The speed of light is relative. Wouldn't two observer in different frame of inertial reference will have different result , that is triggered by the light. Say, There is an ground observer and an observer in a moving car, the car headlight will get ON, when it hit a horizontal metal, placed at a certain point on the road. There is an time recorder instrument placed at a specific point on the road, which get activated when the metal is hit by the car or when the car head light get ON. This time instrument would stop when the light from the car headlight reaches the instrument. Both observer will not agree on the distance the light would travel on the road in a given time. That is the car observer will say after a given time the light would reach certain point B on the road, where as ground observer would say the light would reach certain point A on the road in that time. Point B is further than point A. Lets say the time instrument is placed at point A on the road. The time instrument according to car observer would record less time, since the light according to him in that given time would have crossed point A and reached point B. Where as for the ground observer, the light would reach point A in that time. Therefore they will not agree on the time recorded on the instrument. What would be the actual time get recorded in the instrument? Thanks.