Fundamental interaction The gravitational force is attributed to the curvature of spacetime, described by Einstein's general theory of relativity. The other three are discrete quantum fields, and their interactions are mediated by elementary particles described by the Standard Model of particle physics. Within the Standard Model, the strong interaction is carried by a particle called the gluon, and is responsible for quarks binding together to form hadrons, such as protons and neutrons. As a residual effect, it creates the nuclear force that binds the latter particles to form atomic nuclei. The weak interaction is carried by particles called W and Z bosons,
Elementary charge known elementary particles, including quarks, have charges that are integer multiples of "e". Therefore, one can say that the "quantum of charge" is "e". In this case, one says that the "elementary charge" is three times as large as the "quantum of charge". On the other hand, all "isolatable" particles have charges that are integer multiples of "e". (Quarks cannot be isolated: they only exist in collective states like protons that have total charges that are integer multiples of "e".) Therefore, one can say that the "quantum of charge" is "e", with the proviso that quarks are not to be