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Eisenhart's theorem and the causal simplicity of Eisenhart's spacetime

We give a causal version of Eisenhart's geodesic characterization of classical mechanics. We emphasize the geometric, coordinate-independent properties needed to express Eisenhart's theorem in light of modern studies on the Bargmann structures (lightlike dimensional reduction, pp-waves). The construction of the space metric, Coriolis 1-form and scalar potential through which the theorem is formulated is shown in detail, and in particular a one-to-one correspondence between Newtonian frames and Abelian connections on suitable lightlike principal bundles is proved. The relation of Eisenhart's theorem in the lightlike case with a Fermat-type principle is pointed out. The operation of lightlike lift is introduced and the existence of minimizers for the classical action is related to the causal simplicity of Eisenhart's spacetime.