Conservation of Momentum is not a singular process and to accept it in the EM drive context consider:1. There is indeed a measurable gravitational distortion which occurs due to the high EM field strength and density. (>2.4x10^7kV/m in recent spehrical/parabolic endplate sims!) E=mc^2 is not the entire story here, "energy density carried by an electromagnetic field can be computed by adding the square of the electric field intensity to the square of the magnetic field intensity." This is in addition to the base gravitational effects of the electrons and photons so you can see that increasing EM field strength and density can start to greatly increase the effect. (pop sci article expanding on this: https://www.scientificamerican.com/article/do-electric-charges-and-m/ ) Though not a primary effect, it certainly has some effects on CoM, definitely pulling the center of mass forward. Gravity talk has been inconclusive in the threads so far but it cannot be ignored.2. Side wall currents carry away part of the energy/momentum which photons impart to the upper end of the cavity. Or rather they would if they were not mostly closed due to strong magnetic fields. The resistance to these currents eventually converts this energy partially to kinetic energy which then leaves the outside of the frustum.3. Resonant cavities form local extrema in EM field densities as a result of modes. The location of the peak energy density is not due to any absorption. Quote from Dr. Rodal: "See the excellent discussion by Greg Eganfor an exact solution. The location of the peak in energy density is due to the mode shape, and the mode shapes are a result of the eigenvalue problem which is dictated by the equations of motion and the boundary conditions, and not by dissipation.Here you have the first 3 TE mode shapes in an electromagnetically resonant truncated cone cavity.There is no change whatsoever in absorption at each end for these modes. The modes are a result of the eigenvalue problem. That's why they are called eigenmodes !"In other words, absorption in a frustum with identical wall material does not improve over time. What does change is:i. Radiation pressure as it "locks in"ii. Energy escaping the cavity as heat (and depending on your preference, also in the form of gravitational potential, the Finnish phase overlapped photons, magnetic fields, copper ions, gas leakage and side wall currents(somehow))This is not a new observation really and it has been bounced around (pardon the pun) extensively for a while.4. Quote from Meems: "Why doesn't a 100 Watt light torch give a reaction force ( kick back ) similar to a mechanical device with a power of 100 Watts?An example of mechanical 100 Watt thruster is : A man throwing 2kg mass projectiles at 10 m/s once per second, every second. Such action would be plenty to accelerate the man in the opposite direction of his projectiles at m/s.But the kickback from 100 W light torch would be effectively zero by comparison.Why? The answer is : only a tiny fraction of the energy in light is as mechanical energy \ momentum. Most of it is stored in a EM wave which is lateral to its motion. In other words, a photon is like a thrown grenade, its kinetic energy is insignificant in comparison to its stored potential energy."In the upper cavity we are seeing resonant mode locked EM waves with constant incidence angles hitting copper whose permittivity will change but not change significantly in a short time and over a short distance. Its relative permeability of around 1 also prevents efficient and timely internal redistribution. We know from Lenz's law that the difference in energy absorbed by electrons in the skin of the side walls along its length will balance as retarded potential. Put simply, this potential borrows off of the EM field momentum from which the energy propagated. However, it is not able to borrow equal amounts of momentum since the lower cavity EM fields are (should be) weaker. To continue meem's metaphor our side wall electrons have suddenly pulled the pin on the grenade. As an aside, Evanescent waves, though interesting, likely do not carry momentum en masse to the lower cavity either.Now synthesis of 1, 2, 3, 4: we have energy which is stuck in the upper part of the cavity and desperately wants to balance out with the bottom part of the cavity but cannot since the EM fields keep "pulling" it back up through the above reaction mechanisms. Incidence angles - which determine wave behaviour greatly https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-013-electromagnetics-and-applications-spring-2009/readings/MIT6_013S09_chap09.pdf - also happen to be facilitating for the upper cavity absorbing more momentum. Dielectrics are added to the weak EM field end to further this imbalance.The concern is how does the energy leave? Is the heat dissipation near the maxima alone enough to balance the imbalance and preserve CoM? Yes. Yes it is. The energy does not have to leave the sidewall/cavity system instantaneously. As we see with slowly decaying experimental thrust measurements over time, when the side wall electrons and internal atmo/electrons are fully excited they will reach system capacity and start to convert more to heat, this conversion does not occur at the same speed as the introduction of more EM waves. What we have is not a flying microwave oven but rather a conversion of photonic momentum into thrust by means of voltage and locked magnetic dipoles at a rate which exceeds the capacity for the sidewalls to dissipate the energy to the outside environment.Focus on the sims and experimental results, we see this quite clearly in the higher thrust/field strength designs.Momentum is conserved but with our low power and short run times we do not easily see it yet. With higher energies more heat and electromechanical stress from the radiation pressure will become apparent and CoE/CoM (two sides of the same coin in this case) will be easily visible to the naked eye. Just to hammer home the point: it is not a perfectly collimated rocket, or a microwave oven. It has a higher thrust because our engineers have pulled the pin on a grenade as yet mostly underutilized.