  It is shown without making use of Lorentz transformation that there exists a
phenomenon of relativistic zero-frequency shift in Doppler effect for a plane
wave in free space, observed in two inertial frames of relative motion, and the
zero shift takes place at a maximum aberration of light. When it is applied to
analysis of a moving point light source, two unconventional physical
implications result: (1) a light source, when it is approaching (moving closer
to) the observer, may cause a red shift; (2) a zero-frequency-shift observation
does not necessarily mean that the light source is not moving closer, and in
contrast, the light source may be moving closer to the observer at a high
speed. This fundamental result of special relativity may provide an alternative
way to experimentally examine the principle of relativity, and might have a
significant application in astrophysics.
