Reactionless
Reactionless propulsion refers to hypothetical propulsion methods that produce thrust without expelling reaction mass, thereby seemingly violating Newton's third law and the conservation of momentum. In classical physics, propelling a vehicle in free space requires exchanging momentum with something outside the system, either propellant or an external field. A truly reactionless drive would allow acceleration without propellant or external interaction.
Over the decades, various proposals have claimed to achieve reactionless thrust. The best known is the microwave
The scientific consensus is that there is no validated reactionless propulsion device. Claimed thrusts have generally
Despite the lack of supporting evidence, the idea persists in some fringe research circles and popular culture.