fractional electric charge

Every charged thing you ever measure in a laboratory carries a whole number of a basic unit of charge: an electron is −1, a proton is +1, an ion might be +2 or −1, but never +0.7 or −1.5. That basic unit is so fixed that physicists treat it as nature's smallest coin. So it came as a real surprise when the theory of quarks demanded that quarks carry charges that are fractions of that coin — thirds of it, in fact.

Quarks split into two camps by charge. The up, charm, and top quarks each carry +2/3 of the basic unit; the down, strange, and bottom quarks each carry −1/3. These thirds are not a sloppy approximation — they are exact in the theory, and they are forced on us by the requirement that the everyday particles built from quarks come out with the whole-number charges we measure. Three quarks of charges +2/3, +2/3, −1/3 give exactly +1, the charge of a proton. A quark and an antiquark of charges +2/3 and +1/3 give 0 or ±1, the charges of mesons.

If quarks have thirds of a charge, why has no experiment ever measured a fraction directly? Because of confinement: quarks are never free, so you only ever measure whole hadrons, whose charges always sum to integers. The fractional charge is still very real — it shows up cleanly in how particles are produced and how often, and the pattern of which hadrons exist makes sense only if the underlying quarks carry exactly these thirds.