What causes gravity?

Though some scientists theorize that gravity is caused by particles called gravitons, most agree that gravity is a product of mass. Everything with mass, from a dust mote to a star, exerts a gravitational pull. The force of the pull, however, increases with mass and proximity to the object. So a small object can only attract another small object that's nearby, but a large object, like a planet or a star, can pull in objects from across a vast distance. Einstein revised this theory further, saying that when a large mass causes a distortion in space, objects that come within that distortion will be attracted to the mass.

Of course, gravity in the movies can be quite different from real-life gravity. Often, weightlessness is depicted in the movies as the absence of gravity; however, the absence of gravity does not actually cause weightlessness. The people inside a spacecraft are actually in a state of what's called "free fall," as is the craft itself. In movies, free fall usually is staged by using wires and pulleys to make the actors seem to float. For Apollo 13 in 1995, the crew actually filmed short bouts of free fall aboard NASA's KC-135 "Vomit Comet" aircraft [source: NASA].

In general, it's easier to film without having to deal with weightlessness, and in real life, too much time spent in micro-gravity conditions can be unhealthy. For both these reasons, sci-fi movies often invent ways to artificially increase or decrease gravity. Spacecraft in both Star Trek and Star Wars movies have gravity fields that can be turned on during flight, and space suits have thrusters to keep astronauts grounded. Some movies, such as 2001: A Space Odyssey and Mission to Mars, depict the use of centrifugal force, which is actually a known way to produce artificial gravityOthers depict future technologies not yet known to us.