IMO EMDrive is device, where high-dimensional aspects of space-time manifest
itself. The high-dimensional phenomena can be observed/described from multiple
observational perspectives of 4D space-time, therefore for description of
EMDrive multiple theories may apply and these theories would be related each
other.
From perspective of the water-surface
analogy of the EMDrive IMO the most relevant is the Annilla's
theory, which considers the partial materialization of photons
of the opposite spin under formation of lightweight "dark matter" particles -
scalar waves, which are ejected from EMDrive as a reactive effect. The indicia
of such stream of particle were given with Juday-White "warp field"
interferometer. This theory is based on momentum balance, Newton laws and
Noether theorems - the scalar waves escaping from EMDrive generate the thrust
here.
In dense aether model the spontaneous change of the mass must be always
balanced with some radiation, because it's sorta matter wave refraction effect.
But if we neglect it, we could achieve the thrust simply with considering the
change of the mass of photons along EMDrive resonator, which would be followed
with change of wavelength. This is the group of more simplistic models supported
with Shawyer and McCulloch. McCulloch theory achieves the change of inertia with
shielding of Unruh radiation, which
would have similar effects, like the emanation of scalar waves and it should be
also detectable with Juday-White interferometer. So far so good...
In Annila's theory the photon must be of the opposite spin for being able to
interact itself. IMO they must have opposite orbital angular momentum, which
introduces additional energy into photons, which travels with reference frame of
photon and which increases their mass, not just momentum. The photons can gain
such a mass by their
polarization, but this effect is quite
minute, in addition, it still applies to photons which aren't connected with
reference frame of EMDrive, as they're flying inside it freely. But recently it
has been found, that the effect of polarization to the mass of photons can be
heavily pronounced, if we apply it to photons
which are already coupled to metal surface in form of surface plasmons.
And this is just the point, in which Shawyer's theory gets common ground with
Mach effect based theory. The Mach effect based theory is coined with Prof.
Woodward and others and it proposes, not the photons, but the EMDrive directly
gains mass by periodic introduction of energy into one end. The coupling of
microwave photons with EMDrive surface would provides the basis for this effect,
but it also means, that the internal surface, not the bulk volume are
significant for this effect. In this respect it may be significant, that Cannae
drive of de Fetta increases the effectiveness of thrust with elimination the length and bulk
volume of resonator on behalf of its internal surface.
The third group of theories could therefore consider the EMDrive as the
elongated AC version of Biefeld-Brown capacitor, which is completely flat, it's
just asymmetric in the sense of electric field intensity. Note that later
versions of Woodward drive also utilized the charged capacitors powered with AC
current in the role of the device, which is changing its mass periodically, yet
they applied the Mach theory for it. In dense aether model the principle of
their function is somewhat different though, as the charged capacitor hosts the
Dirac fermions, which interact with ZPE fluctuations like the paddle. You may
see the connection point with McCulloch's shielding theory here again, because
paddle must also serve as a shielding element and it will eject the stream of
particles in opposite direction.
The boundary between both models probably represents the length of resonator:
once it gets smaller than the wavelenght of CMBR, then the asymmetric capacitor
"paddle" model applies - otherwise the reactive "scalar wave rocket" models
become more relevant. The Mach effect based models fall somehow between both
these two boundary cathegories - but we should realize, that both rocket, both
paddle model require also changing mass in this way or another. Whereas for me
it's not difficult to imagine all of it together, for deterministically thinking
physicists (which indeed favor well defined models) the EMDrive will be probably
remain source of conceptual confusion in similar way, like the cold fusion
and/or overunity effects.