Comment on superluminality in general relativity

General relativity provides an appropriate framework for addressing the issue of sub- or superluminality as an apparent effect. It is shown that even though a massless particle travels on the light cone, its average velocity over a finite path measured by different observers is not necessarily equal to the velocity of light, as a consequence of the time dilation or contraction in gravitational fields. Contrary to the naive expectation, this phenomenon occurs in either direction (increase or depletion) according to the location of the observer and irrespective of the details and strength of the gravitational interaction. (paper)