I just finished a neat little paper with John Moffat. Jordan-Brans-Dicke theory is a generalization of relativity theory, in which the gravitational constant becomes variable, and the resulting (scalar) field has kinetic energy. First described by Pascual Jordan in his 1952 book Schwerkraft und Weltall, it was perhaps the first serious, “modern” modification of Einstein’s gravity. Yet it is rejected in the solar system, because it predicts an observable parameter that is grossly out of whack with observation. Then again… in the original form, the scalar field arises as a result of changes in spacetime curvature, only indirectly responding to the presence of matter. If we add matter as a direct source, the picture changes. Even more interesting, if matter and curvature kind of cancel each other out, we get a (still variable, still dynamical) scalar field that’s just like a field in vacuum, and agreement with solar system observations is restored.
Incidentally, Pascual Jordan was one of the greatest 20th century German physicists. Unfortunately he was also an ardent Nazi, member of the NSDAP and an SA volunteer. This may have been the reason why he never received the Nobel prize. Goes to prove that politics and science are really not a very good mix.