Editing Electronvolt (section)

=== Distance ===
In [[particle physics]], a system of natural units in which the speed of light in vacuum ''c'' and the [[Planck constant|reduced Planck constant]] ''ħ'' are dimensionless and equal to unity is widely used: {{nowrap|1=''c'' = ''ħ'' = 1}}. 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 length]]s are often presented using a unit of inverse particle mass.

Outside this system of units, the conversion factors between electronvolt, second, and nanometer are the following:
<math display="block">\hbar = 1.054\ 571\ 817\ 646\times 10^{-34}\ \mathrm{J{\cdot}s} = 6.582\ 119\ 569\ 509\times 10^{-16}\ \mathrm{eV{\cdot}s}.</math>

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 {{nowrap|1=Γ = ''ħ''/''τ''}}. For example, the [[B meson|{{Subatomic particle|B0}} meson]] has a lifetime of 1.530(9)&nbsp;[[picosecond]]s, mean decay length is {{nowrap|1=''cτ'' = {{val|459.7|u=μm}}}}, or a decay width of {{val|4.302|(25)|e=-4|u=eV}}.

Conversely, the tiny meson mass differences responsible for [[Neutral particle oscillation|meson oscillations]] are often expressed in the more convenient inverse picoseconds.

Energy in electronvolts is sometimes expressed through the wavelength of light with photons of the same energy:
<math display="block">\frac{1\; \text{eV}}{hc} = \frac{1.602\ 176\ 634 \times 10^{-19} \; \text{J}}{(6.62\ 607\ 015 \times 10^{-34}\; \text{J} {\cdot} \text{s}) \times (2.99\ 792\ 458 \times 10^{11}\; \text{mm}/\text{s})} \thickapprox 806.55439 \; \text{mm}^{-1}.</math>