Gravity A short Guide of Einstein's Theory
In 1905, Albert Einstein published his famous Special Theory of Relativity and overthrew common sense assumptions about space and time. Relative to the observer, both are altered near the speed of light: distances appear to stretch; clocks tick more slowly.
A decade and a year later, Einstein further challenged conventional wisdom by describing gravity as the warping of spacetime, not a force acting at a distance. Since then, Einstein's revolutionary insights have largely stood the test of time. One by one, his predictions have been borne out by experiment and observation.
But it wasn't until much later that scientists accepted one of the most dramatic ramifications of Einstein's theory of gravitation: the existence of black holes from whose extreme gravity nothing, not even light, can escape. Major advances in computation are only now enabling scientists to simulate how black holes form, evolve, and interact. They're betting on powerful instruments now under construction to confirm that these exotic objects actually exist.
Imagine that the universe has two spatial dimensions instead of three, and
that there are flat creatures living on its surface. Now imagine that the surface
they are living on is subject to deformations, something like a bedsheet. The
creatures living on the bedsheet can only see length and depth, they can only
see within the bedsheet. They cannot even imagine the concept of height.
Now imagine that the bedsheet is draped over a basketball, and the creatures are very small. If the creatures attempt to travel along a straight line within the fabric of the bedsheet they will be deflected by the presence of the basketball. Although on their very small scale the bedsheet appears to be flat, their path through it will be altered by the presence of the basketball, distorting the geometry of their world.
Because of this, if we have three of these creatures travelling along parallel straight lines, and the middle creature's path takes him across the top of the ball, the course of the creatures to his right and left will be deflected inward. Because of curvature effects, these three initially parallel paths will meet. A similar effect can occur in spacetime. If two light rays, initially parallel, pass on either side of a black hole their paths will converge.
Gravity causes spacetime to curve, and this curvature in turn affects the motion of objects in spacetime in much the same way that the curvature of the bedsheet affects the paths of motion of creatures wandering within it.
To understand how these distortions create gravity you need to think of parallel worldlines. These worldlines are not drawn on a flat page, but are drawn on a curved surface. Because the surface is curved the intially parallel lines can be drawn together. What we perceive as gravity is the deflection in the path of worldlines caused by their being traced on a curved surface.