The year was 1915 when Albert Einstein debuted a theory that would alter the foundations of our understanding of the universe. Enter Einstein's general theory of relativity.
But what is the general theory of relativity and how is it the foundation for all the cosmos? The general theory of relativity is really all about gravity, the force that keeps us stuck to the ground rather than floating up into space. Leading up to Einstein's theory, Newton presented gravity as two masses—a planet and an apple for instance—tugging on each other. Whichever one has the bigger "tug" wins.
But what Einstein imagined was something entirely different. Rather than nearly infinite games of tug-of-war happening throughout the cosmos, Einstein thought of gravity like pool balls sitting atop a bedsheet. The pool balls—planets, stars, asteroids, supernovas and every other thing in the universe—form dimples in the sheet. These "dimples" are gravity.