is not helpful for several reasons. The square root of the operators cannot be used as it stands; it would have to be expanded in a power series before the momentum operator, raised to a power in each term, could act on ψ. As a result of the power series, the space and time derivatives are completely asymmetric : infinite-order in space derivatives but only first order in the time derivative, which is inelegant and unwieldy. Again, there is the problem of the non-invariance of the energy operator, equated to the square root which is also not invariant. Another problem, less obvious and more severe, is that it can be shown to be nonlocal and can even violate causality : if the particle is initially localized at a point r 0 so that ψ (r 0, t = 0) is finite and zero elsewhere, then at any later time the equation predicts delocalization ψ (r, t) ≠ 0 everywhere, even for | r | > ct which means the particle could arrive at a point before a pulse of light could. This would have to be remedied by the additional constraint ψ (| r | > ct, t) = 0.