Some people think that the reason behind the upper limit of light speed is the inner properties of the matter.
So – called force carriers or messenger particles or intermediate particles are particles that give rise to forces between other particles.
Explanation of limit of the speed of light implies that these partricles move inside material bodies with the same velocity as they move outside material bodies.
Now, let‘s think whether we can send a massless particle back and forth immediatelly. What „immsediatelly“ means? In the physical sense, “instantaneously” means that no processes and variations occurred between these times – even at the microlevel. The dispatch time and the return time must merge together in this instance. After all, if any processes and variations occurred in a body between these times, then by occurring in time, the processes and variations required a specific amount of time. This means that the times are separated by a time interval to which the arbitrarily accurately running clock must react by a change in their readings.
One young man was even awarded (400 K bucks) for the idea that sounds like that!
As I understand, if you measure velocity of light with a lightclock, you will always measure the same velocity, because accumulated time (number of oscillations) only depends on travelled by lightpulse distance L and distance between lightclock‘s mirrors l.
And in simple words all the matter (on microlevel) consists of small lightclocks.