What IS the speed of light?
The speed at which light travels from one event in Spacetime to another Event.
An event being a location at a point in time.
The separation between two events, in space and in time is the same in ANY Frame of Reference - it is only the coordinates that change.
Remember: being a location (a point in space) at a moment in time there is no movement, there can be no movement an event is fixed and occurs in every Frame of Reference.
So as light always travels at the same speed between any two events, and being massless nothing can travel faster; for an observer who is travelling with respect to those events to measure that it has travelled faster is only complicating the measurement by introducing the relative velocity of the observer into the measurement; changing the conditions of the measurement, NOT the speed of the light!
So the only valid measurement of the speed of light is that of the distance between events divided by the time between those same stationary events, within a single Frame of Reference.
Length Contraction and Time dilation are merely effects that change the measurements of space AND time to show how the speed of light is not exceeded. The unit sizes are contracted, while the unit quantities are dilated, both by the factor gamma, thus the total magnitude, unit size x unit quantity, remains constant.
So wherever and however the speed of light is measured, the speed of light remains constant.
