Having covered special relativity in recent weeks, let's now move on to general relativity. Special relativity pertains only to objects moving at constant velocity, whereas general relativity applies to acceleration, with a focus on gravitational acceleration. As noted in this Einstein timeline, special relativity came out in 1905, whereas general relativity did in 1915-16.
The Wikipedia has a nice page on general relativity, including a section explaining the connection between special and general relativity. A key concept from the earlier discussion of special relativity involved frames of reference, how different observers might view the same event. According to the Wikipedia document:
In relativity theory, all events are referred to a reference frame ...all motion is defined and quantified relative to other matter. In the special theory of relativity it is assumed that reference frames can be extended indefinitely in all directions in space and time. The theory of special relativity concerns itself with reference frames that move at a constant velocity with respect to each other (i.e. inertial reference frames), whereas general relativity deals with all frames of reference.
Perhaps the best-known element of general relativity is Einstein's conception of warped gravity.
If you were to find some soft, flat surface in your home such as your bed or a large seat cushion on your couch, and place a heavy object (such as a bowling ball) in the center of it, the surface would depress in the center while remaining flat along its outer perimeter. The resulting downhill curvature toward the center (like a funnel) is a simplified analogy to warped gravity. What makes one object have a tendency to fall toward another, such as you or me toward earth, is an invisible funnel-shaped slide.
A University of Winnipeg webpage provides an excellent visual illustration and discussion of warped gravity.
Warped gravity also explains the orbit of the earth around the sun and what would happen were the sun to disappear. As described in a Perimeter Institute document:
According to Newton’s theory, if the Sun were to suddenly disappear when the Earth was at [a given point in its orbit], the force [of gravitation] would instantly disappear, allowing the Earth to break out of its circular orbit and continue on [a] straight line... Einstein, on the other hand, argued that there is no way the Earth could know that the Sun had disappeared until at least eight minutes after..., eight minutes (or thereabouts) being the time it takes light (or any other signal moving at the speed of light) to travel from the Sun to the Earth. In other words, Einstein reasoned that the Earth should continue on its circular orbit for at least another eight minutes after [the Sun's disappearance] before breaking orbit.
Earlier this year when I was visiting with family in Los Angeles, my mother and I went to the Einstein exhibit at the Skirball Cultural Center. Among the displays were demonstrations of warped gravity.
One final note: Although the name "Einstein" is synonymous in many people's minds with the pinnacle of genius, even geniuses sometimes need help. In formulating general relativity, in fact, Einstein needed to draw upon the mathematical wisdom of Marcel Grossman. According to a Vectorsite document on the web, when it came to the curvature aspect of general relativity:
This was a revelation, but Einstein's intuition wasn't enough, he had to be able to "do the math". He didn't have a precise idea of where to start, so he contacted his friend Marcel Grossman, now a professor of math, who scratched his head for a bit and found what Einstein was looking for, which was a mathematical theory known as "tensor calculus", known in modern times as "differential analysis". Grossman told Einstein it was nasty, too nasty for a physicist to want to tinker with, but there wasn't anything else that would do the job. Einstein struggled with it using his friend's help, and in 1915 he announced the result, his theory of General Relativity, to the world.