  If the presence of a gravitational field breaks the Lorentz symmetry valid
for special relativity, an "absolute motion" might be detectable. We summarize
a scalar theory of gravity with a such "ether", which starts from a tentative
interpretation of gravity as a pressure force. The theory also admits that our
physical standards of space and time are affected by gravitation similarly as
they are affected by a uniform motion. General motion is governed by an
extension of Newton's second law to the curved space-time which is thus
obtained. Together with the scalar field equation of the theory, this leads to
a true conservation equation for the total energy. The law of motion also leads
to an alternative 4-component equation governing the dynamics of a continuum in
terms of its energy-momentum tensor. That new equation implies that mass
conservation is obtained as a limiting behaviour for a weak and slowly varying
gravitational field and/or at a low pressure. In the presence of the Lorentz
force field, the new dynamical equation gives the second group of the
gravitationally-modified Maxwell equations in the investigated theory. This is
consistent with the geometrical optics of the theory as governed by the
proposed extension of Newton's second law. The theory has the correct Newtonian
limit; it predicts Schwarzschild's exterior metric of general relativity and
geodesic motion in the static situation with spherical symmetry. A
post-Newtonian approximation of this theory shows that no preferred-frame
effect occurs for photons at the (first) post-Newtonian approximation. It is
argued that the existence of preferred-frame effects in celestial mechanics,
comparable in magnitude with the "relativistic" effects, does not a priori
invalidate the theory.
