In physics, length scale is a particular length or distance determined with the precision of at most a few orders of magnitude. The concept of length scale is particularly important because physical phenomena of different length scales are said to decouple[1][2], i.e. they can be separated and studied independently. In other words, the decoupling of different length scales makes it possible to have a self-consistent theory that only describes the relevant length scales for a given problem. Scientific reductionism says that the physical laws on the shortest length scales can be used to derive the effective description at larger length scales.
The idea that one can derive descriptions of physics at different length scales from one another can be quantified with the renormalization group.
Length scales are usually the operative scale (or at least one of the scales) in dimensional analysis.[3] For instance, in scattering theory, the most common quantity to calculate is a cross section which has units of length squared and is measured in barns. The cross section of a given process is usually the square of the length scale.
Examples
The atomic length scale is ℓa ~ 10−10 m and is given by the size of hydrogen atom (i.e., the Bohr radius, approximately 53 pm).
The length scale for the strong interactions (or the one derived from QCD through dimensional transmutation) is around ℓs ~ 10−15 m, and the "radii" of strongly interacting particles (such as the proton) are roughly comparable. This length scale is determined by the range of the Yukawa potential. The lifetimes of strongly interacting particles, such as the rho meson, are given by this length scale divided by the speed of light: 10−23 s.[4] The masses of strongly interacting particles are several times the associated energy scale (500 MeV/c2 to 3000 MeV/c2).
The electroweak length scale is shorter, roughly ℓw ~ 10−18 m and is set by the rest mass of the weak vector bosons, which is roughly 100 GeV/c2.[5] This length scale would be the distance where a Yukawa force is mediated by the weak vector bosons. The magnitude of weak length scale was initially inferred by the Fermi constant measured by neutron and muon decay.
^Zee, Anthony (2010). Quantum field theory in a nutshell (Second ed.). Princeton Oxford: Princeton University Press. p. 360. ISBN978-0-691-14034-6.
^Misner, Charles W.; Thorne, Kip S.; Wheeler, John Archibald; Kaiser, David I. (2017). Gravitation (First Princeton University Press ed.). Princeton Oxford: Princeton University Press. p. 955. ISBN978-0-691-17779-3.
^Zee, A. (2020). "Part IX". Fly by night physics: how physicists use the backs of envelopes. Princeton: Princeton University Press. ISBN978-0-691-18254-4.
^Griffiths, David J. (2007). Introduction to elementary particles. Weinheim: Wiley. pp. xiv. ISBN978-0-471-60386-3.
^Griffiths, David J. (2007). Introduction to elementary particles. Weinheim: Wiley. pp. xiii. ISBN978-0-471-60386-3.
