As the world spins, it warps space and time, creating a gravitational pull into the warped distortion. Earth's gravitational pull attracts all matter toward itself at a constant strength. It's a relatively weak force, so a person can defy it very easily by tossing a ball into the air or standing up. This was first discovered by Einstein, whose simple theory of relativity defined the mechanisms of the universe.
The strength of a planet's gravitational pull depends on the size and density of the planet. Smaller, more gaseous planets have significantly less gravitational pull than a large planet, such as Jupiter. Mars, for example, has 62 percent less gravitational pull than Earth because of its size.
Black holes have an extremely strong gravitational pull; even sunlight can't escape. This gravitational strength is created by the high level of density in the center of the black hole. When a star runs out of nuclear fuel at its core, it no longer has the strength to push mass out against gravity. The star dies but is still rotating enough to warp spacetime, so gravity pulls the surrounding mass inward, creating a small mass with an enormous density.
For the mathematics behind string theory to work, there must be at least 11 dimensions of existence. Humans can perceive three of these dimensions, but according to string theory eight more exist. This is because the tiny strings that are part of everything bend and twist around themselves, creating multiple levels of existence that can't be perceived by a human. Physicists theorize that in some of these dimensions gravity does not exist, and what the world perceives as normal laws of physics don't apply. But as of 2011, physicists haven't discovered any evidence of what types of physical laws govern these dimensions.