Instead of imposing the constraint itself on the boundary one might try to set some linear combination of its normal and time derivatives to zero, obtaining a constraint-preserving boundary condition that does not involve zero speed fields. Unfortunately, this trick only seems to work for reflecting boundaries; see [405Jump To The Next Citation Point] and [106Jump To The Next Citation Point] for the case of general relativity. In our example, such boundary conditions are given by ∂tAt = ∂xAx = ∂tAy = ∂tAz = 0, which imply ∂t(∂νA ν) = 0