The above-mentioned formulae use the conventions for the definition of the electric and magnetic field used with the SI, which is the most common. However, other conventions with the same physics (i.e. forces on e.g. an electron) are possible and used. In the conventions used with the older CGS-Gaussian units, which are somewhat more common among some theoretical physicists as well as condensed matter experimentalists, one has instead F = q G ( E G + v c × × B G ) , where c is the speed of light. Although this equation looks slightly different, it is equivalent, since one has the following relations: q G = q S I 4 π π ε ε 0 , E G = 4 π π ε ε 0 E S I , B G = 4 π π / μ μ 0 B S I , c = 1 ε ε 0 μ μ 0 . where ε 0 is the vacuum permittivity and μ 0 the vacuum permeability. In practice, the subscripts "G" and "SI" are omitted, and the used convention (and unit) must be determined from context.