**Fundamental interactions**, also known as

**fundamental forces**, are the interactions in physical systems that do not appear to be reducible to more basic interactions. There are four conventionally accepted fundamental interactions—

gravitational,

electromagnetic,

strong nuclear, and

weak nuclear. Each one is understood as the dynamics of a

*field*. The gravitational force is modelled as a continuous

classical field. The other three are each modelled as discrete

quantum fields, and exhibit a measurable unit or

*elementary particle*.