EmDrive

EmDrive is a proposed propulsion technology that has been described as a potential “reactionless” drive. First brought to public attention in the early 2000s by British researcher Roger Shawyer, it supposedly generates thrust by bouncing microwaves inside a closed, asymmetrical resonant cavity. Proponents contend that the geometry of the cavity creates a net momentum transfer to the cavity walls, enabling thrust without expelling propellant. Various iterations use conical or frustum-shaped cavities with a microwave input feed.

The claimed mechanism rests on the idea that internal photon reflections inside a tapered cavity produce directional forces that do not cancel, effectively producing thrust along the cavity’s axis. In practice, the device requires electrical power to operate, and the amount of thrust reported by proponents is extremely small relative to the input power, making measurement and interpretation challenging. Technical designs have included different cavity shapes and materials, but the central claim remains a controversial departure from conventional propulsion physics.

Experimental evidence has been contentious. Early claims prompted several laboratory tests, including NASA’s Eagleworks laboratory, which reported small thrust signals in some tests but with substantial uncertainty and questions about reproducibility. Independent researchers have pointed to possible sources of error such as thermal expansion, outgassing, electromagnetic interactions, or measurement artifacts. To date, there is no widely accepted, independently replicated demonstration of sustained, propellantless thrust in vacuum in a manner consistent with conservation of momentum.

As a result, EmDrive remains a controversial and unsettled topic. The broader physics and spaceflight communities generally regard it as unproven, with no consensus that it can function as a practical propulsion technology.