How light get its speed ???

The answer for this question is only three levels above the basic fundamental dynamic level of physical reality. It requires an explanation of a form of basic Timing to regulate speed of intrinsic energy to C and a general (except for the relative varying local parameters) universal linear propagation at speed C.

A photon of light is an intensification in one direction of the intrinsic energy composing a virtual particle.

If the OP or any other person has a further interest in this subject, then the downloading of my 36 page e-pub book titled "Explaining the Fundamental Dynamic Nature of Gravity" from http://Lulu.com , attempts to supply all relevant physical details, beginning with Gravity and continuing on to the nucleus of atoms and gravity and Gravitational Thermodynamic Effect or GTE of our Galaxy. There is no spooky Black Hole at the central volume of our Galaxy. The GTE supplies the physics that now is supposed to result from the presence of a Black Hole.

Mainstream science does humanity a disservice by ignoring the GTE because of its ability to provide a cardinal reason as to Earth’s climate changing cycles, Also more culpable because of the dangerous thermal conditions to life forms, due to a close bypass of Earth by a object from outer space, with a diameter much like that of our moon. The close capture of our Moon would have caused a major extinction