Anti-gravity is the idea of creating a place or object that is free from the force of gravity. It does not refer to countering
the gravitational force by an opposing force of a different nature, as a helium balloon does; instead, anti-gravity
requires that the fundamental causes of the force of gravity be made either not present or not applicable to the place
or object through some kind of technological intervention. Anti-gravity is a recurring concept in science fiction,
particularly in the context of spacecraft propulsion. The concept was first introduced formally as "Cavorite" in H. G.
Wells' The First Men in the Moon, and has been a favorite item of imaginary technology since that day.

In the first mathematically accurate description of gravity, Newton's law of universal gravitation, gravity was an external
force transmitted by unknown means. However in the early part of the 20th century Newton's model was replaced by
the more general and complete description known as in general relativity. In general relativity, gravity is not a force in
the traditional sense of the word, but the result of the geometry of space itself. These geometrical solutions always
cause attractive "forces". Under general relativity, anti-gravity is highly unlikely, except under contrived circumstances
that are regarded as unlikely or impossible. The term "anti-gravity" is also sometimes used to refer to hypothetical
reactionless propulsion drives based on certain solutions to general relativity, although these do not oppose gravity
as such.

There are numerous newer theories that add onto general relativity or replace it outright, and some of these appear to
allow anti-gravity-like solutions. However, according to the current widely accepted physical theories, verified in
experiments, and according to the major directions of physical research, it is considered highly unlikely that
anti-gravity is possible.


Source: wikipedia
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