Special Relativity begins with two basic assumptions which are fundamental to all the conclusions that can be drawn from it.
1) The laws of physics are identical in all inertial frames, or equivalently, the outcome of any physical experiment is the same when performed with identical initial conditions relative to any inertial frame.
This assumption is an amazing generalization of the laws of physics. It had been previously assumed that mechanics where all identical in all inertial frames (conditions where the velocity of the reference, such as inside a car or the Earth is constant). However, Einstein assumed that all the laws of physics, thermodynamics, electricity, magnetism, optics, remained the same in all inertial reference frames. As it turns out, this assumption is required for the second assumption to be true.
2) There exists an inertial frame in which light signals in vacuum always travel rectilinearly at constant speed c (2.9979245 × 10^8 m/s), in all directions, independently of the motion of the source.
This assumption has far reaching consequences which fundamentally change our perception of how the universe works. These consequences will be later explained. An idea of what this means can be given in a brief example. Suppose that you are in a car traveling at forty miles per hour relative to the ground. If you were to throw a ball out of the car in front of you at five miles per hour relative to you, the ball would be traveling at forty five miles per hour relative to the ground. This is Galilean relativity and, until Einstein, was believed to be an entirely accurate representation of mechanics.
However, let's say that instead of a ball we have a beam of light which
we shine out of the car. Under Galilean relativity, the speed of the light
relative to the ground would be c plus the speed of the car, c + 40mph.
If we believe Einstein's assumption, then this cannot be true. The car
would read the speed of the light as being c, but the ground would read
it as c too! Special Relativity attempts to explain the consequences of