Background
Albert Einstein was a German-born American physicist who is best known as the creator as the general and special theories of relativity and for his revolutionary hypothesis on the particle nature of light. He earned numerous honors and awards, including the Nobel Prize in physics in 1921. He was not only brilliant but also the most well known scientist of the 21st century. As a child Einstein did not like traditional ways of teaching and so he taught himself much of what he knew. At age twelve he taught himself Euclidian geometry and withdrew from school when he was 15. A year later he enrolled secondary school and then went on to the Swiss National Polytechnic. He disliked the teaching methods there also and frequently cut class to study physics on his own or play his beloved violin. His professors did not think very highly of him and after his graduation in 1900 was not recommended for a university position. These important years in his youth allowed gave him the background knowledge that was necessary in the development of his revolutionary theories.
Einstein's third major paper "On the Electrodynamics of Moving Bodies" which he completed in 1905 contained the concept that became known as the special theory of relativity. This theory answered the questions of many scientists who were trying to understand the nature of matter and radiation and how they interact in the unified world picture. Einstein explained that looking at the world using just mechanical laws or electromagnetic laws would not adequately explain the radiation and matter interact when viewed simultaneously from different inertial frames of reference or in other words by an observer at rest and an observer moving at a uniform speed.
Einstein had considered these problems for over 10 years and finally came to the conclusion that the problem lay not in a theory of matter but in a theory of measurement. He finally came to the realization that all measurements of time and space depend on judgments as to whether two distant events occur simultaneously. After considering all these revelations Einstein based his theory on two key postulates: that physical laws are the same in all inertial reference systems (the principle of relativity), and the principle of the invariance of the speed of light which states that the speed of light in a vacuum is a universal constant. Einstein was able to provide a consistent and correct description of physical events in different inertial frames or reference. The most important part of Einstein's theory was that it made no special assumptions about the nature of matter or radiation, or how they interact. If you are confused by this theories explanation, don't feel bad because practically no one understood Einstein's argument when he first presented it.
After Einstein had completed the work on his Special Theory of Relativity he decided to take a new challenge. He began work in extending and generalizing the theory of relativity to all coordinate systems. To do so, he emphasized the principle of equivalence which states that gravitational fields are equivalent to accelerations of the frame of reference. A good example to put this in perspective is a person in a moving elevator. As the elevator goes down the person can not tell if the force that is acting on them is caused by gravitation or by the acceleration of the elevator. The theory explained circumstances that used to be attributed to gravitational forces. It stated that the interaction of bodies are explained as the influence of bodies of the geometry of space-time. Space-time is a four dimensional space, having three dimensions from Euclidian space (length, width and height) and time as the fourth dimension. Einstein did not publish the general theory of relativity until 1916.
This theory was especially important to astronomers because it accounted for the previously unexplained variations in planetary orbital motion. Many scientists were skeptical of Einstein's latest theory but he found a way to eliminate their uncertainty. Using his theory he predicted that when starlight was in the vicinity of a massive body such as the Sun it would bend. This prediction and his theories were confirmed during a total eclipse of the Sun in 1919. Scientists were able to take photographs that showed this phenomena. News spread of Einstein's feat and he became famous worldwide.
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