
Originally Posted by
keeseguy
If we were in a bus traveling at c. I sitting in the back turn on my flash light, you would never see the beam, right?
Let's change this to a bus moving at 0.99c, since, as already point out in an earlier post, massive objects cannot travel at c.
For us, traveling in the bus, the light will travel at c relative to the bus. If the bus is 10 m long, it will take ~ 1/30,000,000 of a sec for the light to travel from back to front.
However, for someone standing on the road, this same light travels at c
relative to the road. For him, the light has to "chase after" the front of the bus and will take 1/300,000 of a sec to reach the front of the bus.
The thing to keep in mind is that we are talking about the
same light, just measured by different observers.
This fact leads to to a number of interesting things.
For instance: Let's put our light source in the middle of the bus and shine it in both directions. For anyone in the bus, the light takes an equal amount of time to reach the front and back of the Bus. If we put clocks at the front and back we can use this to synchronize the clocks (imagine each clock is designed to start running from 12:00 once it is hit by the light.).
But from the roadway, it works differently. The light traveling from midpoint to front chases the front of the bus, while the back of the bus rushes towards the light heading towards the back. The result is that the rearward light hits its clock before the forward light hits its clock. This means that the rear clock stats running before the front one does. Since each clock starts running from 12:00, The front clock, once it starts running will always run behind the rear clock. Thus for anyone in the the train, the clocks always read the same time, but for someone on the road, the rear clock runs ahead the front clock. This is known as the Relativity of Simultaneity.
As unnatural as this seems, this is the natural way of things. It only seems unnatural to us because we are used speeds that at very very small when compared to that of light. At these speeds, the difference in comparative clock readings between bus riders and roadway standers is so small it is all but immeasurable. Thus common experience misleads us to think that the idea of simultaneity is universal and is the same for the bus as it is for the road, even though it really isn't. This is usually the biggest stumbling block for people when they first try to understand Relativity; They continue to think in terms of absolute simultaneity and try to apply the idea of "at the same time" when it is just not applicable to the situation.