I wanted to understand something about Inertial frames especially as they are talked about in SR. It appears that the laws of physics should hold the same in them. Now I understand that inertial frames only involve things moving relative to each other and moving in constant speed so you can never really discern which one is moving. Is this a simple test to verify this? If you drop a ball down straight it will hit directly below it, so this should happen in another inertial frame of reference. Both FOR would not be accelerating. Another way of thinking here is there is a person stationary on the ground and a car moving at a constant 50mph relative to him. He Throws a baseball in his car at 5 mph. Now he should read that the ball is going 5mph but the guy on the ground should read 55mph. Correct? Now how do you test this same principle for light in inertial frames? How do you test the for the person on the ground and the guy in the car measure the same speed for light?