 As mentioned above, in these geometric variables, as in any other canonical formulation of general relativity, one is faced with constraints, which encode the fact that the canonical variables cannot be specified independently. A familiar example of a constraint is Gauss’s law from ordinary electromagnetism, which states that, in the absence of charges, ∇·E(x) − 4πρ = 0 at every point x. It means that the three components of the electric field at every point must be chosen so as to satisfy this constraint, which in turn means that there are only two “true” degrees of freedom possessed by the electric field at any given point in space. (Specifying two components of the electric field at every point dictates the third component.) Thus, not all components of the Maxwell equations propagate the fields in a physical sense. 