by modifying it so as to take into account the existence of a fundamental rest frame in the universe." In 1985 Rosen tried again to remove singularities and pseudo-tensors from General Relativity. Twice in 1989 with publications in March and November Rosen further developed his concept of elementary particles in a bimetric field of General Relativity. It is found that the BR and GR theories differ in the following cases: propagation of electromagnetic waves the external field of a high density star the behaviour of intense gravitational waves propagating through a strong static gravitational field. The predictions of gravitational radiation in Rosen's theory have been shown since 1992 to be in conflict with observations of the Hulse–Taylor binary pulsar. Massive bigravity Since 2010 there has been renewed interest in bigravity after the development by Claudia de Rham, Gregory Gabadadze, and Andrew Tolley (dRGT) of a healthy theory of massive gravity. Massive gravity is a bimetric theory in the sense that nontrivial interaction terms for the metric can only be written down with the help of a second metric, as the only nonderivative term that can be written using one metric is a cosmological constant. In the dRGT theory, a nondynamical "reference metric" is introduced, and the interaction terms are built out of the matrix square root of . In dRGT massive gravity, the reference metric must be specified by hand. One can give the reference metric an Einstein–Hilbert term, in which case is not chosen but instead evolves dynamically in response to and possibly matter. This massive bigravity was introduced by Fawad Hassan and Rachel Rosen as an extension of dRGT massive gravity. The dRGT theory is crucial to developing a theory with two dynamical metrics because general bimetric theories are plagued by the Boulware–Deser ghost, a possible sixth polarization for a massive graviton. The dRGT potential is constructed specifically to render this ghost nondynamical, and as long as the kinetic term for the second metric is of the Einstein–Hilbert form, the resulting theory remains ghost-free. The action for the ghost-free massive bigravity is given by As in standard general relativity, the metric has an Einstein–Hilbert kinetic term proportional to the Ricci scalar and a minimal coupling to the matter Lagrangian , with representing all of the matter fields, such as those of the Standard Model. An Einstein–Hilbert term is also given for . Each metric has its own Planck