  At high energies relativistic quantum systems describing scalar particles
behave classically. This observation plays an important role in the
investigation of eigenfunctions of the Laplace operator on manifolds for large
energies and allows to establish relations to the dynamics of the corresponding
classical system. Relativistic quantum systems describing particles with spin
such as the Dirac equation do not behave classically at high energies.
Nonetheless, the dynamical properties of the classical frame flow determine the
behavior of eigensections of the corresponding operator for large energies. We
review what a high energy limit is and how it can be described for geometric
operators.
