Modern particle physics has been, in a sense, driven by the search for the fundamental, or elementary particles. Democritus started the whole process thousands of years ago when he began thinking that matter was ultimately made up of eternal, indistinguishable and unchangeable particles that he named “atoms”, a term that comes from Greek and means “uncuttable” or more accurately, indivisible.

It’s one of the ironies of science that the objects we call atoms (the elements) are in fact not atomic at all. They can certainly be divided into constituent parts, they are capable of being changed into other particles and they are not eternal.

For a while it was thought that protons, electrons and neutrons were actually atomic, but once Gell-Mann et al discovered the existence of quarks, we realized that our best candidates today are electrons, up and down quarks and electron neutrinos (at least in terms of the common fermions.)

But, recognizing that all of the above “atom” particles actually seem to belong to a family (up and down are lower energy manifestations of charm and strange and top and bottom, electrons are related to muons and tauons, and there are tau and muon neutrinos) perhaps we’re better off not thinking about particles at all but rather of excitations of some fundamental field entity. This is what got people so excited about string theory – it is basically a mathematical expression of this very idea.

But that’s over simplifying things. Gordon McCabe, a philosopher of physics takes a much closer look at the question:

“According to modern theoretical physics, the fundamental types of things which exist are quantum fields, and particles are merely excited states of the underlying quantum field. Given that these modes of excitation satisfy the principles of quantum theory, they are often dubbed ‘excitation quanta’. Even when there are no particles present, the quantum field is simply in its lowest-energy state, and this non-zero energy of the so-called ‘vacuum state’ duly has a detectable effect.

Because particles are excitation quanta of an underlying field, their identity conditions are more akin to waves on the surface of a body of water than miniature snooker balls. If one begins with a number of waves on the surface of a pond, and they merge together, then the individual identities of the constituent waves appears to be lost, and this has some similarity with quantum phenomena. Hence, there are conditions under which one can say that there is an N-particle state of a quantum field, but in which it appears to be impossible to individuate N distinguishable particles; there are states of a quantum field in which there are an indefinite number of particles present; and a quantum field state in which no particles are present in one reference frame, is the same state in which there are many particles present in an accelerated reference frame.

[…]So these are the mereological questions which beset elementary particles, but even if we successfully elucidate the quantum concepts of parts and wholes, we are still left with the question, ‘What is an elementary particle?’. Attempts to answer this question have employed the notions of intrinsic properties and extrinsic properties.

An intrinsic property of an object can be defined to be a property which the object possesses independently of its relationships to other objects. In contrast, an extrinsic property can be defined to be a property which an object possesses depending upon its relationships with other objects. Thus, one might deem that a particle’s mass and charge are intrinsic properties, whilst its velocity is an extrinsic property, depending as it does upon the reference frame chosen. “

In other words, McCabe is attempting to find a proper way to express Democritus’ idea in terms of our present understanding to relativistic quantum theory. He does a very interesting job of it, and if you’re interested I commend his article to you. The solution to the problem of expressing exactly what we mean when we seek true elementary entities in the Universe will lead to a better of understanding of what a theory must provide to adequately explain why some candidate for an atomic entity is actually atomic.

Read the full article here.