Three of the four fundamental forces play a significant role in the interactions of elementary particles. A common feature that electromagnetism, the strong force, and the weak force share is that the gauge particles that mediate these forces are all vectors, which means that they carry spin-1. A space-time vector describes the coordinates of a point in three spatial dimensions and time. The Lorentz group (the set of boosts from one inertial reference frame to another together with the set of rotations) acts on a vector in a specified way. A particle is called a vector if it transforms just like a space-time vector does under the action of the Lorentz group.

Consider an electron and its anti-particle, the positron. In a collision, they annihilate to form a virtual boson. A boson is a particle with an integer spin (1,2,3...). If the interaction is electromagnetic in nature...