Although the function θ(x) describes a wave, the laws of quantum mechanics require that it also have particle properties. In the case of electromagnetism, the particle corresponding to electromagnetic waves is the photon. In general, such particles are called gauge bosons, where the term "boson" refers to a particle with integer spin. In the simplest versions of the theory gauge bosons are massless, but it is also possible to construct versions in which they have mass. This is the case for the gauge bosons that carry the weak interaction: the force responsible for nuclear decay.