In 1915 Einstein broadened his special theory of relativity to include gravity. In general relativity, light always takes the shortest possible route from one point to another.
The equivalence of gravity and acceleration:
Without external clues, it’s impossible to tell if you’re being pulled downward by gravity or accelerating upward; our legs will feel the same pressure; a ball will fall precisely the same way.
The realization that gravity and acceleration are equivalent was a key insight that eventually allowed Einstein to construct his theory of general relativity.
Relativity and gravity:
According to relativity, gravity is not a force; it’s a warping of space-time (which is an amalgam of time and space) that happens in the presence of mass. The warping is analogous to the bending of a rubber sheet when a weight is placed on it.
When starlight passes near a massive body, such as the sun, the shortest route is a curved line that follows the curvature of space-time. Thus, the startlight appears to be coming from a different point than it’s actual origin. The observation of this effect in 1919 convinced physicists that Einstein’s strange theory was right.
If a mass is concentrated enough, the curvature of space-time becomes infinite. This phenomenon is known as a black hole because a light beam that comes too close will never escape.
- Time Magazine, Dec 31, 1999