  Mach's principle fits into the wider "relational principle", advocating that
not only inertia, but also space and time emerge from the interaction of
matter. Concepts of a Machian/relational theory are proposed, where inertia and
energy are defined as mutual properties between pairs of objects. Due to
Berkeley, only radial motion represents kinetic energy between (point) masses,
which is the basis of anisotropic inertia, which in turn underlies the
relational principle. The Newtonian definition of potential energy is
considered a model for Machian inertia, leading to a frame independent
definition of Machian kinetic energy, which comprises of the Newtonian terms
(relative to the "fixed stars") and small anisotropic Machian energy terms
between objects. The latter account for relativistic trajectories, such as the
anomalous perihelion precession and Lense-Thirring frame dragging. However,
relativistic effects of remote observation (e.g. time dilation) demand an
isotropic model. A relational spacetime metric is derived, which provides an
isotropic coordinate transform of the anisotropic Machian model, yielding a
relational model which matches GR expressions for relativistic trajectories and
effects of remote observation. Therefore, the experimental verification of GR
in these cases holds automatically for the relational model. The relational
model fits the relational principle (including Mach's principle) and it is
argued that it includes GR as a special case. The relational metric provides
both contraction and (unbound) expansion as a function of relative potential,
i.e. without invoking dark energy.
