum leap, required it to absorb or emit a quantum of energy that was equivalent to the difference in energy between the two levels.
"It is difficult to underestimate the scientific importance of the discovery of the nucleus," says Sean Freeman, professor of nuclear physics at Manchester University.
"Rutherford's insight, imagination and attention to detail enabled him to make revolutionary discoveries using rather rudimentary technology by modern standards. He was a true pioneer."
One of his most important achievements was made in his spare time while Rutherford was developing methods for detecting submarines during the First World War – he split the atom. Arriving late for a committee meeting one day, Rutherford didn't apologise, but announced: "I have been engaged in experiments which suggest that the atom can be artificially disintegrated.
"If it is true, it is of far greater importance than a war!"
It was 1919 before he published the results that showed the nucleus contained positively charged particles he called protons by knocking them out of nitrogen nuclei using alpha particles – thereby effectively splitting the nucleus and hence the atom.
It was the last work he did at Manchester before moving to Cambridge to take over from Thomson as head of the Cavendish Laboratory.
It was there that in 1932 his colleagues James Cockcroft and Ernest Walton "split the atom" using the world's first particle accelerator. Also at the Cavendish, James Chadwick used Rutherford's suggestion that there was probably another constituent to heavier nuclei to discover the neutron. The particle plays the central role in establishing a nuclear chain reaction. The three men were among the 11 former students and colleagues of Rutherford who would win the Nobel prize.
Another of those 11 was Niels Bohr, who said that Rutherford never spoke more angrily to him than he did one evening at a Royal Society dinner.
He had overheard Bohr refer to him by his title (Rutherford was awarded a peerage in 1931) and angrily asked the Dane loudly: "Do you Lord me?"
Rutherford never cared for the honours and was indifferent to academic or social standing. What mattered most to him were the results of experiments. "I was brought up to look at the atom as a nice hard fellow, red or grey in colour, according to taste," he once said.
It was a model he replaced with an atom that began the nuclear age.