  The Maxwell-Proca electrodynamics corresponding to a finite photon mass
causes a substantial change of the Maxwell stress tensor and, under certain
circumstances, may cause the electromagnetic stresses to act effectively as
"negative pressure." The paper describes a model where this negative pressure
imitates gravitational pull and may produce forces comparable to gravity and
even become dominant. The effect is associated with the random magnetic fields
in the galactic disk with a scale exceeding the photon Compton wavelength. The
presence of a weaker regular field does not affect the forces under
consideration. The stresses act predominantly on the interstellar gas and cause
an additional force pulling the gas towards the center and towards the galactic
plane. The stars do not experience any significant direct force but get
involved in this process via a "recycling loop" where rapidly evolving massive
stars are formed from the gas undergoing galactic rotation and then lose their
masses back to the gas within a time shorter than roughly 1/6 of the rotation
period. This makes their dynamics inseparable from that of the rotating gas.
The lighter, slowly evolving stars, as soon as they are formed, lose connection
to the gas and are confined within the galaxy only gravitationally. Numerical
examples based on the parameters of our galaxy reveal both opportunities and
challenges of this model and motivate further analysis. The critical issue is
the plausibility of formation of the irregular magnetic field that would be
force free. Another challenge is developing a predictive model of the evolution
of the gaseous and stellar population of the galaxy under the aforementioned
scenario. It may be interesting to also explore possible broader cosmological
implications of the negative-pressure model.
