- The **proton** is the positively charged particle in the [[Atomic nucleus]]. How many protons an [[Atom]] has *is* what element it is: one proton makes hydrogen, six makes carbon, 92 makes uranium. It carries exactly the opposite [[Electric charge]] of an [[Electron]], which is why ordinary matter is electrically neutral.+ The **proton** is the positively charged particle in the [[Atomic nucleus]]. How many protons an [[Atom]] has *is* what element it is: one proton makes [[hydrogen]], six makes carbon, 92 makes [[uranium]]. It carries exactly the opposite [[Electric charge]] of an [[Electron]], which is why ordinary matter is electrically neutral.- A proton is not fundamental. It is three [[Quark]]s — two up and one down — bound by the strong force.+ A proton is not fundamental. It is three [[Quark]]s — two up and one down — bound by the [[strong force]].- | rest mass | 1.672 621 925 95 × 10⁻²⁷ kg | CODATA 2022 |+ | rest [[mass]] | 1.672 621 925 95 × 10⁻²⁷ kg | CODATA 2022 |
History of Proton
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- For scale, the universe is about 1.4 × 10¹⁰ years old. That second limit is over a trillion trillion times longer.+ For scale, the universe is 13.797 ± 0.023 billion years old (PDG 2024, *Astrophysical Constants*). That second limit is over a trillion trillion times longer.
+ The **proton** is the positively charged particle in the [[Atomic nucleus]]. How many protons an [[Atom]] has *is* what element it is: one proton makes hydrogen, six makes carbon, 92 makes uranium. It carries exactly the opposite [[Electric charge]] of an [[Electron]], which is why ordinary matter is electrically neutral.+ A proton is not fundamental. It is three [[Quark]]s — two up and one down — bound by the strong force.++ ++ ## Properties++ | property | value | note |+ |---|---|---|+ | electric charge | +1 e = +1.602 176 634 × 10⁻¹⁹ C | exact by SI definition since 2019 |+ | rest mass | 1.672 621 925 95 × 10⁻²⁷ kg | CODATA 2022 |+ | rest mass (energy) | 938.272 089 43 MeV | same mass, physicists' units |+ | rest mass (atomic units) | 1.007 276 466 621 u | PDG 2024 |+ | mass vs the electron | 1836.152 673 426 × heavier | CODATA 2022 |+ | rms charge radius | 8.407 5 × 10⁻¹⁶ m = 0.840 75 fm | CODATA 2022 |+ | magnetic moment | 2.792 847 344 6 μ<sub>N</sub> | PDG 2024 |+ | spin | ½ | a fermion |+ | quark content | u u d | PDG 2024 |+ | mean life | no decay ever seen | see below |++ ## Almost all of its mass is not its parts++ Add up the three quarks using the PDG's 2024 masses:++ | piece | mass |+ |---|---|+ | up quark | 2.16 MeV |+ | up quark | 2.16 MeV |+ | down quark | 4.70 MeV |+ | **sum of quarks** | **9.02 MeV** |+ | **the proton** | **938.27 MeV** |++ The quarks account for about **1 %** of the proton. The other 99 % is energy — the motion of the quarks and the gluon field binding them — showing up as mass through E = mc². The proton is mostly not made of stuff. It is made of a knot in a force field.++ ## Does it ever decay?++ Nobody has ever watched a proton fall apart, and people have watched very hard. Detectors full of thousands of tonnes of water sit underground waiting for a single event. The limits, from PDG 2024:++ - **> 9 × 10²⁹ years** for the proton simply vanishing into invisible products.+ - **> 2.4 × 10³⁴ years** for the classic grand-unified prediction p → e⁺ π⁰.++ For scale, the universe is about 1.4 × 10¹⁰ years old. That second limit is over a trillion trillion times longer.++ Proton decay is not ruled out — many grand unified theories require it. It just has not happened where anyone was looking.++ ## Where to read next++ - [[Neutron]] — the proton's neutral partner, and what turns one into the other+ - [[Quark]] — the three pieces inside+ - [[Atomic nucleus]] — how protons and neutrons stick together despite repelling each other+ - [[Isotope]] — same proton count, different neutron count