Why is the speed of light constant instead of say, the speed of lead? What makes light the gold standard of movement? One reason relates to the role of light in causality. Imagine a rocket going to a planet at nearly light speed and then returning to earth. If the rocket’s speed altered the speed of light, a message sent on the way to the planet might arrive after one sent coming 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 keeps the causality implied by light messages in the right order.
It turns out that if a rocket could leave the earth faster than light, it could in theory go back in time to return before it left. As Buckley points out, given faster than light travel, relativity, and causality, a universe can support two but not all three at once. Going faster than light would breach the causality of events we observe, but it doesn’t happen because light, the messenger of causality, never reports events backwards.
This suggests that light moves differently from matter. If matter and light moved the same way, then light would need a push to go faster again when it slowed down in water say, but needless to say, it doesn’t. Light speeds up when going from water to a vacuum with no push needed.
In general, it takes work to move matter, and 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 on a rocket going at half the speed of light, light goes forward at exactly the speed of light! How then does light, and only light, do this?