Before a GPS satellite is launched, its atomic clock is set to the wrong frequency. On purpose. The clock is supposed to tick at 10.23 million times a second. Engineers set it to 10.22999999543 million. That’s slower by about 4.5 parts in ten billion. Without that change, the clock would gain about 39 microseconds a day on a ground clock, enough for light to travel more than 11 kilometers. Here’s why. ...
Why is the speed limit of the universe tied to light and not to gravity or something else? Does light really travel at that limit? And what if the photon turned out to have a tiny mass? These questions intrigue me, and they turn out to share one answer. It starts with an idea: c isn’t really about light. It’s the speed limit on cause and effect, the fastest anything can travel from one event to another and still change what happens there. Light just happens to be able to reach it. I’ll build up to that from something software engineers already know. ...