When a sphere is an equipotential surface, it requires zero energy to travel
from one point to the next. The shortest distance to travel from the
sphere's center to its surface is a straight radial line. EMR propagates in
this fashion only because by doing so it goes from a high potential
reservoir to a lower potential sink. Circumstances are in a predisposition
for the EMR to propagate, lowering its energy level in the process. Maybe
because when energy is FORCED to dissipate, it chooses the most economical
path, radially outward on the surface of a sphere. Like a pebble dropped
in a pond disturbing the medium (water), EMR pushes and pulls (warps) space
(the medium) because the energy is FORCED through space, having no other
means to dissipate its energy than radial propagation on the surface of an
expanding sphere. Some EMR is harmless to life (radio waves) and some EMR
cooks flesh (microwaves).
A mass is dropped in a pan of water creating ripples. It then knocks out a
hole in the bottom and drains the pan in a vortex. EMR propagates at first
but after the water drains it's lost its medium (the water). The system has
then deteriorated into a trough of lowest potential.
http://jons-math.bravehost.com/sourcesink.html