section_path: Classical field theories/Electromagnetism
section_name: Electromagnetism
paragraph_text: Electromagnetism
  
  In classical electromagnetism, the vacuum of free space, or sometimes just free space or perfect vacuum, is a standard reference medium for electromagnetic effects. Some authors refer to this reference medium as classical vacuum, a terminology intended to separate this concept from QED vacuum or QCD vacuum, where vacuum fluctuations can produce transient virtual particle densities and a relative permittivity and relative permeability that are not identically unity.
  
  In the theory of classical electromagnetism, free space has the following properties:
  
  The vacuum of classical electromagnetism can be viewed as an idealized electromagnetic medium with the constitutive relations in SI units:
  
  relating the electric displacement field D to the electric field E and the magnetic field or H -field H to the magnetic induction or B -field B. Here r is a spatial location and t is time.