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Gravity is one of the four fundamental forces of nature, and by far the weakest of the four.
Gravity is the attraction between any two objects that have mass. The strength of gravity depends on the masses of the two objects and the distance between them. Albert Einstein, in his theory of relativity, showed that gravity can actually be considered a curvature in the space-time continuum. Since gravity is thought of as a force, it can also be considered an acceleration. Gravity is the big hurdle to be overcome in space travel.