Why Does E=mc2_ - Brian Cox [63]
The energy released in chemical reactions has been the primary source of power for our civilization since prehistoric times. The amount of energy that can be liberated for a given amount of coal, oil, or hydrogen is at the most fundamental level determined by the strength of the electromagnetic force, since it is this force that determines the strength of the bonds between atoms and molecules that are broken and reformed in chemical reactions. However, there is another force of nature that offers the potential to deliver vastly more energy for a given amount of fuel, simply because it is much stronger.
Deep inside the atom lies the nucleus—a bunch of protons and neutrons stuck together by the glue of the strong nuclear force. Being glued together, it takes effort to pull a nucleus apart, just as it does for atoms and molecules, and its mass is therefore less than the sum of the masses of its individual proton and neutron parts. Entirely analogous to the goings-on in chemical reactions, we might wonder whether it is possible to make nuclei interact with each other in such a way that allows this mass difference to be emitted as useful energy. Breaking chemical bonds and releasing the stored energy in the atoms can be as easy to achieve as lighting a match, but releasing the energy bound up in a nucleus is an entirely different matter. It is often hard to access and usually requires some clever apparatus. Not always, though; there are occasions where nuclear energy is liberated naturally and spontaneously, with extremely important and unexpected consequences for planet Earth.
The heavy element uranium has 92 protons and, in its most stable naturally occurring form, 146 neutrons. In this guise, it has a half-life of around 4.5 billion years, which simply means that in 4.5 billion years, half of the atoms in a lump of uranium will have spontaneously split up into lighter things, the heaviest of these being the element lead, and liberated energy as a result. In the language of E = mc2, the uranium nucleus splits into two smaller nuclei, whose combined mass is a little less than the mass of the original nucleus. It is that loss of mass that manifests itself as nuclear energy. The process whereby a heavy nucleus splits up into two lighter nuclei is called nuclear fission. Along with the 146-neutron form of uranium, there also exists a less-stable naturally occurring form with 143 neutrons that splits into a different form of lead with a half-life of 704 million years. These elements can be used to accurately date rocks almost as old as the earth itself, which is around 4.5 billion years old.
The technique is beautifully simple. There exists a mineral known as zircon that naturally incorporates uranium into its crystalline structure, but not lead. It can therefore be assumed that any lead present in the mineral comes from the radioactive decay of uranium, which allows the date of formation of the zircon to be measured with high precision simply by counting the number of lead nuclei present and knowing the rate of decay of the uranium. The heat generated when uranium splits up also plays a crucial role in keeping the earth warm, and that heat helps provide the power that drives plate tectonics and pushes up new mountains. Without this impetus, fueled by nuclear energy, the land would crumble into the sea as a result of natural erosion. We shall say no more about nuclear fission. It is now time to zoom in on the atomic nucleus and learn a little more about its stored energy and the other important process that can occur to facilitate its release: nuclear fusion.
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