General Relativity is the field equation that Einstein developed to deal with changing frames of reference in the case of gravity. He had already worked with velocity and decided to work on acceleration afterwards. He noticed that gravity and acceleration worked the same, and his field theory equation showed they did exactly that. If you are in an accelerating frame, you cannot tell the difference from gravity, unless you have some visual clue. Artificial gravity can be created in space by creating a rotating hub.

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