the basic assumption on which the whole theory of relativity is based is
that the speed of light is always the same no matter how fast you travel yourself. (as you can not determine your own speed if there isn't any refence point).
Some people argue that this basic assumption follows from a different assumption:
the laws of physics are the same no matter your own speed and derive the speed of light from the electromagnetic theory (maxwell equations)
The result of this statement is that there is need for a transformation from system that are "at" rest to systems which "move". (this is offcourse ambigous as for the moving system they are at rest and the other system moves.)
It goes quite deep to explain the mathematics and how to derive lengthcontraction and time increase but these are the results of moving.
To get to the example of the cars that move towards each other.....
50 km/h to left is one car
60 km/h to the right one car
if you would measure the speed of one car compared to the other the result would be
110 km/h
this is wrong according to the theory of relativity.
it's less then 110. I will not explain how and why but it's a result not many people know
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Flowing with the stream of life
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