From Centauri Dreams:
" In this case, the work goes toward a so-called Mach Effect Thruster (MET). Mach effects are transient variations in the rest masses of objects as predicted by standard physics where Mach’s principle applies. Proponents believe they offer the possibility of producing thrust without the ejection of propellant, as discussed in James Woodward’s Making Starships and Stargates: The Science of Interstellar Transport and Absurdly Benign Wormholes (Springer-Verlag, 2012).Here's the book mentioned.
What Fearn proposes is to investigate such thrusters by continuing the development of laboratory-scale devices while designing and developing power supply and electrical systems that will determine the efficiency of the Mach Effect Thruster. The analytical task is to improve theoretical thrust predictions and build a reliable model of the device. At the theoretical level, this team is definitely talking deep space, with part of the proposal being to:
'Predict maximum thrust achievable by one device and how large an array of thrusters would be required to send a probe, of size 1.5m diameter by 3m, of total mass 1,245 Kg including a modest 400kg of payload, a distance of 8 light years (ly) away.'"
I downloaded the book-sample to my Kindle app and so far it's both well-written and interesting. Unlike the 'EM Drive', which was widely criticised and seems to violate conservation of momentum, the Mach Effect appears to be a valid consequence of General Relativity when combined with Mach's principle - at least, no-one so far has come forward with a convincing theoretical refutation.
Experimental effects so far appear to be small (if they exist at all) and unproven, but NASA evidently considers there could be something to it.
I'll probably read the book a little later. At least Jerry Pournelle will be pleased.