Our picture of the physical world at its most fundamental level, a model that also has a very high degree of experimental support, runs along the following lines. There are only three types of interaction: QCD (Quantum ChromoDynamics), which binds quarks into hadrons, that is, nuclear particles like protons and neutrons, pions and so on; the electroweak interaction, which is the unification of electromagnetism with the weak nuclear force (responsible for beta decay); and gravity. The first two interactions are understood in the context of quantum field theory, more particularly gauge field theory, and the interactions are transmitted by the field quanta, which are gluons (for QCD), and the photon and the W and Z bosons which mediate electroweak interactions. Gravity is described by General Relativity. What is immediately obvious about this statement is that General Relativity is, conceptually, a completely different sort of theory from the other field theories, because of its explicitly geometric nature. The whole enterprise of physics, however, is to reduce the number of fundamental theories and concepts to the absolute minimum, and as a consequence a large number of physicists now work on unification schemes of one sort or another – supergravity, superstring theory, brane worlds, and so on. One guiding principle at work in these endeavours is to unite the three fundamental interactions into one interaction, and another, equally important, aim is to find a quantum theory of gravity; it is clear that General Relativity is a classical theory since it never at any point employs notions involving wave–particle duality or Planck's constant.