1. Relativity
H2: Concepts and postulates of special relativity

2. Inertial frames of reference
Inertial reference frames are reference frames in which Newton’s laws hold
From Newton’s 1st law:
If an object experiences no net force, the object either remains at rest or continues with constant speed in a straight line
The Earth is not an inertial frame (it rotates) but is close enough to be considered an inertial frame

3. Inertial frames of reference
Inertia is a property of matter
It is that property of matter which opposes changes in velocity
So, one must centre one's physics reasoning on these thoughts:
An object's velocity will not change all on its own.
Pushes, or pulls, are necessary to change an object's velocity.

4. Inertial frames of reference
There are several ways to describe an inertial frame. Here are a few descriptions:
An inertial frame of reference is a frame of reference with constant velocity.
An inertial frame of reference is a non-accelerating frame of reference.
An inertial frame of reference is a frame of reference in which the law of inertia holds.
An inertial frame of reference is a frame of reference in which Newton's laws of motion hold.
In an inertial frame of reference no fictitious forces arise.

5. Postulates of the Special Theory of Relativity
All motion is relative, not to any stationary hitching post in the universe, but to arbitrary frames of reference.
A familiar experience to a passenger on a train who looks out his window and sees the train on the next track moving by his window.
The important point: If you were in a train with no windows, there would be no way to determine whether the train was moving with uniform velocity or was at rest.

6. First of Einstein 's postulates of the special theory of relativity:
All laws of nature are the same in all uniformly moving frames of reference.
Any number of experiments can be devised to detect accelerated motion, but none can be devised, according to Einstein, to detect a state of uniform motion.
Therefore absolute motion has no meaning.
No experiment, mechanical or electrical or optical, has ever revealed absolute motion.
That is what the first postulate of relativity means.

7. Second postulate in his special theory of relativity:
The speed of light in free space has the same measured value for all observers, regardless of the motion of the source or the motion of the observer; that is, the speed of light is a constant.

9. To illustrate this statement, consider a rocket ship departing from the space station
A flash of light traveling at 300,000 kilometers per second, or c, is emitted from the station.
An observer in the rocket sees the flash of light pass her at the same speed c
If a flash is sent to the station from the moving rocket, observers on the station will measure the speed of the flash to be c.
All observers who measure the speed of light will find it has the same value c.

10. Simultaneity:
An interesting consequence of Einstein's second postulate occurs with the concept of simultaneity.
Two events are simultaneous if they occur at the same time.
Consider, for example, a light source in the exact center of the compartment of a rocket ship.

11. From the point of view of the observer who travels with the compartment, light from the source travels equal distances to both ends of the compartment and therefore strikes both ends simultaneously.

12. The events of light striking the front and back of the compartment are not simultaneous from the point of view of an observer in a different frame of reference. Because of the ship’s motion, light that strikes the back of the compartment doesn’t have as far to go and strikes sooner than light strikes the front of the compartment