  It is a little known fact that while he was developing his theory of general
relativity, Einstein's initial idea was a variable speed of light theory.
Indeed space-time curvature can be mimicked by a speed of light $c(r)$ that
depends on the distribution of masses. Einstein's 1911 theory was considerably
improved by Robert Dicke in 1957, but only recently has the equivalence of the
variable speed of light approach to the conventional formalism been
demonstrated (Broekaert, 2008). Using Green's functions, we show that
Einstein's 1911 idea can be expressed in an analytic form, similar to the
Poisson equation. Using heuristic arguments, we derive a simple formula that
directly relates curvature $w$ to the local speed of light, $w= -c^2 \Delta
\frac{1}{c^2}$. In contrast to the conventional formulation, this allows for a
Machian interpretation of general relativity and the gravitational constant
$G$. Gravity, though described by local equations, has its origin in all other
masses in the universe.
