Within the Standard Model of particle physics, one finds three almost identical types of particles: the electron, the muon and the tau. Their interactions with the electromagnetic force, the weak nuclear force and the strong nuclear force are exactly the same. In particular, all three have electric charge -1 (which in first-year physics classes we would usually write as “-e”).
They’re not entirely identical, however. For instance, they have different masses.
- Electron: 0.000511 GeV/c2
- Muon: 0.105658 GeV/c2
- Tau: 1.777 GeV/c2
Compare these to a hydrogen atom, which has mass 0.938783 GeV/c2 . (For the definition of “GeV”, which is an amount of energy, click here.) [Specifically, these are their “rest masses”. Rest mass is the type of mass that is intrinsic to objects and does not change with speed; see Chapter 5 of Waves in an Impossible Sea.]
Why does nature have these three similar particles, collectively called the “charged leptons“? We don’t know. But they’re not alone.
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