Editing Length scale (section)

{{Short description|Particular length or distance determined with the precision of a few orders of magnitude}}
{{References needed|date=September 2020}}

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 cannot affect each other{{citation needed|date=March 2016}}{{clarify|date=May 2016}} and are said to [[coupling (physics)|decouple]].   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]].

In [[quantum mechanics]] the length scale of a given phenomenon is related to its [[de Broglie wavelength]]
{{nowrap|1=''&ell;'' = ''ħ''/''p''}}, where ''ħ'' is the [[reduced Planck constant]] and ''p'' is the momentum that is being probed. In [[relativistic mechanics]] time and length scales are related by the [[speed of light]]. In [[relativistic quantum mechanics]] or [[relativistic quantum field theory]], length scales are related to momentum, time and energy scales through the Planck constant and the speed of light.   Often in [[high energy physics]] [[natural units]] are used where length, time, energy and momentum scales are described in the same units (usually with units of energy such as [[GeV]]).

Length scales are usually the operative scale (or at least one of the scales) in [[dimensional analysis]]. For instance, in [[scattering theory]], the most common quantity to calculate is a [[cross section (physics)|cross section]] which has units of length squared and is measured in [[barn (unit)|barn]]s. The cross section of a given process is usually the square of the length scale.