Relativistic wave equation for spin-0 particles
(1/c^2 * d^2/dt^2 - nabla^2 + m^2c^2/hbar^2) * psi = 0
Dispersion: omega^2 = c^2*k^2 + (mc^2/hbar)^2
Energy: E = +/- sqrt(p^2c^2 + m^2c^4)
