femtometers

Femtometer, symbol fm, is a unit of length in the metric system equal to 10^-15 meters. It is widely used in nuclear and particle physics to express subatomic distances. The term is also known by the historic name fermi, after Enrico Fermi; while “fermi” remains common in some contexts, “femtometer” is the standard modern term.

In nuclear and hadronic physics, femtometers describe the size of nuclei and nucleons. Nuclear radii are typically a few femtometers; a common empirical relation is R ≈ r0 A^(1/3), with r0 about 1.2 fm. The proton charge radius is about 0.84–0.88 fm, and heavier nuclei have radii toward several fm (for example, uranium-238 has a radius around 7.4 fm).

Measurements of radii and distributions rely on scattering and spectroscopic techniques. Elastic electron scattering probes charge distributions in nuclei and protons; muonic atom spectroscopy and other methods inform matter distributions and form factors. The resulting radii are essential for understanding the strength and range of the strong nuclear force and the spatial structure of matter at subatomic scales.

As an SI-derived non-SI unit, the femtometer remains standard in fields dealing with nuclear and particle scales. It is used alongside other scale descriptors such as form factors and interaction ranges in theoretical models of quantum chromodynamics.