how is time relative?
How do you know what time it is? You look at your watch. That shows a fundamental relationship between light and time. It turns out that because light does not travel at an infinite speed you must take what you see and realize that what you are seeing is not happening right “now” but “happened” earlier when the light started out toward your eye. So to know what really happened at any given time you must take what you see and adjust it with a calculation realizing that things that are far away will be seen now from an earlier time than those that are close since the light had shorter distance to cover. Now to do this calculation and determine when something actually happened you need to know the speed that light travels at. But to know how fast light (or any signal) travels at you must be able to know what the time is at two different points in space – the point when the light started out from and the point when it arrived. You need this to calculate the speed by dividing the distance b
