is called the Lorentz factor and c is the speed of light in free space. The equations above are in SI. In CGS these equations can be derived by replacing 1 c 2 with 1 c , and v × × B with 1 c v × × B , except γ γ . Lorentz factor ( γ γ ) is the same in both systems. The inverse transformations are the same except v → − v.