A few points: at this stage in our primitive understanding:
(a) Time is an arrow travelling from here to the next stage (the future). This explains causality. For time to travel backwards it could only do so without involving humanity (the killing your father or grandfather shows this to be impossible - no not a paradox). The only way round this is the infinite reality or universe hypothesis (if that is the correct term) but that seems like a fairy-tale-like get out to me.
(b)The velocity of light is NOT constant (only in a vacuum, allegedly and probably right). It varies according to the material it passes through here on earth. Hence the refraction effect as it passes through water, glass etc.
Armchair philosophers are a waste of time and space.
I speak of course as an empiricist or classical/natural scientist - at least that approach yields useful dividends. To me metaphysics has a tendency to mean what you want it to except in it's claim to transend classical physics, chemistry, biology etc.
Regards,
SIQ.