The problem of gravity propagation has been subject of discussion for quite a
long time: Newton, Laplace and, in relatively more modern times, Eddington
pointed out that, if gravity propagated with finite velocity, planets motion
around the sun would become unstable due to a torque originating from time lag
of the gravitational interactions.
Such an odd behavior can be found also in electromagnetism, when one computes
the propagation of the electric fields generated by a set of uniformly moving
charges. As a matter of fact the Li\'enard-Weichert retarded potential leads to
a formula indistinguishable from the one obtained assuming that the electric
field propagates with infinite velocity. Feyman explanation for this apparent
paradox was based on the fact that uniform motions last indefinitely.
To verify such an explanation, we performed an experiment to measure the
time/space evolution of the electric field generated by an uniformely moving
electron beam. The results we obtain on such a finite lifetime kinematical
state seem compatible with an electric field rigidly carried by the beam
itself.Comment: 23 pages, 15 figure
