Algebra of quantum mechanics \textit{via} classical phonons. II: Klein-Gordon dynamics, the Heisenberg formalism, the Dirac canonical commutation rule and the Poincar\'e algebra through the continuous Poisson bracket formalism

In the first part of this series we have shown how the Schrodinger equation for a single particle and the corresponding non relativistic quantum observables can be obtained from a purely classical phonon model through the Newtonian equations of motion.

In this work we focus instead on how the classical Hamiltonian formalism applied to the same phonon system allows to recover the feature of relativistic quantum mechanics for a single spinless particle.

Using the classical nature of the phonon model, we naturally define continuous Poisson brackets between classical observables, which allows to recover the dynamics of such observables, i.e. the Ehrenfest relations associated to real-valued Klein-Gordon fields.

The Poisson brackets also permits to obtain the generic form of constants of motions, thus generalizing the concept of inner products and momentum on Klein-Gordon fields.

We then connect the formalism of real-valued classical functionals with that of hermitian operators and complex-valued wave functions.

This is done through the introduction of a non-local complex-valued change of variables which allows to rewrite the real-valued Klein-Gordon equation in a form akin to the Schrodinger equation, and the classical observables as quantum expectation values.

Then, we show how this change of variables allows to rewrite the classical Poisson brackets as commutators of hermitian operators.

This points out the strict equivalence between the Heisenberg formalism and the formalism of classical Poisson bracket.

Eventually, we illustrate how the Poisson brackets allows to recover the transformations of Poincaré group in 1+1 dimension together with its algebra.

The latter makes the link between the Lorentz invariant inner product of Mostafazadeh and the Casimir invariant associated to the mass of particle.