This " is " relativistically invariant, yet this equation alone isn't a sufficient foundation for RQM for a few reasons; one is that negative-energy states are solutions, another is the density ( given below ), and this equation as it stands is only applicable to spinless particles.
12.
Alternatively, since we wish to build a Lagrangian out of relativistically invariant quantities, take the action as proportional to the integral of the Lorentz invariant line element in spacetime, the length of the particle's world line between proper times " ? " 1 and " ? " 2,
