 One of the most unusual consequences of the speed of light being a constant for all observers is that the flow of time itself is not immutable. It too must be relative to the inertial frame's motion. Here's a good way to see why this must now be the case. Here's a simple light clock. Light travels between mirrors at the end of a known length. For our purposes, we'll make it a very long length, say, 150,000 kilometers. That way, one round trip will mark one full second. Now if we put the clock into motion with respect to a ground-based observer, we see that the light has to travel further, and because its speed is constant, it will take longer. The moving light clock is therefore running slower than a stationary duplicate. This is called time dilation. It's the geometry of the situation that gives us the conversion factor. If I were on a 10-hour flight from LA to Paris, traveling at 800 kilometers per hour, I would lose just under a nanosecond, way too small to notice. Except for faster-moving spacecraft, it can be quite significant and critically important. For example, orbiting GPS satellites travel at 3,874 meters per second with respect to the Earth. They use cesium atomic clocks to keep time. They measure time to within one second in 1,400,000 years. But these on-board clocks lose 7,214 nanoseconds a day due to time dilation. If on-board clocks aren't corrected regularly based on Lorenz's time dilation formula, the position data they produce would be off by kilometers in less than a day.