There are only four fundamental forces and they explain the physical behaviour of all matter in the universe. These are gravity, the electromagnetic force, and the strong and weak nuclear forces.
The force of gravity causes matter to condense into planets, stars and galaxies. The electromagnetic force is responsible for holding the overall structure of the atom together and also causes atoms to bind together to form various compounds.
I dealt with both these forces in last week's article. This week I describe the strong and weak nuclear forces. Unlike gravity and the electromagnetic force that can act across large distances, the effects of the nuclear forces are only felt within atoms.
Almost all the mass of the atom is concentrated in a small central region called the nucleus. The nucleus contains two types of particles called protons and neutrons. Each proton has a positive charge; the neutron has no charge.
Revolving around the nucleus at a distance is a cloud of particles called electrons. Each electron has a negative charge. The number of protons in an atom equals the number of electrons - overall the atom is electrically neutral. The face that the atoms shows to the outside world is the electron cloud.
As I described in the last article, like electric charges repel each other and opposite charges attract each other by the electromagnetic force. This attractive force holds the electrons in orbit around the nucleus. The electrons do not spiral down into the nucleus because of quantum effects, but how are the positively charged protons packed together into the tiny volume of the nucleus?
Our knowledge of the electromagnetic force would predict that the nucleus would fly apart from repulsive forces. Part of the reason this does not happen is supplied by the neutral neutrons, which act to "dilute" the repulsive effects of the positive protons.
However, this alone is not enough. There is also a strong attractive force that operates between protons and neutrons when they are packed closely together. This strong nuclear force of attraction is 100 times stronger than the electromagnetic repulsive force and so holds the nucleus together.
What is the nature of the strong nuclear force? Protons and neutrons are not really fundamental particles, but in turn are composed of smaller particles called quarks. So far as is known, quarks are fundamental and cannot be broken into smaller parts. The strong nuclear force operates between the quarks.
Quarks come in several varieties. They carry electric charges that are either one- or two-thirds the size of the standard unit charge carried by the proton and the electron. Quarks are found in combinations such that their electric charges always sum up to zero or to whole numbers.
Quarks also have another property called colour charge that underlies the strong nuclear force. This has nothing to do with the ordinary meaning of colour, but receives its name because it appears to come in three varieties, analogous to the three primary colours in light.
The colour charge, like the electric charge, can also add up to zero, e.g. the proton contains a red, a green and a blue quark which add up to a colourless condition just as happens when you mix these three primary light colours in the everyday world.
What carries the strong nuclear force? You will recall from the last article the field of electromagnetic force that can be drawn between two electric charges. Similarly, lines of strong nuclear force can be drawn between two quarks. The electromagnetic force is exchanged between charged particles, according to the theory of quantum electrodynamics (QED) by the emission and absorption of virtual photons - ghostly massless particles.
Similarly the theory of quantum chromodynamics (QCD) visualises the strong nuclear force being transmitted by "gluons", also massless particles. However, they differ from the photons in one important respect.
The photons carry no charge and do not interact with each other. The gluons carry a colour charge and can interact with each other. This makes the lines of force between quarks very tightly defined.
When you pull two electric charges further apart the electromagnetic force between them decreases. When you pull two quarks further apart the strong nuclear force between them does not decrease. It is therefore not possible to pull lone quarks out of a proton or a neutron.
BUT the strong nuclear force alone cannot explain everything that happens in the nucleus. One example is radioactivity, where the nucleus is unstable and spits out bits of itself in an attempt to achieve stability, e.g. the transformation of a neutron into a proton with the emission of an electron from the nucleus.
The neutron can be thought of as a close combination of a proton and an electron. The transformation of a neutron into a proton is mediated by the weak nuclear force, a force that is 10,000 times weaker than the strong force.
Relatively little is heard about the weak nuclear force in accounts of the fundamental forces, although it is very important. In addition to explaining radioactivity, it is the force underpinning the phenomenon of nuclear fusion in stars enabling them to shine and emit enormous amounts of energy. All life on Earth depends on energy radiated to us from the sun. Our existence depends on the weak nuclear force.
Physicists have shown that at very high temperatures the weak nuclear force and the electromagnetic force merge into a single force, the electro-weak force. Both forces are just different aspects of the same fundamental electro-weak force and "freeze out" into separate apparently different identities at ordinary temperatures.
Think of the ice crust that forms on top of a lake in very cold weather. The solid ice and the liquid beneath are just different forms of the same substance: water. Physicists are searching for a fully unified theory that will describe all the four fundamental forces with one set of equations.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC