Properties of free space

The concept of free space is an abstraction from nature, a baseline or reference state, that is unattainable in practice^{[dubious – discuss]}, like the absolute zero of temperature. It is characterized by the parameter μ₀ known as the permeability of free space or the magnetic constant, by the parameter ε₀, called the permittivity of free space or electric constant, and by the speed of light in vacuum, c, the three being related through Maxwell's equations by:^[1][4][5]

Because of the definitions of the ampere and metre in the SI system of units, μ₀ and c have exact defined values. Based on these values, the parameter ε₀ also has an exact value:

F m⁻¹

H m⁻¹ or N A⁻²

All of these quantitative properties of free space are only the result of the arbitrary definitions of physical units that humans have decided to use to express or measure these properties with. They are not intrinsic parameters of free space, but only a reflection of the units used to express these quantities.

The parameter ε₀ also enters the expression for the fine-structure constant, usually denoted by α, which characterizes the strength of the electromagnetic interaction.

In the reference state of free space, according to Maxwell's equations, electromagnetic waves, such as radio waves and visible light (among other electromagnetic spectrum frequencies) all propagate at the speed of light, c. The electric and magnetic fields in these waves are related by the value of the characteristic impedance of vacuum Z₀, given by:

Ω

In addition, in free space the principle of linear superposition of potentials and fields holds: for example, the electric potential generated by two charges is the simple addition of the potentials generated by each charge in isolation.