Force carrier

Gravity is not a part of the Standard Model, but it is thought that there may be particles called gravitons which are the excitations of gravitational waves.

The status of this particle is still tentative, because the theory is incomplete and because the interactions of single gravitons may be too weak to be detected.

The description of forces in terms of virtual particles is limited by the applicability of the perturbation theory from which it is derived.

This effect is only explainable if light can behave as a stream of particles, and it convinced the physics community of the existence of Einstein's light-particle.

Lastly, in 1926, one year before the theory of quantum mechanics was published, Gilbert N. Lewis introduced the term "photon", which soon became the name for Einstein's light particle.