Editing Subatomic particle (section)

=== By mass ===
In [[special relativity]], the [[Mass–energy equivalence|energy of a particle at rest equals its mass times the speed of light squared]], {{nowrap begin}}''E'' = ''mc''<sup>2</sup>{{nowrap end}}. That is, [[mass]] can be expressed in terms of [[energy]] and vice versa. If a particle has a [[frame of reference]] in which it lies [[rest (physics)|at rest]], then it has a positive [[rest mass]] and is referred to as ''massive''.

All composite particles are massive. Baryons (meaning "heavy") tend to have greater mass than mesons (meaning "intermediate"), which in turn tend to be heavier than leptons (meaning "lightweight"), but the heaviest lepton (the [[tau particle]]) is heavier than the two lightest flavours of baryons ([[nucleon]]s). It is also certain that any particle with an [[electric charge]] is massive.

When originally defined in the 1950s, the terms baryons, mesons and leptons referred to masses; however, after the quark model became accepted in the 1970s, it was recognised that baryons are composites of three quarks, mesons are composites of one quark and one antiquark, while leptons are elementary and are defined as the elementary fermions with no color charge.

All [[massless particle]]s (particles whose [[invariant mass]] is zero) are elementary. These include the photon and gluon, although the latter cannot be isolated.