In physics, a fundamental interaction (depending on the nature of the interaction, it might also be called a fundamental force) is a process by which elementary particles interact with each other. An interaction is often described as a physical field, and is mediated by the exchange of gauge bosons between particles. For example, the interaction of charged particles takes place through the mediation of electromagnetic fields, whereas beta decay occurs by means of the weak interaction. An interaction is fundamental when it cannot be described in terms of other interactions. There are four known fundamental interactions in nature: The electromagnetic, strong, weak and gravitational interactions. The weak and electromagnetic interactions are unified in electroweak theory, which is unified with the strong force in the standard model.