The Dean drive is a device intended to be a reactionless thruster that was invented by Norman L. Dean. Dean claimed that it was able to generate a uni-directional force, in violation of Newton's Third Law of Motion. Such a violation is generally considered to be impossible in physics. While it is theoretically possible for a mass that moves in one direction to have its momentum balanced by something other than a reaction mass (e.g. see Nuclear photonic rocket), there is no known theoretical mechanism for a mass to be moved one way while nothing moves the other way, at least in a flat spacetime.
An actual functioning "reactionless thruster" would, of course, have enormous application, completely changing human transport, engineering, space travel and more.
According to Dean, his drive is a reactionless thruster, and his models were able to demonstrate this effect. He received two patents for related devices that are known to be unable to generate a uni-directional force, but he occasionally demonstrated devices that were different. Dean's claims of reactionless thrust generation have subsequently been shown to be in error; the thrust generated is understood to be reliant on friction with the surface on which the device is resting.
Read more about Dean Drive: Early Publicity, Purported Weight Loss, Interested Parties, Further Developments, Similar Work By Others
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