Albert Einstein was born in 1879 and died in 1955. He didn't start talking until he was three, and at age nine he still didn't talk very well. Everyone thought he was retarded. He had trouble in school. However, he got smarter. One of the many impressive things that he did was to invent special relativity and illustrate and explain it with thought (in german "gedanken") experiments. These are idealized experiments in which one thinks of situations that capture the essence of the point trying to be understood.
In 1820, Thomas Young performed an experiment that indicated that light
is composed of waves. However, common sense told the physicists of that day
that every wave needs some sort of medium to wave through.
For example, ocean waves wave through water, and sound waves wave through air
(as well as water and other materials).
Thus, physicists believed that light waves also needed some medium to wave through.
They called this medium the ether.
In 1831 two American scientists, Michelson and Morley, set up an experiment
to detect the ether.
The idea behind their experiment was that as the earth moves through space
it would at times be moving with the ether and at other times against the ether.
Suppose the earth were moving against the ether.
Then the situation would be analogous to moving upstream in a boat.
In that case, anything you threw downstream would move away from you faster than
something that you threw upstream.
Thus, physicists reasoned that as the earth moved through the ether,
the speed at which light moved in one direction would be different
from the speed in another direction.
The experiment of Michelson and Morley was designed to detect this difference in speed,
and thus, confirm the existence of the ether.
However, when performed, the Michelson-Morley experiment detected no variation
in the speed of light.
As a result, scientists gradually discarded the idea of the ether
(since it couldn't be detected), and they began to accept the idea
that the speed of light is the same in all directions.
Thus, Einstein began his theory of special relativity with two assumptions:
1. The Principle of Relativity: One cannot tell by any experiment whether one is at rest or moving uniformly (that is, moving in a straight line with constant velocity). In other words, there is no such thing as absolute rest. All motion or rest is only in relation to other observed objects (i.e. I can consider myself not to be moving with respect to the earth while at the same time I am moving very rapidly with respect to the sun).
2. The Constancy of the Velocity of Light: The speed of light has the same value c with respect to any observer either at rest or moving uniformly.
What Einstein then discovered was that in order for all observers to measure the same velocity for a beam of light, time would have to "flow" differently for different observers, and space would sometimes have to contract.