Wonders of the Universe - Brian Cox [47]
US DEPARTMENT OF ENERGY / SCIENCE PHOTO LIBRARY
Years before the Manhattan project designed and delivered the most destructive weapon used in anger in the history of warfare, two of the greatest physicists of the age had already lost interest in the idea. Edward Teller and Enrico Fermi were friends and colleagues who would both go on to be members of the Manhattan team, but in 1941, before any type of nuclear bomb had been assembled, their minds were already wandering beyond the bomb that would later be dropped on Hiroshima and Nagasaki with devastating effect.
The Hiroshima and Nagasaki bombs were fission bombs, which work by splitting the nuclei of very heavy elements (uranium in the case of the Hiroshima bomb and plutonium for the Nagasaki bomb), into lighter elements such as strontium and caesium. This is the assembly of the elements in reverse. Each time a nucleus of uranium or plutonium splits, neutrons are released which trigger the splitting of other nuclei. In this way a nuclear chain reaction ensues. Each time a heavy nucleus splits, a large amount of energy is liberated – this ‘nuclear binding energy’ is stored in the strong nuclear force field that sticks the protons and neutrons together inside the nucleus.
However, even in the very early stages of the Manhattan project, years before the idea of a fission bomb was a physical reality, Enrico Fermi postulated that there was the very real possibility of creating a far more powerful type of bomb. Edward Teller became obsessed with his friend’s idea and spent the next decade designing and building a device that would create the most powerful explosions ever made on Earth. It earned Teller the title ‘father of the hydrogen bomb’.
On 1 November 1952, the fruits of Fermi’s conversation with Teller were realised. Ivy Mike was the codename given to the first successful testing of a hydrogen bomb on Enewetak, an atoll in the Pacific Ocean. The explosion was estimated to be 450 times more powerful than the bomb dropped on Nagasaki, producing a fireball over five kilometres (three miles) wide, a crater two kilometres (one mile) wide and wiping the tiny atoll off the map. Teller had collaborated with another Manhattan scientist, Stanislaw Ulam, to design the bomb, but he wasn’t present for the explosion. Instead he sat watching a seismometer thousands of miles away in his office in Berkeley, California. The explosion was so powerful that he was able to clearly see the shockwave from the comfort of his office. ‘It’s a boy!’, he cryptically told his colleagues to inform them of the success.
The Ivy Mike test was the first man-made nuclear fusion reaction. Nuclear fusion is the direct opposite of fission; it is the process by which two atomic nuclei are fused to form a single heavier element. The hydrogen bomb reproduces the process that occurred in the first seconds of the evolution of the Universe – the assembly of hydrogen into helium.
The Teller–Ulam design for the hydrogen bomb that exploded on Enewetak is the basic design employed by all five of the major nuclear weapon states today. Although the fusion element of the design is only part of its explosive power, combined with the other stages contained within the bomb it creates destruction on an unparalleled scale.
Here are two completely different ways of creating new elements and releasing vast amounts of energy. The first, fission, involves taking a heavy element and splitting it. The second, fusion, involves taking lighter elements and sticking them together. But how can both these processes result in energy being released? Isn’t there a contradiction here? There isn’t, of course, because this is how nature works. It’s all down to the delicate balance between the electric repulsion of the protons in the nucleus and the power of the strong nuclear force to stick the protons and neutrons together. Since there are two competing forces, one trying to blow the nucleus apart and one trying to glue it together, you might think there must be some kind of balancing