Impedance of free space

In electromagnetism, the impedance of free space, $Z 0$ , is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, $Z_{0}={\frac {|\mathbf {E} |}{|\mathbf {H} |}},$ where $| E |$ is the electric field strength, and $| H |$ is the magnetic field strength. Its presently accepted value is

Z 0 = 376.730 313412 (59) Ω

,

where Ω is the ohm, the SI unit of electrical resistance. The impedance of free space (that is, the wave impedance of a plane wave in free space) is equal to the product of the vacuum permeability $μ 0$ and the speed of light in vacuum $c$ . Before 2019, the values of both these constants were taken to be exact (they were given in the definitions of the ampere and the metre respectively), and the value of the impedance of free space was therefore likewise taken to be exact. However, with the revision of the SI that came into force on 20 May 2019, the impedance of free space as expressed with an SI unit is subject to experimental measurement because while the speed of light in vacuum remains exactly defined, the use of an exact value of $μ 0$ as the definition of the ampere has been replaced by a fixed value for the elementary charge, i.e. the charge on the electron.