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FRANCIS ASTON’S inheritance had put him in an enviable position. He no longer had to work for a living. Instead he could travel the world and try any glamorous sport that took his fancy. But by his early thirties, his glitzy lifestyle was beginning to wear thin. In 1909, when he had mastered surfing at Waikiki and was at a loss to know what to try next, the hollowness of his existence suddenly struck him. Back at his hotel in Honolulu, he sent a telegram enquiring about a position at Cambridge and booked a berth on the next steamer back to England.

Aston’s interest in weighing atoms had been triggered by J. J. Thomson, the discoverer of the electron. When Aston landed a job as his assistant in 1910, Thomson set him the task of finding out why neon atoms did not weigh in at whole-number multiples of the weight of a hydrogen atom, the presumed fundamental building block of all atoms.

Aston solved Thomson’s puzzle by laboriously feeding neon gas through porous pipe clay over and over again, noticing that the neon that emerged each time was less dense than before. The result made sense if neon consisted of a light form, which percolated relatively quickly through the clay, and a heavier form, whose progress was more sluggish. Aston found that neon atoms were a mixture of two versions or “isotopes” – a light one, neon-20, which weighed 20 times as much as a hydrogen atom, and neon-22, which weighed 22 times as much.

The percolation method was painfully slow. Aston was convinced there had to be a better way of separating out and weighing atoms. After the first world war, and a stint at the Royal Aircraft Factory, he found it.

Aston invented and built a mass spectrograph, which used a magnetic field to bend the trajectory of atoms by an amount which depended on their mass. Precisely how much their trajectory was altered was registered by a photographic plate placed in the path of the atoms. Helium atoms made a fuzzy spot at one point on the plate, calcium atoms at another, and so on.

The first element Aston checked was neon. Sure enough, two dots appeared on the photographic plate, one corresponding to neon-20 and the other to neon-22. The mass spectrograph worked. Before too long, Aston had succeeded in showing that all atoms had a weight which was a whole-number multiple of the mass of the hydrogen atom, confirming a prescient claim made by London physician William Prout in 1815. Prout’s hypothesis had appeared to be undermined by the existence of atoms such as chlorine, which had a mass 35.5 times that of hydrogen. Now Aston showed that anomalous substances like chlorine, in common with neon, were mixtures of isotopes. Prout’s rule held – but for individual isotopes. The twist, which Prout could never have anticipated, was that nature had supplied two atomic building blocks with the mass of a hydrogen atom. Not only was there the proton, there was the neutron too, whose existence would be confirmed by James Chadwick in 1932.

At this point, Aston could have rested on his laurels. But he was an incorrigible tinkerer and he continued to make improvements to his mass spectrograph until eventually it could measure the mass of an atom to an astonishing accuracy of 0.001 per cent. It was then that nature dropped its atomic bombshell.

In 1919, with his vastly improved mass spectrograph, Aston discovered peculiar departures from Prout’s cast-iron rule. Bizarrely, the mass of every atom was marginally less than the sum of the masses of its constituent building blocks. The discrepancy was most marked for helium, the next heaviest atom after hydrogen. Each helium atom weighed almost 1 per cent less than the combined mass of its building blocks.

Aston knew that helium was made of four building blocks. Finding that it was 1 per cent lighter than it ought to be was like putting four 1-kilogram bags of rice on a set of scales and finding that they weighed 3.96 kilograms.

Prout’s hunch had been that all atoms had at one time been built up from the basic building block – now known to be two different building blocks. But if Aston’s findings were right, this atom-building process resulted in a fraction of the original mass vanishing into thin air. What was going on?

The answer had been anticipated in 1905 by a 25-year-old patent clerk called Albert Einstein. The cores or “nuclei” of atoms were held together by a tremendously strong force – it had to be strong to counteract the huge electrical repulsion trying to drive the positively charged protons apart. During atom-building, the force would slam together the building blocks at tremendous speed. Somehow, the enormous energy of motion they acquired would have to be lost to the system. And it was – in the form of gamma rays and heat. But, as Einstein discovered, when a body loses energy it loses mass – this is the meaning of his equation, E = mc². So the bound atom that formed would be lighter than its unbound constituents.

It turns out that everything has less mass when bound than free. Even the combination of the Earth plus a person standing on it has less mass than the Earth plus the same person floating out in deep space. But the mass difference is usually far too small to measure. Only in the case of atoms – where the binding force is 10,000 trillion trillion trillion times stronger than gravity – is it significant.

What Aston had stumbled on was nothing less than the elusive source of atomic energy – the destruction of mass or, more precisely, the transformation of mass-energy into other forms of energy such as heat. Weight for weight, it was an energy source a million times more concentrated than chemical fuels such as coal or oil, or even dynamite.

The factor of a million was not lost on the prominent English astronomer Arthur Stanley Eddington. Calculations first carried out in the 19th century had showed that if the sun were a lump of coal it would have burnt out in a mere 6000 years. The evidence from geology and biology, however, was that the sun had been burning for around a million times longer than this. The factor of a million was too much of a coincidence for Eddington. He was sure the sun must be building atoms, a process that had been demonstrated at Cambridge by Ernest Rutherford while Aston was puzzling over his discrepancies in mass. And the most efficient atom-building process was the creation of helium from hydrogen, where almost 1 per cent of the mass would be lost as energy. Here, at last, was the ultimate source of sunlight.

Aston had never been cut out for the life of an international playboy: he was pathologically shy. Science suited him far better. Once he had given up the fast life, he devoted himself single-mindedly to one problem – weighing atoms – and unusually for an experimental scientist, to one instrument. But his dedication brought a big pay-off. For his invention of the mass spectrograph, he received the 1922 Nobel prize in chemistry. He died in 1945 – the year that a few grams of matter went missing above Hiroshima, replaced by a fireball hotter than the sun.