https://en.wikipedia.org/wiki/Radiation_pressure
Radiation pressure is the
pressure exerted upon any surface exposed to
electromagnetic radiation. Radiation pressure implies an interaction between electromagnetic radiation and bodies of various types, including clouds of particles or gases. The interactions can be
absorption,
reflection, or some of both (the common case). Bodies also emit radiation and thereby experience a resulting pressure.
The forces generated by radiation pressure are generally too small to be detected under everyday circumstances; however, they do play a crucial role in some settings, such as
astronomy and
astrodynamics. For example, had the effects of the sun's radiation pressure on the spacecraft of the
Viking program been ignored, the spacecraft would have missed Mars orbit by about 15,000 kilometers.
[1]
This article addresses the macroscopic aspects of radiation pressure. Detailed quantum mechanical aspects of interactions are addressed in specialized articles on the subject. The details of how photons of various wavelengths interact with atoms can be explored through links in the
See also section.
The microwaves must be interacting with other radiation to create a forward momentum.
http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html
http://hyperphysics.phy-astr.gsu.edu/hbase/mod2.html#c3
There must be a value in the EM Drive that creates a rotating and torsion effect that other radiation interacts with to create forward momentum. Basically the microwave field would be similar to the wheels on a water wheel for a grist mill. When water is added to each bucket the wheel rotates.
The more water that is added to the bucket means more weight is present that turns the wheel. The larger the bucket and the speed of the stream filling the bucket means the buckets are filled at a faster rate causing the wheel to turn much faster. The torque of the wheel turning is then converted into energy that is used to turn the heavy grist stone that crushes the wheat, barely or corn, etc. into a fine powered.
https://en.wikipedia.org/wiki/Microwave
Basically if you have a large barrel, the EM Housing and were to "swish" the water, the microwaves, around the inside of the barrel in a pattern where the most pressure of the microwave swish is exerted on the front inside of the EM Drive housing without interacting with other surfaces that would negate a forward momentum, the EM Drive would show a trust without showing any exhaust.
What also might be happening is that the EM Drive is causing particles to phase in and out of existence causing the electrons or particles at the quantum level to jump to another level that would increase the energy level output.