  Maxwell's equations are obeyed in a one-parameter group of isotropic
gravity-free flat space-times whose metric depends upon the value of the group
parameter. An experimental determination of this value has been proposed. If it
is zero, the metric is Minkowski's. If it is non-zero, the metric is not
Poincare invariant and local frequencies of electromagnetic waves change as
they propagate. If the group parameter is positive, velocity independent red
shifts develop and the group parameter plays a role similar to that of Hubble's
constant in determining the relation of these redshifts to propagation
distance. In the resulting space-times, the velocity dependence of Doppler
shifts is a function of propagation distance. If the group parameter and
Hubble's constant have the same order of magnitude, observed frequency shifts
in radiation received from stellar sources can imply source velocities quite
different from those implied in Minkowski space. In these space-times,
electromagnetic waves received from bodies in galactic Kepler orbits undergo
frequency shifts which are indistinguishable from shifts currently attributed
to dark matter and dark energy in Minkowski space, or to a non-Newtonian
physics.
