QR5.2.3 Maintaining Causality

Why is the speed of light constant instead of say, the speed of lead? What makes light the gold standard of movement? The answer lies in 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 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 keeps the causality given 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, the universe can support two but not all three at once. It can’t go faster than light because that would breach the causality of events we observe. It doesn’t happen because light, the constant messenger of causality, never gets it backwards.

It follows that light moves differently from matter. If matter and light moved the same way, then light would need a push to go faster again after it slowed down in water say but needless to say, it isn’t so. Light speeds up when going from water to a vacuum with no push needed. In general, it takes work to move matter but to stop light moving takes work. 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, torchlight goes forward at exactly the speed of light! How does light, and only light, do this?