In simple terms with an analogy you'll understand... 1. get a buddy with a cap gun 2. stand approx. 1000m apart 3. use a stop watch to time the smoke vs. the sound you hear 4. this is the difference between light (seeing the smoke) and sound (what you hear in your ears) 5. This same principle applies to light but you'd have to use such a massive distance and accurate timer to see the affects. Essentially light travels so fast we're not able to detect the "lag" if you will.