# Lorentz invariance of the Klein-Gordon equation action

What I will say is not exclusively true for the KG equation, but let's take it as a simple example. When proving the invariance of its action under a Lorentz transformation, it suffices to show that the Lagrangian density is covariant, because the absolute value of the Jacobian determinant is 1, and the integral giving the action is over the whole Minkowski spacetime, so we don't need to worry about the boundary of the region of integration. But the whole proof rests on the assumption that we are integrating over the whole spacetime manifold, thus over the whole time axis. But for ordinary particle action, we can take it to be from $$t_1$$ to $$t_2$$ whatever their values are. And if we did the same in field theory, we will have to start worrying about the change of the boundary of integration after we do a Lorentz transformation, and the usual proof doesn't hold. So, do I have a blind spot here that I cannot see? Am I mistaken in something? What's happening? I would appreciate your help very much!

• What's wrong with the action for finite time intervals transforming under Lorentz boosts? For example, the Hamilton-Jacobi equation tells you that the variation of the action w.r.t. the ending time is equal to minus the Hamiltonian, and you know that the Hamiltonian transforms non-trivially under boosts. Commented Dec 14, 2020 at 5:48