The fundamental physical theory should be nonlinear (to describe interactions) but that leads to problems, infinities in solutions – both for classical and quantum theories (not so toy, able to include gravity). String theory changes the very topology of Feynman diagrams, but still remains just a program, a design on a (complete) theory. On the other hand, Einstein’s research did not stop on Special and General relativities – he also considered (compatible, second order) field equations whose symmetry group united symmetries of both relativity theories. Among such equations (mostly non-Lagrangian) one can find a unique one (no free parameters, D=5) which solutions last forever (no breaks on singularities). Many features of this equation and its solutions are interesting indeed (three linearly unstable polarizations, plus longitudinal one, which add nothing to D-momentum; topological charges and quasi-charges) and give rise to a new interpretation of Quantum Mechanics and new 4th-order Lagrangian gravity.
Ivan L Zhogin