mass (as a particle label)

A shopping cart is harder to get moving than a skateboard, and once moving it is harder to stop. That resistance to being pushed around is what we mean, in everyday life, by having more mass. Every particle has a fixed amount of this property, and mass is the third of the three basic labels — alongside charge and spin — that say what kind of particle something is.

A particle's mass is its rest mass: the mass it has when sitting still, an unchanging number that is the same for every particle of that type and that does not depend on how fast the particle moves. Because of mass-energy equivalence, this mass is most naturally quoted in energy units; physicists say the electron's mass is about 0.5 MeV, the proton's about 938 MeV, and the top quark's a hefty 173 GeV. Some particles, notably the photon, have exactly zero rest mass, and these always travel at the speed of light. Mass spans an enormous range across the particle zoo, with no understood pattern to it — a puzzle physicists call the mass hierarchy.

Mass matters because it controls almost everything about how a particle behaves: how it moves, whether it can decay into lighter particles, and how much energy is needed to create it in the first place. There is a deep and often-misstated subtlety here. The Higgs field is what gives the fundamental particles their rest masses, but it is not the source of most mass you encounter: the vast majority of a proton's mass — and hence of your body's weight — is the energy of the strong force binding its quarks, not the small Higgs-given masses of the quarks themselves.