, and that effectively rules out bimetric theories. Another class of metric theories is the scalar–tensor theories, such as Brans–Dicke theory. For all of these, . The limit of means that would have to be very large, so these theories are looking less and less likely as experimental accuracy improves. The final main class of metric theories is the vector–tensor theories. For all of these the gravitational "constant" varies with time and is non-zero. Lunar laser ranging experiments tightly constrain the variation of the gravitational "constant" with time and , so these theories are also looking unlikely. There are some metric theories of gravity that do not fit into the above categories, but they have similar problems. Accuracy from experimental tests Bounds on the PPN parameters from Will (2006) and Will (2014) † ‡ Based on from Will (1976, 2006). It is theoretically possible for an alternative model of gravity to bypass this bound, in which case the bound is from Ni (1972). See also Alternatives to general relativity#Parametric post-Newtonian parameters for a range of theories Linearized gravity Peskin–Takeuchi parameter The same thing as PPN, but for electroweak theory instead of gravitation Tests of general relativity References Eddington, A. S. (1922) The Mathematical Theory of Relativity, Cambridge University Press. Misner, C. W., Thorne, K. S. & Wheeler, J. A. (1973) Gravitation, W. H. Freeman and Co. Will, C. M. (1981, 1993) Theory and Experiment in Gravitational Physics, Cambridge University Press. . Will, C. M., (2006) The Confrontation between General Relativity and Experiment, https://web.archive.org/web/20070613073754/http://relativity.livingreviews.org/Articles/lrr-2006-3/ Theories of gravity Formalism (deductive) General relativity