How to Destroy the Universe (15 page)

One such radioactive material is the chemical element thorium. In 1901, British physicist Ernest Rutherford and his colleague Frederick Soddy found that thorium emits alpha particles—and that as it does this the thorium gradually converts itself into a chemical element called radium. One chemical element was being converted into another—exactly what the ancient alchemists had sought to achieve. Soddy called the process “transmutation” (despite Rutherford's protestations that the name sounded too reminiscent of alchemy). However, quite what was going on—or how the process could be controlled—was still a mystery. That would have to wait for a better understanding of atoms and how they work. But it wasn't far away.

Scientists had already figured out that atoms were broadly composed of a piece with positive electrical charge and lots of little pieces with negative electrical charge, called electrons. But how they fitted together was anyone's guess. At the turn of the 20th century, the smart money was on an idea called the “plum pudding model”—which said that atoms consisted of a positively charged blob (the pudding) with negatively charged electrons embedded within it (the plums). But in 1911, that all went out of the window following a landmark experiment by Ernest Rutherford and his colleagues Hans Geiger and Ernest Marsden.

Rutherford, Geiger and Marsden bombarded a thin sheet of gold foil with particles of alpha radiation. They were expecting the positive charge of the alpha particles to interact with the electrical charges inside the atoms, causing the particle trajectories to be deflected, which they hoped would reveal details about how charge is distributed within each atom. The experiment did just that, though the results proved to be the death knell for the plum pudding model. Rather than seeing each radiation particle deflected by a few degrees, as they were expecting, most of the particles passed straight through the foil unaffected. However, now and again a lone alpha particle bounced back from the foil toward the radiation's source. To Rutherford, the meaning was clear. The positive charge of the atom is concentrated into a tiny volume, so that most of the time the alpha particles passed through undisturbed. But on the rare occasions one strayed near to one of these clumps of positive charge, the particle's own positive charge was repelled, catapulting it back in the direction it had come from. Rutherford's team had discovered the atomic nucleus, a pinprick of positive charge at each atom's core. Indeed, it's so small that if an atom was scaled up to the size of the Albert Hall, the nucleus would only be the size of a pea.

Rutherford was intrigued by his team's discovery. If the positive charge of the atom was concentrated into a nucleus at the center then what exactly was this blob of positive charge made of? In 1919, he carried out an experiment that revealed the answer. He fired alpha particles into a cloud of nitrogen gas, and found that some of the particles were absorbed by the nitrogen atoms, which in turn spat out a nucleus of the element hydrogen. Rutherford correctly concluded that the hydrogen nucleus must be a fundamental component of all atomic nuclei. He consequently named it the proton—it has positive electric charge, equal but opposite to that found on the electron. Each atom then has a number of protons in its nucleus together with an equal number of electrons orbiting in a cloud around the outside, making its overall electric charge zero.

The picture of the atom was gradually taking shape. But what distinguishes an atom of one element from an atom of another? A Dutch physicist by the name of Antonius van den Broek already had the answer to that one. A few years before the discovery of the proton, he had noticed that different elements seem to be specified by the charge on their atomic nuclei. Experiments soon confirmed his theory. Rutherford's discovery then made it crystal clear how you go about changing an atom of one element into another—simply alter the number of proton particles that it contains.

It didn't take long for Rutherford to twig that he'd already done this. Each atom of the nitrogen gas in his experiment had seven protons in its nucleus. His alpha particles each contained two protons and each time one of them was absorbed by a nitrogen nucleus it emitted a hydrogen nucleus—which is just one proton. This meant that what was left must be an element with eight (7 + 2-1) protons at its center. This is the gas oxygen. Rutherford had actively created oxygen from nitrogen.

There was still a problem with the structure of the nucleus that physicists couldn't quite figure out. Bunching a large number of positively charged protons together in this way should cause the nucleus to fly apart—because two or more electrical charges of the same polarity repel each other. British physicist James Chadwick solved the problem in 1932 when he discovered a second component of the nucleus. Called the neutron, it had previously gone undetected because it carries no electrical charge. But it does have a mass roughly the same as the proton. Chadwick exploited this property to spot neutrons as they knocked protons—which could be detected—from a piece of paraffin wax.

Neutrons nestle inside atomic nuclei between the protons, stopping these positively charged particles
from getting so close that their electric repulsion might fling the nucleus apart. With the discovery of neutrons, physicists were in a position to classify each chemical element by a small set of numbers describing its atomic nucleus. The number of protons in the nucleus is called the “atomic number,” denoted by the symbol Z. The total “atomic mass” of the nucleus is specified by a number A, just given by adding together the number of neutrons and the number of protons. Since protons and neutrons each weigh about the same—1.6 million-billion-billionths of a gram—the total mass of the nucleus is just this number multiplied by A. Finally, a third number, N, gives the number of neutrons in the nucleus. It is related to the other two by the formula A = Z + N. For example, simple carbon has A = 12, N = 6 and Z = 6.

Only Z is needed to specify the particular chemical element; it is possible to vary N (and hence A) while keeping Z the same by adding or subtracting neutrons from the nucleus. Such atoms are known as isotopes. For example, carbon can exist in the form of isotopes that have N = 7 and A = 13, and N = 8 with A = 14. Alpha particles also contain two neutrons. So Rutherford's first transmutation experiment in 1919 can be explained by taking nitrogen (A = 14, Z = 7) and adding an alpha particle (A = 4, Z = 2) to make hydrogen (A = 1, Z = 1) plus an isotope of oxygen (A = 17, Z = 8).

In the late 1930s, scientists realized that neutrons don't always help an atom stay together. Very heavy atomic nuclei are actually destabilized by the addition of a neutron, causing the nucleus to split in half to form two lighter elements, and releasing a great deal of energy. The process became known as nuclear fission. It is the basis of modern nuclear power stations, and was the principle underpinning the first nuclear weapons (see
How to build an atomic bomb
).

One of the ongoing problems with nuclear power is disposing of the radioactive waste it produces. This material is filled with elements such as plutonium, neptunium and americium, which remain radioactive for tens of thousands of years, meaning any nuclear waste repository presents a long-term radiation hazard. A solution that scientists are currently investigating is to transmute these radioactive waste materials into more benign elements. They believe this can be done by placing the waste in a specially designed reactor and bombarding it with neutrons to produce new elements that are either non-radioactive or whose radioactivity decays away after a few tens of years. Research projects are underway around the world to make transmutation of nuclear waste a practical, working technology. If they are successful, it could mean clean nuclear energy with fewer dangers attached.

But what of our original quest to turn ordinary metals into glittering gold? It is indeed possible. In 1980, US scientist Glenn Seaborg made a small amount of gold (Z = 79) from lead (Z = 82) in a nuclear reactor. Sadly, however, he found the process requires a massive amount of energy—so much so, it cost more per kilogram of gold produced than the metal's market value. In a cruel twist, it is actually much easier to turn gold into lead, which is probably why lead is plentiful while gold is so rare.

Gold is not the only precious metal, however. The metals rhodium and ruthenium are also extremely expensive, and both are produced in nuclear power plants as a bi-product of the energy that's released to generate electricity. And they only remain radioactive for a few years. Japanese scientists now have a plan to begin extracting these metals with a view to selling them in order to offset the large cost of processing nuclear waste. It's not quite the alchemists' dream. But it is a sobering reminder, if one were needed, that claims which sound too good to be true often are just that.

• Chain reactions

• Mass deficits

• Fission bombs

• Critical mass

• Fusion bombs

• Nuclear explosions

They remain the most fearsome weapons of war ever created. A single nuclear bomb—built around a piece of radioactive material no bigger than an orange—can flatten an entire city and kill hundreds of thousands of people. A nuclear weapon is frighteningly simple to make. So much so that, today, the fear is that they may be used not only by nation states but by terrorists as well.

Nuclear weapons are a development that was inspired by a science fiction story. In 1914, H.G. Wells published a novel called
The World Set Free
. It tells the story of scientists who find a way to accelerate the rate of decay of radioactive materials, such as radium, enhancing the
amount of radiation they give off. Hungarian physicist Leo Szilard read Wells's novel in the early 1930s. By then, the structure of the atom was well understood: a condensed nucleus containing neutrons and positively charged proton particles, with negatively charged electrons orbiting the outside. Radioactivity was known to be caused by an instability of some atomic nuclei, causing them to spontaneously throw out particles and radiation every now and again. Szilard wondered whether Wells's vision might actually be possible if the particles given out by the decay of one atomic nucleus could stimulate other nearby nuclei to decay in the same way, setting up a chain reaction.

Meanwhile, other physicists were noticing something strange about atomic nuclei, namely that their masses were less than the mass you get when you add up all their constituent particles. For example, a standard carbon nucleus is made of six protons and six neutrons, but its total mass is found to be less than six times the proton mass plus six times the neutron mass. They called this difference a mass deficit. But what did the mass deficit mean? The answer lay in Albert Einstein's special theory of relativity. Published in 1905, this was a theory describing the motion of objects moving at close to the speed of light. As well as giving us the best description yet of superfast motion, Einstein's theory
also threw up what is probably the most famous equation in the whole of physics:
E = mc
²
. It essentially said that mass (
m
) and energy (
E
) are just different aspects of the same fundamental entity, linked together by the speed of light (
c
). Physicists interpreted the mass deficits to mean that some of the mass of the constituent particles in atoms gets turned into energy and released as they come together to form a complete nucleus. This is known as binding energy because it is the energy required to hold the nucleus together. Fire in this amount of energy and you shatter the nucleus apart. Assemble the same nucleus and the binding energy is released as a burst of radiation.

There was another surprise to come when they plotted a graph of the binding energy per particle against the total number of particles in the nucleus. Rather than just a flat line, they found a curve that rose sharply up to a peak then gradually dropped off again as the total number of particles in the nucleus became large. Any nuclear reactions in which the binding energy of the nuclei that come out is more than the binding energy of the nuclei that go in will release energy. A quick look at the graph made it clear to the physicists that they could do this in one of two ways: by joining together two light atomic nuclei to the left of the peak to make a heavier one, a process called fusion; or by splitting apart a heavy nucleus to the right of the peak to make two lighter ones, called
fission. This latter possibility was the one physicists chose to pursue first.