Quasi-particles

A recent BBC Horizon documentary on the smallest things in the universe, claimed to present experimental proof that the electron has been split.

With the tone of indulgent bemusement that characterises contemporary television accounts of modern physics, the viewer was told that, in principle, it is possible to split the fundamental properties of the electron. These properties, we were told, are (intrinsic) spin, (electric) charge and orbital (angular momentum), and in the experiment in question, "when the x-ray beam strikes, the electron split into new quasi-particles, called spinons, orbitons and holons, which carry the properties of the electron, and can travel off in different directions."

In point of fact, the electron hasn't been split at all. Quasi-particles are collective excitations in the state of condensed matter, typically the atomic lattice of a crystal. Some of these collective states possess properties which formally resemble properties possessed by states of the individual electron. In 1996, holons and spinons were produced for the first time in the collective state of a crystal, and now a new collective state, containing orbitons and spinons, has been created.

Quasi-particles, however, are certainly of very deep interest, not least because they reveal that particle-like entities can be created as nothing more than transient disturbances within a substrate. As philosopher of physics David Wallace points out, quasi-particles "can be created and annihilated; they can be scattered off one another; they can be detected (by, for instance, scattering them off 'real' particles like neutrons); sometimes we can even measure their time of flight...We have no more evidence than this that 'real' particles exist...and yet they consist only of a certain pattern within the constituents of the solid-state system in question," (p51, The Emergent Multiverse, OUP, 2012).

Moreover, according to quantum field theory, elementary particles such as electrons are merely excited states of underlying quantum fields. This in itself should undermine confidence in the fundamentality of so-called elementary particles, but unfortunately quantum field theory provides a rather sparse characterisation of what a quantum field actually is, merely specifying it to be a self-adjoint operator-valued field on space-time (technically an operator-valued 'distribution'). The significance of quasi-particles is that they provide a very concrete substrate upon which particle-like entities can be realised, and a discrete substrate at that.

Perhaps, then, there is no lowest level to the structure of the universe; no foundations and no basement level, just an infinite multi-storey subterranean car-park.