Point particle in general background fields and generalized equivalence   principle
  The model of point particle in general external fields is considered and the generalized equivalence principle is suggested identifying all backgrounds which give rise to equivalent particle dynamics. The equivalence transformations for external fields are interpreted as gauge ones. The gauge group appears to be a semidirect product of all phase space canonical transformations to an abelian ideal of "hyperWeyl" transformations and includes U(1) and general coordinate symmetries as a subgroup. The implications of this gauge symmetry are considered and a connection of general backgrounds to the infinite collection of Fronsdal gauge fields is studied. Although the result is negative and no direct connection arises, it is discussed how higher spin fields could be found among general external fields if one relaxes somehow the equivalence principle. Besides, the particle action in general backgrounds is shown to reproduce the De Wit-Freedman point particle -- symmetric tensors first order interaction suggested many years ago, and generalizes their result to all orders in interaction. 
A. Yu. Segal