Prior to Einstein, the laws of Isaac Newton were used to understand the physics of motion.
In 1687, Newton wrote that gravity affects everything in the universe. The same force of gravity that pulled an apple down from a tree kept the Earth in motion around the sun. But Newton never puzzled out the source of gravity.
Philosopher David Hume’s 1738 “A Treatise of Human Nature” was a big influence on Einstein’s thinking about space and time. Hume was an empiricist and skeptic, believing that scientific concepts must be based on experience and evidence, not reason alone. He also held that time did not exist separately from the movement of objects.
“It is very well possible that without these philosophical studies I would not have arrived at the solution,” Einstein wrote.
In 1905, Albert Einstein based a new theory on two principles. First, the laws of physics appear the same to all observers.
Second, he calculated that the speed of light – 186,000 miles per second (299,338 kilometers per second) – is unchanging. Prior to Einstein, scientists believed that space was filled with luminiferous aether that would cause the speed of light to change depending on the relative motion of the source and the observer.
As a result of these principles, Einstein deduced that there is no fixed frame of reference in the universe.
Everything is moving relative to everything else, hence Einstein’s theory of relativity.
Source SPACE.com: All about our solar system, outer space and exploration
Source: http://www.space.com/28738-einstein-theory-of-relativity-explained-infgraphic.html
