The mass equivalent of is  For example, an electron and a positron, each with a mass of , can annihilate to yield of energy. The proton has a mass of . In general, the masses of all hadrons are of the order of , which makes the GeV (gigaelectronvolt) a convenient unit of mass for particle physics:  The unified atomic mass unit (u), almost exactly 1 gram divided by the Avogadro number, is almost the mass of a hydrogen atom, which is mostly the mass of the proton. To convert to electron volts, use the formula:  In high-energy physics, the electronvolt is often used as a unit of momentum. A potential difference of 1 volt causes an electron to gain an amount of energy (i.e., ). This gives rise to usage of eV (and keV, MeV, GeV or TeV) as units of momentum, for the energy supplied results in acceleration of the particle. The dimensions of momentum units are . The dimensions of energy units are . Then, dividing the units of energy (such as eV) by a fundamental constant that has units of velocity (), facilitates the required conversion of using energy units to describe momentum. In the field of high-energy particle physics, the fundamental velocity unit is the speed of light in vacuum "c". By dividing energy in eV by the speed of light, one can describe the momentum of an electron in units of eV/"c". The fundamental velocity constant "c" is often "dropped" from the units of momentum by way of defining units of length such that the value of "c" is unity. For example, if the momentum "p" of an electron is said to be , then the conversion to MKS can be achieved by:  In particle physics, a system of "natural units" in which the speed of light in vacuum "c" and the reduced Planck constant "ħ" are dimensionless and equal to unity is widely used: . In these units, both distances and times are expressed in inverse energy units (while energy and mass are expressed in the same units, see mass–energy equivalence). In particular, particle scattering lengths are often presented in units of inverse particle masses. Outside this system of units, the conversion factors between electronvolt, second, and nanometer are the following:  The above relations also allow expressing the mean lifetime "τ" of an unstable particle (in seconds) in terms of its decay width "Γ" (in eV) via .