  This paper reviews the history and origin of the Abraham-Minkowski
controversy and points out that it is a continuation of the controversy over
the speed of light in medium. Upon considering an aircraft flying at a constant
speed along the great-circle route from the perspective of a geosynchronous
space station, we show that the A-M controversy arises from non-local
observation. The relative motion refractive index and the gravitational field
refractive index are defined by the space-time metric tensor, which reveals the
A-M controversy hidden in special and general relativity. As another example,
we take light propagating over the surface of the sun, and show that Minkowski
and Abraham forces are responsible for the gravitational deflection of light
and the Shapiro time delay, respectively. Overall, we heuristically conclude
that non-local observation is the cause of the A-M controversy.
