Relativity 1: Special Theory of Relativity

This first theory published by Einstein in 1905 showed that nothing can move faster than the speed of light, and that observations of events are relative. One could experience the same event in a different way from another person, depending on where the people are compared to the initial occurrence. The theory deals with time dilation (the clocks tick slower), length contraction (things get shorter) as well as changes in mass and energy of objects travelling at the speed of light.

The special theory of relativity also has laid the basis for the most important theory  Einstein ever put forward. This theory has had a large impact on the ideas people had of the Universe: the General Theory of Relativity.

Nothing can move faster than the speed of light.
The speed of light has a crucial role in the Theory of Relativity, the theory says that nothing can move faster than the speed of light. The strange impacts of this becomes clear with the following examples:

A boy throws a snowball in the direction of a person on a bike, the ball is thrown with a velocity of 20 km/hour, cyclist is riding in the direction of the boy, his speed of 30 km/hour. The cyclist will in that case see the ball coming towards him with a velocity of 50 km/hour.

In case the boy would have thrown the ball to the cyclist, while this one was riding away from the boy, then the ball would hit the cyclist with a velocity of 10 km/hour. All this is in accordance with Newtonian Mechanics, which deals with velocities of everyday magnitude.

Suppose that an electron travels through the Universe at a velocity of 200,000 km/s, and that it sends a flash of light to the Earth with a speed of 300,000 km per second.

The light approaches the Earth in all cases with a speed of 300,000 km per second, regardless the velocity or direction of movement of the electron. As these high velocities (near the speed of light) the Newtonian
Laws are not applicable.

Observations of events are relative
According to the theory of Einstein, all observers will automatically measure identical speeds of light. This has bizarre results, which is mostly explained with the following example (of Einstein himself):

Suppose somebody outside the train standing in a station, sees a flash of light in the middle of a train riding along at the very high speed. As the train is moving, he will see the light a fraction sooner in the back of the train than in the front.

Somebody inside the train sees the same flashlight, however he sees the light travelling at exactly the same speed to the back and the front of the train. The distance of the flash of light to the front and the back are equal, therefor back and front will be lighted-up at the same time.

The conclusion one can derive from this that an event, which is experienced by somebody at the same time, may be experienced by somebody else at different time intervals.