In 1905, the great physicist, Albert Einstein, put forward
a new theory called "The Special Theory of Relativity".
It is very simple and it says only these two principles
1. The laws of physics are
the same in all inertial system.
2. There is a speed limit, and it is the speed
of light.
Suppose two inertial systems, one of which, S, is the
rest system and another, S', is moving at the velocity v to S.
images from http://casa.colorado.edu/~ajsh/sr/time.html
Images of relative motion
In above pictures, red and blue represent
S and S', white balls moving are light, green boards are mirrors.
Since
the speed of light is constant,
(ct)2
= (vt)2 + L2
(1)
ct' = L
(2)
where
t is the time on S, t' is the time on S', c is the velocity of light,
L is the distance between the red ball and
the mirror. Calculating
this phenomenon mathematically, the following
equation about the time is derived.
dt' =(1- (v/c)2 )1/2
·dt
(3)
This equation says that the
speeds of the time are different if the systems are not the same.