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 its role 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 could arrive after one sent coming back. Hence, if the rocket exploded after rounding the planet, one might first see the blast then get a crew message that all is well, like getting a cheery email from a person after attending their funeral. In our world, causality is always maintained because the speed of light is always constant.
In theory, a rocket that left earth faster than light could 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 can’t happen because light, the messenger of causality, doesn’t allow it.
Does light then move 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, and light speeds up when going from water to a vacuum with no push needed.
In general, it takes effort to move matter but it takes effort to slow light down. A brick thrown at 10mph from a car going at 100mph leaves at about 110mph, but light leaves a rocket going at half the speed of light at exactly the speed of light! How then does light, and only light, do this?