In 1905 Einstein published his theory of special relativity. It is called "special" because it does does not account for reference frames which are experiencing acceleration; it therefore does not apply to a frame being pulled by gravity. The guiding principle behind the formulation of special relativity was Einstein's intuition that the laws of physics should be identical for all observers; special relativity leaves out a major class of observers- all the residents of planet Earth. The theory begged to be expanded.
For years Einstein struggled with ways to combine his theories with Issac Newton's concept of gravity. He had many false starts until the day that he had what he called "the happiest moment of my life". His great revelation was deceptively simple; he realized that "if a person falls freely he will not feel his own weight". He further realized that if you fall freely, not only will you not feel your own weight, all effects of gravity will seem to disappear. Imagine this scenario: for some stupid reason, you decide to jump off a cliff. As you are falling, you drop some rocks from your hands. According to Gallileo, they will fall at the same rate as your body; they will therefore, not moving relative to your body, seem to float beside you. From your point of view, there is no way to distinguish between your condition and the (equally treacherous) state of being in the gravity-free vacuum of outer space; the laws of physics are identical, and here we have the beginning of General Relativity, which expands Special Relativity to include the effects of gravity.
With this new development, Einstein began to make some astounding predictions, including the assertion that gravity affects the flow of time.
After working on the problem for some time, he became frustrated with his lack of progress. So, he took some time off from relativity to win the Nobel Prize.
When he returned to the problem, he focused on a gap in his previous reasoning: he had ignored tidal forces. Tidal forces are the name given to the forces that result from differences in the strength of the gravitational forces on an object. He had ignored these forces in his earlier work, but their existence invalidated his theory because they allow someone in free-fall to observe the effects of gravity; if you observe your body to be stretching to enormous length, you can be sure you're being pulled by gravity and not floating in empty space. How could Einstein explain tidal forces? The answer lies in curved space.
Imagine being on the surface of the Earth (that shouldn't be too hard to picture). In each hand, you hold a ball, and, at the same instant, you let them drop. They travel parallel paths to the ground. Now, imagine what would happen if the balls were made of a fantastic substance that can pass right through the solid matter of the planet. The tidal forces acting on them would bring them to collide with one another exactly at the center of gravity: the center of the earth. How can objects travelling parallel paths collide? They are in curved space, where Euclidian geometry is turned on its head and parallel lines always cross. Gravity is not really a force at all, but the effect of the curvature of space. Therefore, Einstein realized, matter causes the space around it to warp and curve; each object in our universe affects the fabric of space-time itself.
Curved space was a critical advancement, and Einstein searched for a mathematical means to express this curvature. He found it in the work of Bernhard Riemann, a mathematician working in the ninteenth century who, decades earlier, had created the mathematical tools to express the curvature of a four-dimensional system, exactly what Einstein needed. After puzzling through Riemann's formulae for months, Einstein finally had what he wanted: a fully realized theory demonstrating the consistency of physical laws. He published his theory of General Relativity in 1915.
His theory made a number of definite predictions; over the course of the next few decades, a number of tests have shown that all these predictions were correct.
General Relativity remains today at the heart of all modern physics.