A thrust from nothing
At NASA's Advanced Propulsion Physics Laboratory, known as Eagleworks, researchers tested a device that appeared to defy a fundamental law of physics. The device, a resonant frequency (RF) cavity thruster, or EM Drive, is a sealed copper cone into which microwaves are fired. According to inventor Roger Shawyer's concept from 2001, the microwaves exert more pressure on the wide end of the cone than the narrow end, producing a net thrust without expelling any propellant. This effect, if real, would violate Newton's third law of motion and the conservation of momentum.
A 2016 paper from the Eagleworks team, led by Harold "Sonny" White, detailed tests conducted in a vacuum at Johnson Space Center. The paper, published in the Journal of Propulsion and Power, reported a consistent thrust-to-power ratio of 1.2 ± 0.1 millinewtons per kilowatt. The researchers tested the device at power levels of 40, 60, and 80 watts and measured forces in the micronewton range—exceedingly small and persistent. The result generated intense debate because it came from a NASA lab, even though the paper itself listed many potential sources of experimental error.
Chasing ghosts in the machine
The scientific community met the Eagleworks results with deep skepticism. The primary criticism centered on experimental artifacts that could mimic a real thrust. The most likely culprit was thermal effects; the device heats up when operating, which could cause minute expansion or interact with residual air molecules in the vacuum chamber, creating false positives.
In 2018 and 2021, a research group at the Dresden University of Technology, led by Martin Tajmar, published a series of papers that systematically addressed the EM Drive claims. They successfully replicated the thrust observed by the NASA team. However, they then demonstrated that this apparent force could be entirely explained by the interaction of the power cables with the Earth's magnetic field. When they improved the shielding and measurement setup, the thrust vanished. Their conclusion stated that all published EM Drive results were false positives, refuting the claims by at least three orders of magnitude. While proponents had offered speculative theories involving quantum vacuum fluctuations, the consensus is that the EM Drive is subtle experimental errors, not new physics.