by Guest writer Aaron Anderson, I Think Too Much
Imagine that you are standing on the ground and you throw a ball and at the same time someone drives by you in a car at a reasonable speed and throws a ball at the same time. From where you are standing, you will see their ball going much faster than your ball because theirs had the added speed from the car.
In the pre-relativity way of thinking about Physics the same is true of light. If you were standing there holding a
flash light and someone drove by you with their head lights on the light coming out of your flash light will be moving slower than the light coming out of their head lights because they are moving faster than you. However, once physicists actually started measuring the speed of light they discovered something odd. Regardless of how fast the object in question was moving, the light that it gave off was always moving at the same speed.
Relativity is the answer that physicists came up with when they asked why this is the case. They figured that if the speed of light isn't being distorted then time must be the thing that is being distorted and everything just goes from there.
So there you go. Now you know where relativity comes from.