Einstein's theory of general relativity is an extension of his theory of special relativity. Special relativity, which came first in 1905, was the origin of the E=MC^2 equation. Special relativity had many applications throughout physics, astronomy and other fields, and it taught us many things, including the fact that the speed of light in a vacuum is constant throughout the universe, that matter can be exchanged for a great release of energy, that the passage of time one experiences is related to the rate at which one is moving, and that the laws of physics apply equally to all matter, no matter how fast it is moving.

Einstein published his theory of general relativity in 1915 and, in doing so, factored in something that didn't make the cut for his 1905 theory of special relativity: gravity. Previously, gravity had been described by Sir Isaac Newton and other thinkers as a natural force, such as the nuclear force that holds atoms together. Though this theory had enjoyed prominence for centuries, there were problems -- it couldn't always predict real-world events, such as planetary movements, with 100 percent accuracy.

Einstein provided a solution to this problem of gravity. Special relativity already detailed how speed affected time. Through the equivalence principle, general relativity states that gravity is equivalent to acceleration, and therefore gravity affects measurements of time and space. In this, general relativity redefines gravity as warping of space-time and not a mere force. Under Newton's theory, gravity was an invisible attraction between objects with mass. Einstein saw gravity as a simple depression in the topography of the universe. Imagine the universe as the 3-dimensional equivalent of the 2-dimensional surface of an inflatable air mattress. If you set a large object on the mattress, it creates a depression. If there are other, smaller objects on the mattress, they will tend to roll down into the depression created by the heavy object. The heavier the object, the deeper the depression, and the stronger the attraction.

Gravity still gives modern physicists some trouble, since it works on a cosmic scale, while electromagnetism and the nuclear forces work on a much smaller scale. Physicists have such a hard time developing a theory of everything because they have to include gravity, yet they can't express gravity in the same terms as the other forces without violating the known laws of physics.