# Imposing Klein-Gordon on Dirac Equation

by Sekonda
Tags: dirac, equation, imposing, kleingordon
 P: 209 Hey, My question is on the Dirac equation, I am having a little confusion with the steps that have been taken to get from this form of the Dirac equation: $$i\frac{\partial \psi}{\partial t}=(-i\underline{\alpha}\cdot \underline{\nabla}+\beta m)\psi$$ to $$-\frac{\partial^2 \psi}{\partial t^2}=[-\alpha^{i}\alpha^{j}\nabla^{i}\nabla^{j}-i(\beta\alpha^{i}+\alpha^{i}\beta)m\nabla^{i}+\beta ^{2}m^{2}]\psi$$ I believe we are imposing the Klein-Gordon (maybe not) on the Dirac Equation to determine the conditions required for a free particle description via the Dirac equation, however I cannot see how this is done from those steps above. I'm not exactly sure what these mean ∇^i and alpha's^i... We are told we apply the 'operator' to both sides of the top equation - I'm not sure what operator this is - I'm guessing it's the Klein Gordon operator though. Any help would be appreciated on how to get from equation 1 to equation 2, Thanks, SK