Einstein revised Newton's theory and envisioned gravity as a distortion of space caused by mass. An object that gets within the distortion of space caused by a large mass (like a gravity well) gets attracted to that mass. The larger the mass, the greater the gravity well. Einstein's theory explained everything that Newton's did and more (the odd behavior of Mercury's orbit, the bending of light by gravity). Einstein's theories also lead to explanations of black holes.
Gravity in Sci-Fi
We see depictions of gravity in many sci-fi films from weightlessness to artificial gravity, and we also see gravitational black holes and asteroids with little gravity. Before we examine how gravitational issues are addressed in sci-fi films, let's look at what gravity is. According to Isaac Newton, gravity is an attractive force between any two masses. Newton's law of gravity says that the force of gravity is directly proportional to the sizes of masses (m1, m2) involved and inversely proportional to the square of the distance (r) between the two masses (Specifically, the centers of the masses. He derived this equation: F = Gm1m2/r2, where G is the universal gravitational constant, 6.67 x 10-11 N-m2/kg2.). The force of gravity increases when the masses involved increase and it decreases as the distances between them gets farther apart.
Weightlessness has been depicted in many sci-fi films. In George Pal's classic "Destination Moon," the crew experiences weightlessness and use magnetic boots to attach themselves to the spacecraft's floor and walls. One crewmember even remarks that he can't swallow well without gravity (This is not true because swallowing relies on muscle contractions of the esophagus rather than gravity. You can swallow quite well in weightlessness.). The absence of gravity does not cause weightlessness, as is often thought. Instead, the occupants of the spacecraft are in a state of freefall with the spacecraft itself. Most sci-fi films depict weightlessness by having the actors attached to wires and pulleys during filming. In Ron Howard's film "Apollo 13," the weightless scenes were shot on board NASA's KC-135 "Vomit Comet" aircraft. This plane flew parabolic arcs repeatedly where the occupants (actors, camera operators, director) experienced many brief, 30-second periods of free-fall. Weightlessness causes many adverse effects; short-term effects include nausea and vomiting, while long-term effects include bone loss, muscle atrophy, fluid loss,and anemia (see How Weightlessness Works).
On spaceships, such as "Star Trek"'s Enterprise or Star Wars' "Millennium Falcon," there is some type of artificial gravity field that allows the occupants to experience normal gravity in flight. This is important to counteract the adverse effects of prolonged weightlessness. It is also easier to film a movie without having to make the occupants appear weightless. How these artificial gravity fields are generated are unknown (remember, sci-fi writers are free to extrapolate). Currently, the only known means of producing artificial gravity is spinning the astronauts in a wheel-like environment. Centripetal acceleration towards the center of the wheel produces centripetal force. The reaction to this acceleration (often called centrifugal force) throws the occupants against the wall and feels like gravity (many amusement parks have rides like this). The films "2001: a Space Odyssey, " "2010: The Year We Make Contact," and "Mission to Mars" all depict this type of artificial gravity correctly.
When you apply a force to an object, it accelerates. Newton's Second Law describes this relationship as F = ma, where F is the force, m is the mass of the object and a is the acceleration. In Star Trek and Star Wars, spacecraft often accelerate from rest or sub-light speed to light speed or more in a matter of seconds. The crews of these spacecraft would experience huge forces of acceleration (G-forces), even more than the G-forces experienced by jet fighter pilots when they accelerate and maneuver their aircraft. To compensate for this, Star Trek writers came up with the idea of inertial dampers, which counteract the forces of acceleration. In "The Physics of Star Trek," Lawrence Krauss speculates how these devices might work, but to date no such device exists.