Szilard believed that recent scientific breakthroughs meant it was now possible to convert mass into energy. And that this could be used to make a bomb. If this were to happen, it would be a terrible realisation of the law of nature Einstein had discovered some 34 years earlier.
September 1905 was Einstein's 'miracle year'. While working as a patents clerk in the Swiss capital Berne Einstein submitted a three-page supplement to his special theory of relativity, published earlier that year. In those pages he derived the most famous equation of all time; e=mc², energy is equal to mass multiplied by the speed of light squared.
The equation showed that mass and energy were related and that one could, in theory, be transformed into the other. But because the speed of light squared is such a huge number, it meant that even a small amount of mass could potentially be converted into a huge amount of energy. Ever since the discovery of radioactivity in the late 19th century, scientists had realised that the atomic nucleus could contain a large amount of energy. Einstein's revolutionary equation showed them, for the first time, just how much there was.
However, at the time Einstein doubted whether that energy could ever be released. By 1935 he was convinced it would never be practical. At the Winter Session of the American Association for the Advancement of Science in Pittsburgh, he is quoted as telling journalists: "The likelihood of transforming matter into energy is something akin to shooting birds in the dark in a country where there are only a few birds."
Einstein was so sceptical because attempts to break open the atomic nucleus always required more far energy be put in than was ever released. Nuclear physicists like Ernest Rutherford were exploring the structure of the atom by bombarding atomic nuclei with alpha particles. Even when machines were built to accelerate the alpha particles to ever higher speeds they had only limited success in breaking apart the nucleus. In 1933 Rutherford dismissed talk of atomic power as 'moonshine'.
One morning in September 1933 Szilard read Rutherford's comments in The Times. Leaving his hotel and crossing the street, he had a brainwave. Alpha particles and the other particles that physicists had been using to bombard the nucleus were simply the wrong tool for the job, because he realised that they, like the nucleus, had a positive charge.
Since like charges repel, Szilard thought, no matter how hard you fire them in, the majority would simply be deflected away. That morning he was one of the first to realise that the recently discovered neutron might be what was needed. The neutron, a subatomic particle like a proton but with no electric charge was discovered in 1932. With no charge, Szilard believed the neutron would simply slip into the heart of the atom undeflected.
But he didn't stop there. Szilard thought that if an atom could be found that is split open by neutrons, not only would it release some of its huge store of energy, it might also release further neutrons, which could then go on and split further atoms, setting up a chain reaction leading to a truly vast release of energy. Szilard immediately saw the possible military applications and sought to patent the idea and have it made an official secret. But in 1933, the chain reaction only existed in Szilard's head. No one had yet found an atom that could be split by neutrons.
These developments were happening against a background of extraordinary political turmoil in Europe. Hitler had come to power in Germany in January 1933. In 1938, less than a year before the outbreak of World War II, just such an atom was found, uranium.
Working at the Kaiser Wilhelm Institute in Berlin, the nuclear chemists Otto Hahn and Fritz Strassman found that when bombarded with neutrons, uranium split into two nuclei of roughly half the size. Not only that, but further calculations showed that a large amount of energy was also released - enough from a single nucleus to move a grain of sand. The first stage of Szilard's chain reaction had been achieved.
When he heard the news Szilard, now in New York and working at Columbia University with Enrico Fermi, set about showing whether, as well as energy, further 'secondary' neutrons were released. By July 1939, when he first knocked on Einstein's door, he knew that they were and so the chain reaction was possible. Also, he and Fermi had settled on a design for the first nuclear reactor.
During the course of their conversations in the summer of 1939, Szilard explained these new developments to Einstein and his fear that the Nazis might use them to create a nuclear bomb. Together they drafted a letter, signed by Einstein, to the American President, Franklin Roosevelt. The letter was delivered to the President on the 11 October 1939 and after reading it the President provided funding for research that would pave the way for the Manhattan Project and lead, ultimately to the construction of the first atomic bomb. After signing the letter, Einstein played no further part in the development of the bomb.
With the first atomic explosion over Hiroshima, the power of e=mc² had been graphically demonstrated to the world. Just 0.6 grams of mass, converted into energy, had been enough to destroy an entire city.
Einstein was horrified when he heard that the bomb had been dropped. When they, wrote to the President, Szilard and Einstein advocated the development of an American bomb purely as a deterrent against the threat of a Nazi weapon. They had not conceived of its use as an offensive weapon, especially after the defeat of Nazi Germany.
Einstein always saw e=mc² as a purely theoretical insight and refuted any responsibility for the bomb but he did feel some responsibility for the letter he'd written to Roosevelt. A letter he would come to describe as "the one mistake" of his life. Einstein saw nuclear weapons and the nuclear arms race as a threat to the future of civilisation. In his final years he devoted much of his time and energy to issues dealing with the world's future - advocating pacifism and campaigning for the control of nuclear weapons, not by individual nations, but by a world government. The last document he signed, just a week before he died, was a manifesto drawn up by Bertrand Russell, renouncing war and nuclear weapons. As Russell said: ""Einstein was not only a great scientist he was a great man. He stood for peace in a world drifting towards war..."
But while the bomb proved e=mc² to be the ultimate equation of destruction, only after his death has the role of Einstein's equation in the creation of the universe become clear. Just as mass can be turned into energy in a bomb, the pure energy generated in the Big Bang condensed into the matter that makes up our world. Almost a hundred years ago, with just six short pen stokes Einstein unlocked one of the most powerful truths about the universe. A truth that would change our world, both for good and ill.