Neither person who claims to have invented it has committed to an explanation for how it could operate as a thruster or what elements define it, making it hard to say definitively whether a given object is an example of an EmDrive.

In 2016, Harold White's group at NASA observed a small apparent thrust from one such test,[13] however subsequent studies suggested this was a measurement error caused by thermal gradients.

[23][24][25][26][27] Martin Tajmar's group at the Dresden University of Technology started testing prototypes in 2015, and by 2021 concluded that observations of thrust were false positives, reporting in the CEAS Space Journal they had refuted all EmDrive claims by "at least 3 orders of magnitude.

[29] Media outlets were later criticized for misleading claims that a resonant cavity thruster had been "validated by NASA"[30] following White's first tentative test reports in 2014.

[32] In 2006, responding to the New Scientist piece, mathematical physicist John C. Baez at the University of California, Riverside, and Australian science-fiction writer Greg Egan, said the positive results reported by Shawyer were likely misinterpretations of experimental errors.

[33] In 2014, White's first conference paper suggested that resonant cavity thrusters could work by transferring momentum to the "quantum vacuum virtual plasma", a new term he coined.

[36] In May 2018, researchers from the Institute of Aerospace Engineering at Technische Universität Dresden, Germany, concluded that the dominant effect underlying the apparent thrust could be clearly identified as an artifact caused by Earth's magnetic field interacting with power cables in the chamber, a result that other experts agree with.

Especially, Egan said he was "gobsmacked by the level of scientific illiteracy" in the magazine's coverage, alleging that it used "meaningless double-talk" to obfuscate the problem of conservation of momentum.

We should have made more explicit where it apparently contravenes the laws of nature and reported that several physicists declined to comment on the device because they thought it too contentious ...

[29]New Scientist also published a letter from the former technical director of EADS Astrium: I reviewed Roger's work and concluded that both theory and experiment were fatally flawed.

[50][51][52][53][54] In 2016, Fetta announced plans to eventually launch a CubeSat satellite containing a version of the Cannae Drive, which would run for 6 months to observe how it functions in space.

In China, researchers working under Yang at NWPU built a resonant cavity thruster in 2008, and tested it for a number of years.

[18][56][57] At the China Academy of Space Technology, Yue Chen filed several patent applications in 2016 describing various radio frequency (RF) resonant cavity thruster designs.

In an interview on CCTV in September 2017, Chen showed some testing of a flat cylindrical device, corresponding to the patent describing stacked short cavities with internal diaphragms.

[64] Assuming homogeneous electric and magnetic fields, it is impossible for the EmDrive, or any other device, to extract a net momentum transfer from either a classical or quantum vacuum.

However, if these vacuum forces are present, they are expected to be exceptionally tiny based on our current understanding, too small to explain the level of observed thrust.

[73][74] In 2004, Shawyer claimed to have received seven independent positive reviews from experts at BAE Systems, EADS Astrium, Siemens and the IEE.

[75] The technical director of EADS Astrium (Shawyer's former employer) denied this in the strongest terms, stating: I reviewed Roger's work and concluded that both theory and experiment were fatally flawed.

In 2014, the group began testing resonant cavity thrusters, and in November 2016 they published a peer-reviewed paper on this work, in the Journal of Propulsion and Power.

The group announced a plan to upgrade their equipment to higher power levels, and to use a test framework subject to independent verification and validation at one or more major research centers.

[22] This was the first such paper published in a peer-reviewed journal, however the experiment was again criticized for its small dataset and missing details about the experimental setup, which was again not independently validated.

[13][84][85] In July 2015, an aerospace research group at the Dresden University of Technology (TUD) under Martin Tajmar reported results for an evaluation of an RF resonant tapered cavity similar to the EmDrive.

[86] Testing was performed first on a knife-edge beam balance able to detect force at the micronewton level, atop an antivibration granite table at ambient air pressure; then on a torsion pendulum with a force resolution of 0.1 mN, inside a vacuum chamber at ambient air pressure and in a hard vacuum at 400 μPa (4×10−6 mbar).

[89][90][15] They were able to run the experiment and show no thrust in any direction, and to reintroduce the previous sources of experimental error to replicate the earlier results.

[55] In December 2016, Yue Chen told a reporter at China's Science and Technology Daily that his team would test an EmDrive in orbit.

Produced by Rogue Space Systems, BARRY-1's primary objective is to test the company's hardware and software, with a future goal of offering hosted payload services.

The satellite's testing program includes a 60-day resting period post-launch to stabilize its orbit, accounting for initial perturbations such as outgassing.

For Rogue, this mission was mostly characterized as a success, however due to "ongoing power-system issues" the experimental drive provided by IVO was never tested in space.

The largest error source is believed to come from the thermal expansion of the thruster's heat sink; as it expands this would lead to a change in the centre of gravity causing the resonant cavity to move.

The paper includes a graphical chart, but it is based on a priori assumptions about what the shapes of the "impulsive thrust" and "thermal effects" should be, and how those signals will superimpose.