Adapted from P. Coles, 1999, The Routledge Critical Dictionary of the New Cosmology, Routledge Inc., New York. Reprinted with the author's permission. To order this book click here: http://www.routledge-ny.com/books.cfm?isbn=0415923549
The four ways in which the various elementary particles interact with one another: electromagnetism, the weak nuclear force, the strong nuclear force and gravity. They vary in strength (gravity is the weakest, and the strong nuclear force is the strongest) and also in the kinds of elementary particle that take part.
The electromagnetic interaction is what causes particles of opposite charge to attract each other, and particles of the same charge to repel each other, according to the Coulomb law of electrostatics. Moving charges also generate magnetic fields which, in the early history of physics, were thought to be a different kind of phenomenon altogether, but which are now realised to be merely a different aspect of the electromagnetic force. James Clerk Maxwell was the first to elucidate the character of the electromagnetic interactions. In this sense, Maxwell's equations were the first unified physical theory, and the search for laws of physics that unify the other interactions is still continuing (see grand unified theories, theory of everything).
The theory of electromagnetism was also an important step in another direction. Maxwell's equations show that light can be regarded as a kind of electromagnetic radiation, and demonstrate that light should travel at a finite speed. The electromagnetic theory greatly impressed Albert Einstein, and his theory of special relativity was constructed specifically from the requirement that Maxwell's theory should hold for observers regardless of their velocity. In particular, the speed of light had to be identical for all observers, whatever the relative motion between emitter and receiver.
The electromagnetic force holds electrons in orbit around atomic nuclei, and is thus responsible for holding together all the material with which we are familiar. However, Maxwell's theory is a classical theory, and it was realised early in the 20th century that, in order to apply it in detail to atoms, ideas from quantum physics would have to be incorporated. It was not until the work of Richard Feynman that a full quantum theory of the electromagnetic force, called quantum electrodynamics, was developed. In this theory, which is usually abbreviated to QED, electromagnetic radiation in the form of photons is responsible for carrying the electromagnetic interaction between particles.
The next force to come under the spotlight was the weak nuclear force, which is responsible for the so-called beta decay of certain radioactive isotopes. It involves elementary particles belonging to the lepton family (which includes electrons). As with electromagnetism, weak forces between particles are mediated by other particles - not photons, in this case, but massive particles called the W and Z bosons. The fact that these particles have mass (unlike the photon) is the reason why the weak nuclear force has such a short range. The W and Z particles otherwise play the same role in this context as the photon does in QED: they are all examples of gauge bosons (see gauge theory). In this context, the particles that interact are always fermions, while the particles that carry the interaction are always bosons.
A theory that unifies the electromagnetic force with the weak nuclear force was developed in the 1960s by Sheldon Glashow, Abdus Salam and Steven Weinberg. Called the electroweak theory, it represents these two distinct forces as being the low-energy manifestations of a single force. At high enough energies, all the gauge bosons involved change character and become massless entities called intermediate vector bosons. That electromagnetism and the weak force appear so different at low energies is a consequence of spontaneous symmetry-breaking.
The strong nuclear interaction (or strong force) involves the hadron family of elementary particles, which includes the baryons (protons and neutrons). The theory of these interactions is called quantum chromodynamics (QCD) and it is built upon similar lines to the electroweak theory. In QCD there is another set of gauge bosons to mediate the force: these are called gluons. There are eight of them, and they are even more massive than the W and Z particles. The strong force is thus of even shorter range than the weak force, Playing the role of electric charge in QED is a property called `colour'. The hadrons are represented as collections of particles called quarks, which have a fractional electrical charge and come in six different `flavours': up, down, strange, charmed, top and bottom. Each distinct hadron species is a different combination of the quark flavours.
The electroweak and strong interactions coexist in a combined theory of the fundamental interactions called the standard model. This model is, however, not really a unified theory of all three interactions, and it leaves many questions unanswered. Physicists hope eventually to unify all three of the forces discussed so far in a single grand unified theory. There are many contenders for such a theory, but it is not known which (if any) is correct.
The fourth fundamental interaction is gravity, and the best theory of it is general relativity. This force has proved extremely resistant to efforts to make it fit into a unified scheme of things. The first step in doing so would involve incorporating quantum physics into the theory of gravity in order to produce a theory of quantum gravity. Despite strenuous efforts, this has not yet been achieved. If this is ever done, the next task will be to unify quantum gravity with the grand unified theory. The result of this endeavour would be a theory of everything. The difficulty of putting the theory of interactions between elementary particles (grand unified theories) together with the theory of space and time (general relativity) is the fundamental barrier to understanding the nature of the initial stages of the Big Bang theory.
Roos, M., Introduction to Cosmology, 2nd edition (John Wiley, Chichester, 1997), Chapter 6. Davies, P.C.W., The Forces of Nature (Cambridge University Press, Cambridge, 1979). Pagels, H.R., Perfect Symmetry (Penguin, London, 1992).