Glues the pieces together

The Particle Century
June 25, 1999

That matter is constituted from atoms was conjectured some 2,500 years ago. However, it is only within this past century that the physical properties of atoms have come to be understood in terms of even more basic entities - elementary particles. As one of the authors in this collection of 19 essays remarks, "It must be understood that at present everything, but indeed everything, is understood in terms of particles.'' This understanding has been achieved, roughly speaking, by knocking matter to pieces and studying the behaviour of the resultant debris, which has involved the use of large electromagnetic machines - particle accelerators - that accelerate particles to extremely high energies. These particles are then used to bombard suitable targets or even collide with other particles that have been accelerated in the opposite direction. Either way, many other particles are produced in the resultant collisions, and their nature and behaviour are then investigated using immensely complicated detectors.

Of course there is an upper limit to the energy to which particles can be accelerated - limited by technical know-how and, not least, by cost. But higher energies are needed if the very small-scale nature of particle interactions is to be studied and also, remembering Einstein's relation between energy and mass (E=mc2), if unknown or conjectured heavier particles are to be created in collisions. Much particle research, for financial reasons, is therefore conducted by international collaboration and future, more powerful particle accelerators will most likely be "world machines". But, besides machines, there is another source of high-energy particles, namely the universe itself, which came into being during the highest energy process of all time - the big bang. The behaviour of the evolving universe, stellar processes and the associated cosmic rays all throw light on the nature of elementary particle processes and, in turn, our understanding of the cosmos is largely in terms of the nature of these processes.

This collection of essays by distinguished particle physicists, including several Nobel laureates, traces the evolution of our understanding of particle physics during the century following the first discovery of an atomic constituent - the electron - in 1897. This understanding is largely incorporated in what is known as the Standard Model. In this, the basic ingredients of all matter are taken to be six heavy particles known as quarks, six light particles (including the electron) known as leptons and a number of "carrier" particles responsible for carrying the interactions experienced by quarks and leptons between each other. The model is formulated in terms of three basic interactions. These are the strong interaction (manifested, for example, in the nuclear force) experienced by quarks but not by leptons, the electromagnetic interaction experienced by quarks and electrically charged leptons, and the weak interaction (responsible, for example, for some forms of radioactivity) experienced by essentially all particles.

It is in terms of these particles and interactions that the discussion in these essays is conducted. The first six essays deal largely with post war developments through to the 1960s. By this time three of the six quarks had been established as the constituents of, for example, protons and neutrons (nuclear "building blocks") and theories of the strong interaction were developed. The next five essays focus mainly on developments in the 1970s. A fourth quark was identified, the weak interaction came to be well understood and its three "carrier" particles were identified. Most importantly, the discovery of an electrically neutral carrier confirmed, as had been hypothesised, that the electromagnetic interaction (which is also carried by a neutral particle) and the weak interaction can be unified - electroweak theory. Finally, in 1979, came clear evidence that the strong force between quarks was carried by eight different particles - gluons, carrying what is called a "colour" charge - such that a coherent picture of the strong, electromagnetic and weak interactions could be formulated.

This is the Standard Model that, together with its deficiencies, likely extension and related experimental evidence, is discussed in the next three essays, which take us through to the present day. So emerged the final two quarks and the probability of the existence of another key particle - the Higgs boson, responsible for particle masses and so far undetected. Further, it is conjectured that the basic particles are paired in doublets with another set of particles (super-symmetry) and that basic particles are not point-like but are more like minute strings (string theory). The benefit of these possibilities is that they may allow the extension of the Standard Model to a Grand Unified Theory including gravity. Particle physics research is now focused on exploring these ideas.

The next two essays recount the development of accelerators and detectors and, finally, the book concludes with three essays dealing with the interaction between particle physics, cosmology and astronomy. As one of the latter authors comments, "Particle physics provides the framework in which fundamental issues in cosmology may be resolved'' and cosmic rays, for example, "are now providing tantalising hints of possible physics beyond the Standard Model". The impressive collection of colour plates relates almost entirely to these three essays.

Inevitably, the essays in a collection of this kind vary in their nature and detail. Some are challengingly technical, while others are highly personal accounts of research. But they manage to present, in this glossy and attractively compiled book, most of the key developments in the field of particle physics during this past century. It should be readily accessible to scientifically literate readers.

Roger Blin-Stoyle is emeritus professor of theoretical physics, University of Sussex.

The Particle Century

ISBN - 0 7503 0543 6
Publisher - Institute of Physics Publishing
Price - £30.00
Pages - 232

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