In 1939, particle physics was scarcely distinguishable from its parent discipline, nuclear physics. Only a handful of the particles that were to occupy the attention of physicists in the latter half of the century were known. Probing the structure of nuclei and of the sub-nuclear particles, and studying their interactions, requires high energies. This is again a consequence of the uncertainty principle: to explore fine detail requires a probe of small wavelength, which implies large momentum, and therefore high energy. For the highest energies, the naturally occurring cosmic radiation was (and continues to be) used as a source. But cosmic radiation is increasingly sparse at higher energies, so artificial accelerators were already being designed and built in the 1930s. At the very highest energies ever observed (the highest energy yet recorded for a cosmic ray is a staggering 3.2×1020 electron-volts - the kinetic energy of a tennis ball at nearly a hundred miles an hour concentrated on a single sub-atomic particle!), cosmic rays are so rare that detectors ranged over 6000 square kilometres would only be expected to detect a few thousand particles a year with energies above 1019 electron-volts . But such an array of detectors is now under being constructed. This is the Pierre Auger Observatory, which consists of two arrays of detectors, one in Utah in the United States and one in Argentina, located so as to allow coverage of the whole sky. Each of these sites will have 1600 particle detectors deployed over 3000 square kilometres.
Modest by comparison with the engineering feats of today, the cyclotrons built by Ernest Lawrence were in the vanguard of the new breed of "atom-smashers". The largest of these was commandeered during the Second World War, to be used to separate the fissile fraction of uranium for the construction of the bomb that devastated Hiroshima. Physicists along with their machines were redeployed on new missions. When peace came, they had achieved stupendous success, not only with new scientific discoveries that had fed the war-machine. These had profoundly troubled the conscience of men like J Robert Oppenheimer who led the Manhattan Project which developed the atom bomb; others had deliberately set themselves aside from project. (I know of only one who actually resigned from the Manhattan Project having joined it to begin with, and that was Joseph Rotblat , who in 1995 was awarded the Nobel Peace Prize. He was a Fellow of Queen Mary, and an Emeritus Professor here.) The physicists had also learnt the skills and methods needed to bring to fruition gigantic collaborative enterprises, designing and engineering new technical devices that both incorporated the fruits of discoveries only just achieved, but also addressing problems at the frontiers of knowledge -- and beyond. They had acquired courage from their confidence in their science, and audacity in its application. Their science had moved from the laboratory bench to the factories of Oak Ridge and Los Alamos; from a team of a few individuals to the co-ordinated efforts of hundreds. Big science was born. As an example of this legacy of big science, the ATLAS collaboration which will use the Large Hadron Collider (or LHC) at CERN - the European laboratory for particle physics (in this aerial view the superposed circles show where the complex of accelerators and storage rings run underground)- already has over 2000 scientists as participants from 150 institutions (including Queen Mary) in 34 countries; their detector is planned to start taking data in 2007.
The juggernaut of big science, as some would see it, though inaugurated in the exigencies of war, has persisted and burgeoned in peacetime. Huge laboratories have grown up, and not only for high energy physics. CERN, for example, which spans the frontier between France and Switzerland at Geneva, is host at any one time to some 2,000 physicists, not only from its 20 member states, but also from dozens of other countries. One of the great rewards for working in this field is to know oneself to be a partner in a truly international enterprise; no flags or other national insignia are permitted inside CERN. It is probably inevitable that all future laboratories of this scale will be international, not only in their scientific personnel, but also in their funding. The United States wrote off an expenditure of some two billion dollars and six years of planning and construction when it abandoned the project to build the Superconducting Super Collider in Waxahachie, Texas, leaving behind a useless incomplete 14.5 mile section of the tunnel which would have housed the machine, and putting hundreds of highly specialised scientists and engineers out of work. The US has now joined the LHC programme at CERN.
The discoveries of particle physics in the past half-century have led us to a model of the structure of matter, a set of fundamental particles and the forces which bind them, which has the scope to embrace all the phenomena of particle physics and so up through the hierarchy of nuclear and atomic physics to chemistry and beyond. (But beware the hubris of supposing that there is nothing to be added at each level of the hierarchy! Nothing however would seem to be contributed from above to this level of understanding). We have then a Standard Model for the fundamentals of particle physics. Standard, because it is accepted by most particle physicists as being correct in its essentials. A model only, because although it provides a marvellous, tightly interlocking framework for the explanation and description of all the observed phenomena, there are still significant loose ends. There are predictions which although in principle are believed to be calculable, have so far eluded our technical capabilities. And there are still too many arbitrary parameters which have to be "fed in by hand" for theorists to be comfortable with this as a truly fundamental theory. (One might reflect on the audacious optimism that discomfort reveals. A century ago, most physicists would have accepted that there were very many quantities which entered their description of the world which should simply be accepted as given. They were to be measured, with ingenuity and accuracy; but it was unreasonable to suppose that they could all be calculated from just a handful of fundamental constants. Now as a new century dawns, most physicists would agree that it ought to be possible, in principle at any rate, to determine all the basis of physics from just such a handful of parameters. And the 26 or so constants needed to define the Standard Model are generally regarded as too many!)
Remember that when I speak of particles, I am also speaking of fields; the particles are the basic quantum excitations of the fields which bear their name. We may distinguish two different varieties of field. One, of which the electron field is an example, has as its quanta particles which obey Pauli's exclusion principle; no two of them may occupy the same quantum mechanical state. Electrons, and other particles like them, spin like tiny gyroscopes or tops, with an amount of angular momentum equal to one half in the natural units of quantum mechanics (Planck's constant h divided by 2 pi). Collections of fermions satisfy a different kind of statistical mechanics from the distinguishable particles of classical mechanics; they are called fermions after Enrico Fermi , who along with Dirac first described their statistics.
Another kind of particle, and the photon is one such, carries spin angular momentum of one unit (the spin of the photon is associated with the polarisation of light.) Still others have zero spin. Particles with zero or integer spin are called bosons after Physicist Satyendranath Bose.
