Physicists measure mass, heat and light with one tiny unit of energy
An electronvolt is the energy one electron picks up crossing a single volt, a sliver of about 1.6×10^-19 joules. Yet particle physicists use it to weigh a proton, plasma researchers to state temperatures of millions of kelvin, and optics labs to describe a photon of green laser light, which carries about 2.33 of them.
The unit was born in accelerator labs, where a charged particle gains energy equal to its charge times the voltage it falls through. Because an electron's charge in coulombs matches one electronvolt in joules, and the 2019 SI revision fixed that charge exactly, the electronvolt now has an exact joule value. It is not itself an SI unit, but it pervades atomic, nuclear, particle and solid-state physics, stretched with prefixes from milli to quetta. Older papers sometimes wrote BeV, the B for billion, a label that survives in the name of the Bevatron accelerator.
Einstein's mass-energy equivalence turns it into a unit of mass. An electron weighs 0.511 MeV divided by the speed of light squared, so an electron meeting a positron can annihilate into 1.022 MeV of energy. A proton comes to 0.938 GeV, and since most particles made of quarks weigh about a GeV, that figure makes a handy yardstick. Physicists often simply drop the factor of c and quote masses in electronvolts.
Temperature works the same way through the Boltzmann constant: one electronvolt corresponds to about 11,605 kelvin. A magnetic-confinement fusion plasma at 15 keV is therefore roughly 174 million kelvin, while the thermal energy of a room at 20 °C is only about 0.025 eV.
For light, energy and wavelength trade off directly. One electronvolt matches an infrared photon of about 1,240 nanometres, and shorter wavelengths carry proportionally more. The same arithmetic lets physicists express the lifetime of an unstable particle as an energy width; the B0 meson, living about 1.53 picoseconds, has a decay width of roughly 0.0004 eV.
Source: Electronvolt
