Editing Physical constant (section)

== Characteristics ==
Physical constants are parameters in a physical theory that cannot be explained by that theory. This may be due to the apparent fundamental nature of the constant or due to limitations in the theory. Consequently, physical constants must be measured experimentally.<ref name=UzanVaryingConstants/>{{rp|9}}

The set of parameters considered physical constants change as physical models change and how fundamental they appear can change. For example, <math>c</math>, the speed of light, was originally considered a property of light, a specific system. The discovery and verification of Maxwell's equations connected the same quantity with an entire system, [[electromagnetism]]. When the theory of [[special relativity]] emerged, the quantity came to be understood as the basis of causality.<ref name=UzanVaryingConstants/> The speed of light is so fundamental it now defines the international [[metre |unit of length]].