Isaac Newton's 1687 description of gravity was considered scientific law until Einstein's General Theory of Relativity, published more than two centuries later. Newton had explained gravity as a force that instantaneously acts over a distance. The result is a pull between any two objects in the universe. This concept of gravity is most familiar today.
Einstein realized that Newton's theory and his own Special Theory of Relativity could not both be true. According to Newton, for Earth's gravity to immediately affect the Moon, the force must travel some 240,000 miles in an instant--faster than the speed of light. But Einstein's theory stated that nothing can exceed light's "cosmic speed limit." Searching for another way to describe how gravity works, Einstein imagined a universe in which gravity is not a force.
Einstein's Tour de Force
Einstein mathematically showed that objects, such as the Sun and planets, bend "space-time," or the four-dimensional arena in which all things exist. Imagine the depression you make by standing in the middle of a trampoline. Roll a ball across the trampoline's surface, and it is redirected by the "valley" your mass forms.
To Einstein, space-time valleys create the effect of gravity. So, the bowl-shaped warp made by Earth's mass, for example, alters the course of an object, like a satellite, that travels into that warp.