Wonders of the Universe

THE MOST POWERFUL EXPLOSION ON EARTH

The now iconic image of a hydrogen bomb explosion. This mushroom cloud was produced by the detonation of XX-33 Romeo on 26 March 1954; it was the third-largest test ever detonated by the USA.

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 point – an ideal mixture of protons and neutrons that is perfectly poised between attraction and repulsion. There are in fact two elements that are very close to the mixture of optimal stability, and these are iron and nickel. Elements lighter than these can be made more stable, releasing energy in the process, by fusing them together. Elements heavier than these can be made more stable, releasing energy in the process, by breaking them apart.

Look up into a clear blue sky and you are bathing in the energy of nuclear explosions on an unimaginable scale.

To be completely accurate, we should mention that there are other factors than just the balance between the electromagnetic and nuclear forces that feed into the stability of the elements. These are to do with the shape of the nucleus itself and that the balance between protons and neutrons is favoured for quantum mechanical reasons. (If you are interested, google ‘Semi-empirical mass formula’ and enjoy!)

Here on Earth, fusion may seem the ultimate human technological achievement but actually it’s the most natural thing in the world. It didn’t only happen at the Big Bang; it’s a process that can be found occurring across the Universe as we speak. In fact, it illuminates the whole Universe and happens all the time millions of miles above our heads.

Fusion is the process that powers every star in the heavens, including our sun. Look up into a clear blue sky and you are bathing in the energy of nuclear explosions on an unimaginable scale. Deep in the Sun’s core, 800,000 kilometres (500,000 miles) below the surface (where temperatures reach fifteen million degrees Celsius), the Sun is busy fusing hydrogen into helium at a furious rate. In just one second the Sun converts 600 million tonnes of hydrogen into helium, releasing as much energy as the human race will use in the next million years. This is the energy that makes the stars shine and fills the Solar System with heat and light.

The shining Sun is one of the most natural demonstrations of the effect of fusion. It, and all the other stars in the heavens, are powered by the fusing of hydrogen and helium.

It is the process of turning hydrogen into helium that creates the energy that allows all life on Earth to exist, but for all its power the Sun only converts hydrogen, the simplest element, into helium, the next simplest. This process is repeated across the night sky; every star in the Universe began its life fuelled by hydrogen and powered by this reaction.

So the assembly of the second-simplest element, helium, is well understood. We know the stars can do it, we know it happened in the very early Universe, and we can even do it ourselves on Earth. But this doesn’t help to explain the origin of the other ninety-two naturally occurring elements. Clearly, somewhere in the Universe there must be a plentiful source of the other elements because they are everywhere, our whole planet is made from them. We are made of billions and billions of atoms; from magnesium, to zinc, to iron and, of course, the one atom that life is more dependent on than any other – carbon. Every human being on the planet is made from about a billion billion billion carbon atoms. That’s an unimaginable number of carbon atoms that simply didn’t exist in the early moments of the Universe. Where did they come from? The answer must be nuclear fusion, and the natural place to look is within the stars themselves