The idea that the world is made up of 'stuff' in the form of lots of small particles is old. Ancient greek philosophers, notably Democritus who first conceived of the idea of 'atoms', devoted considerable energy contemplating this question. During the rise of early-modern science, under the influence of Descartes and Newton in the sixteenth century, particle theories of natural phenomena became all the more en vogue. Even light, according to Newton, should be interpreted as a particle phenomenon, but, as is often the case, the question of the nature of these particles went unanswered.
Continuous Nature A totally different concept, namely that of continuous media carrying waves, also owes its scientific origins to the same era. Huygens, in whose honour the principle of wave-propagation is named, proposed that light was to be thought of as something continuously flowing and altering in form in a very specific way. A wave-front could be considered as a result of each point in space within the wave being a source of waves itself. In his day optic devices were far too inaccurate to discriminate between these ideas decisively, but, two centuries later, the 'light as a wave phenomenon' emerged as the favoured view. In fact, the whole view of nature became a lot more 'continuous' and even a concept like "heat" was phrased in terms of a continuous caloric fluid. The extent to which this view permeated scientific thought is exemplified by the resistance amongst the nineteenth century physics community to embrace the concept of molecules.
'Modern particles' The view most non-physicists have of the nature of particles dates from the late nineteenth and early twentieth centuries. The Austrian physicist Ludwig Boltzmann showed that the old "continuous" view of nature could be very well reproduced by working out the laws of classical mechanics for very large numbers of very small particles, dubbed molecules. The emerging science of chemistry showed that molecules could be thought of as comprising the right pairings of more fundamental units called atoms. The great triumph came a short time later when it was revealed that most of the properties of atoms could be understood in terms of three simple constituent particles: protons and neutrons, which reside in the atomic nucleus, and electrons. What a nice place the world was - three little billiard balls making up this whole astonishing variety of stuff that we see around us.
'Post-Modern Particles' The Post-Modern world started before the modern world had ended, at least in physics. It turned out that the three fundamental building blocks of stuff were not at all like billiard balls. Quantum mechanics is the desperate attempt of mankind to come to terms with this. In the first half of the twentieth century wave ideas came crashing back into physics to explain the oddities of how these particles behave. Niels Bohr (figure 1) subsequently sought to break down the discussion on the "true nature" of particles by describing them as being "both waves as well as particles".
Of course, if you call this the "complementarity-principe" it sounds more credible, and if you add a little Yin-Yang all is well that ends well. I believe we could just as well say that they are neither … neither particles nor waves. So what is the nature of these elusive things that we call particles?
Let us ignore for the moment that the original simplicity of the electronic-protonic-neutronic trinity has given way to a little zoo of 6 quarks, 3 types of electrons, and three types of neutrinos. Still not bad though for an explanation of all of stuff. No, the real enigma is their nature, not their numbers. The discovery of their wave-properties was not the last word, for these waves turned out to be wholly different than any waves we ever had encountered before. Indeed, quantum waves are not at all like the waves you see at the beach. They are, at best, waves of probability. These waves of probability come in package deals that we can either take or leave. It is those packages that we refer to as particles - hardly a game of billiards now is it ?
But those probability waves aren't really probability waves at all! They are only like that if the probable velocities of the particles are low. But if they are high, i.e. approaching the speed of light, these particles reveal their true (?) nature as excitations of an underlying field! Now where did that come from? Modern particle physics uses the language of quantum fields; continuous agents (fields) that somehow manage to make a very discrete impression (particles). These are the sort of things a royal household dearly needs, but physics? More and more aspects that we once considered to be sturdily embedded in the nature of material particles are evaporating before our eyes. Even the strings in string theory are not strings.
At the End of All Things... Stuff is not made from little billiard balls. That is, unless you want to extend the definitions of balls to include things that have no size, no exact location, no exact velocity, and no precise number. Things that can change their nature, their charges, that have distinct left and right hands (weak interactions). Things that appear "hot" to an accelerated observer in the vacuum (Unruh Effect). A family of Things of which some are confined to invisibility for most of their lives (strong interactions). I guess physics, which was long thought to be a materialistic science dealing only with the dead and inanimate subjected to soulless forces and interactions has come to the end of all things. At its deepest level known to man, nature seems infinitely more subtle and alien than ever before. The great miracle is that we were actually able to understand it this far at all.