Evanescent waves
From EM Drive
As proposed by Aero on the NSF forums, the possibility that the thrust is occurring as a result of the generation/leakage of evanescent waves.
Status
No longer considered a viable source of thrust.
Evidence
Initial calculations by Aero were showing an efficiency rating 2-3 times that of a perfect photon drive, which is beneath the observed ratings of Q-thruster test articles.[1]
Utilizing MEEP, the conclusion was that due to rapid dropoff at the frustum surface, evanescent waves were of insufficient magnitude to explain the observed thrust.[2]