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 cavity thruster, often called an EmDrive, which purportedly generates thrust with microwaves in a resonant cavity but has not yielded reproducible, peer‑reviewed results. Other ideas include the Woodward effect (Mach effect), which posits transient mass fluctuations, and more speculative concepts involving interactions with the quantum vacuum or spacetime. None of these proposals has gained broad acceptance in the physics community, and experimental claims remain controversial or unverified.

The scientific consensus is that there is no validated reactionless propulsion device. Claimed thrusts have generally been attributed to conventional, mundane effects such as thermal gradients, gas leaks, electrostatic forces, or measurement artifacts. In the form of robust, replicated evidence, mainstream physics maintains that momentum must be conserved in inertial frames, and no method to bypass this principle has been demonstrated under controlled conditions.

Despite the lack of supporting evidence, the idea persists in some fringe research circles and popular culture. Reactionless propulsion continues to be discussed as a provocative topic about fundamental physics, measurement limits, and the challenges of validating tiny forces in vacuum.