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About these units
Femtometer (fm)
A femtometer, equal to 10⁻¹⁵ meters, is the scale at which the structure of atomic nuclei becomes measurable. Also known historically as a "fermi," this unit is used extensively in nuclear physics to describe the radii of protons, neutrons, and nuclei, which typically span 1–10 femtometers. At this scale, the strong nuclear force dominates interactions, and classical intuition breaks down almost entirely—quantum mechanics provides the only meaningful framework. The femtometer also plays a role in high-energy particle experiments, where the wavelengths of probing particles (like high-velocity electrons) may be expressed in femtometer increments. These small wavelengths allow researchers to resolve sub-nuclear structures. While invisible to any optical instrument, distances in the femtometer range can be inferred through scattering experiments, such as those performed in particle accelerators.
Exameter (Em)
An exameter, equal to 10¹⁸ meters, reaches into the interstellar and even intergalactic scale. It is useful for describing the dimensions of star clusters, distances between major astronomical features, or large-scale cosmological structures. Only a handful of physical phenomena require expression in exameters, and even then, astronomers typically prefer parsecs or light-years. The unit sees more use in theoretical physics, cosmology, and data modeling where SI uniformity is necessary. For example, mathematical simulations of the universe's evolution may employ exameters in their internal computations. Although too large for everyday use, the exameter highlights the vastness of the universe and the adaptability of the metric system to describe phenomena across incomprehensible scales.