Why is the speed of light constant not say the speed of lead? Why is light the gold standard of movement? The answer is causality. Imagine a rocket going at nearly the speed of light to a planet and then returning to earth. If the rocket’s speed affected the speed of light, a message sent on the journey to the planet might arrive after one sent on the way back. If the rocket exploded after rounding the planet, one might first see the blast then get a message from the crew that all is well, like getting a cheery Facebook message from a person after attending their funeral.
Relativity even tells us that a rocket that left the earth could use faster than light travel to go back in time and return before it left. As Buckley points out, given the options of faster than light travel, relativity and causality, the universe can support two but not all three at once. Faster than light travel would allow the natural causality of events in our world to be breached. But it doesn’t happen because light, as the constant messenger of reality, never gets causality backwards.
One must also conclude that matter moves differently from light. If matter and light moved in the same way, then some particle would be needed to push light to go faster again after it slowed down when it moved through water say. Needless to say, this is not so, and in general, it takes work to move matter but it takes work to stop light moving. If I drive at 100mph and throw a brick forward at 10mph, it goes at about 110mph but if I shine a torch, light goes at exactly the same speed! How does light, and only light, do this?