stylistic:style_transfer, structural:scholar_edition, quality:correction
In electromagnetism, the impedance of free space, Z0, is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, where |E| is the electric field strength, and |H| is the magnetic field strength. Its presently accepted value is where Ω is the ohm, the SI unit of electrical resistance. The impedance of free space is equal to the product of the vacuum permeability μ0 and the speed of light in vacuum c0. Before 2019, the values of both these constants were taken to be exact, and the value of the impedance of free space was therefore likewise taken to be exact. However, with the redefinition of the SI base units that came into force on 20 May 2019, the impedance of free space is subject to experimental measurement because only the speed of light in vacuum c0 retains an exactly defined value.