COMMENT: Physical versions of nonlinear gravity theories and positivity of energy
Abstract
Brans (1988) has criticized claims that nonlinear metric theories of gravity are physically equivalent to general relativity with additional fields. Conceding that the physical metric is determined by its interaction with matter, it is emphasized that at present the available data are too scarce for it. It is argued that, for the theories which are mathematically equivalent under a conformal rescaling of the metric (this is the case of Lagrangian solely depending on the scalar curvature), a unique choice of physical variables can be based on a theoretical criterion  that of positivity of energy.
 Publication:

Classical and Quantum Gravity
 Pub Date:
 December 1989
 DOI:
 10.1088/02649381/6/12/029
 Bibcode:
 1989CQGra...6.2045S
 Keywords:

 Cosmology;
 Gravitation Theory;
 Relativistic Theory;
 SpaceTime Functions;
 Cauchy Problem;
 Conformal Mapping;
 Theoretical Physics;
 Wave Functions;
 Astrophysics