Dawn of the nuclear age

A century ago, Pierre Curie and his assistant isolated enough of a mysterious metal - radium - to measure its properties. The astonishing results, says Marcus Chown, changed the world for ever

The problem with radium was getting enough of it. Marie Curie had discovered the "radioactive" element, which glowed menacingly in the dark, by the Herculean effort of dissolving, filtering and crystallising many tons of the uranium ore pitchblende. But her backbreaking toil had yielded a mere speck of the stuff. It took a further five years to accumulate enough to measure its detailed properties, which is what Pierre Curie and his young assistant, Albert Laborde, set out to do in their Paris laboratory one day in 1903.

The first thing that Curie and Laborde noticed was that their sample of radium was doing more than simply pumping out light. It was pumping out heat, too - enough to keep the sample in a permanently molten state. It was when they measured just how much heat was flooding out that they got their shock. The sample of radium was generating enough heat to boil its own weight of water every 45 minutes. In the next 45 minutes, it could boil another kilogram of water. And in the next, another. In fact, the radium could continue boiling a kilogram of water every 45 minutes for thousands upon thousands of years...

Few had suspected that radioactivity was associated with such a tremendous release of energy. Although the phenomenon had been discovered by Henri Becquerel in 1896, and radium - the most radioactive substance - had been isolated by Marie Curie in 1898, it had been thought to be no more than an inconsequential curiosity. Certainly, radioactivity had not created the sort of worldwide media frenzy aroused by the discovery of X-rays in 1895.

Curie's and Laborde's discovery changed everything. In 1904, a single grain of radium created a sensation when it was displayed at the International Electrical Congress in St Louis. "Its power will be inconceivable," wrote the St Louis Post Dispatch. "By means of the metal, all the arsenals in the world would be destroyed. It could make war impossible by exhausting all the accumulated explosives in the world... It is even possible that an instrument might be invented that, at the touch of a key, would blow up the whole Earth and bring about the end of the world."

Curie and Laborde had dropped a bombshell into the heart of the physics world. Locked inside ordinary matter was an energy supply to surpass the imagination. But where was the energy coming from? The short answer was: from the central core of atoms. In 1911, the New Zealand physicist Ernest Rutherford discovered that an atom was an almost entirely empty space. At the centre of each, orbited by "electrons", sat a tight knot of matter called the "nucleus". No one has described the atom discovered by Rutherford better than the playwright Tom Stoppard: "Now make a fist, and if your fist is as big as the nucleus of an atom, then the atom is as big as St Paul's, and if it happens to be a hydrogen atom, then it has a single electron flitting about like a moth in an empty cathedral, now by the dome, now by the altar."

As early as 1903, Rutherford and the English chemist Frederick Soddy had suggested that a radioactive atom such as uranium or radium was simply one that was unstable, seething with excess energy. Desperate to shed the surplus, it eventually ejected bits of atomic shrapnel, in the process transforming itself into a lighter, hopefully more stable, atom. After Rutherford's discovery of the nucleus, physicists modified that picture, realising that it was the nucleus, not the atom as a whole, that was the seat of the enormous energy. But knowing that the energy came from the atomic nucleus did not explain why it was being released.

The answer was discovered by a shy Cambridge physicist, who built a hypersensitive instrument to "weigh" atoms. What Francis Aston discovered in 1919 defied belief. When a radioactive atom disintegrated, he found, the sum total of the bits - the lighter atom plus the shrapnel - weighed less than the original atom. It was a bit like splitting a 1kg bag of sugar into two and finding that the sum of the halves was less than 1kg. Every time a radioactive atom of, for instance, radium disintegrated, mass appeared to be vanishing. But mass, as Einstein discovered in 1905, is merely another form of energy. And energy can never be destroyed, but only changed from one guise into another - for example, from electrical energy into light energy. So, the mass-energy that was going missing was actually changing into another form - the kinetic energy of the nuclear shrapnel that rocketed out of the nucleus and ultimately dumped its energy as heat in the radium.

The formula that gave the amount of energy contained in a mass, m, of matter was the most famous in science: E=mc2, where c is the speed of light. It was a simple matter for Aston to put in the figures. The result was staggering. Pound for pound, radium contained a million times the destructive power of dynamite.

The factor of a million did not go unnoticed by astronomers. For millennia, people had wondered what made the Sun shine. In the 19th century - the age of coal - physicists had naturally wondered whether the Sun was a giant lump of coal. But they had found that, if it was, it would burn out in about 5,000 years. The trouble was that the evidence from geology and biology was that the Earth - and, by implication, the Sun - was at least a million times older. The coincidence was too great.

In 1903, John Joly, an Irish geologist, and George Darwin, the astronomer son of Charles Darwin, suggested that the Sun was powered by radioactivity. They were almost right. We now know that the Sun derives its heat not from radioactivity but from a closely related form of "nuclear energy".

However, the ultimate demonstration of what Curie and Laborde discovered on that fateful day in 1903 would not come until 6 August 1945. That was the day, as the mushroom cloud rose above the Japanese city of Hiroshima, that the world finally learnt what the physicists had long known - the appalling truth about the atom.

Marcus Chown is the author of 'The Universe Next Door: Twelve Mind-blowing Ideas from the Cutting Edge of Science', published by Headline (£7.99)