Mass is electromagnetic force
It is known that there are two kinds of masses. It is an inertial mass and a gravitational mass. The equivalence principle is that both are the same. The matter that make up are electrons, protons, and neutrons. Neutron is electron + proton, so let's think about electrons and protons. Protons and electrons may not have mass. Something that is perceived as mass creates something, so it may seem that there is mass.
Think about only electrons. When a force is applied to an electron, a magnetic field is generated in the rear. The magnetic field attracts electrons in the opposite direction to the force applied to the electrons. In other words, applying force to an electron generates a magnetic field, which creates resistance. The same thing happens with protons. Resistance is generated in the direction opposite to the applied force when viewed from the whole substance. The generated magnetic field is canceled out by protons and electrons and it does not come out.
Is this resistance similar to mass? Electromagnetic mass is a phenomenon considered at the beginning of the 20th century, but it disappeared from main stream because it can not explain neutrons. However, we now know that neutrons are protons + electrons. It is reasonable to explain mass by electromagnetic force.
The following are my guesses. Electromagnetic mass is the principle of EM drive. EM drive irradiates microwave inside the metal tube, but the metal tube is shaped like a cone cut. Microwave is reflected internally. Thrust is generated toward the large area surface
When microwaves are reflected inside the metal pipe, electromagnetic masses are generated toward the incoming direction. Then it will be in the same state as when a force is applied from the outside with a large face and a small face. Therefore, large surface - small surface force will be generated. When thinking more carefully, when the microwave is hit by an electron, the direction of the electric field is random. The direction of the generated electromagnetic mass should also be random. In order to explain the generation of force by electromagnetic mass, it is necessary to align the electric field on only one side in some way.
In order to align the electric field, it is possible to create a coherent microwave, precisely adjust it and irradiate it to electrons. But this method is not realistic. The other way is to apply a positive bias voltage to the irradiated metal and shake the electric field of the microwave to the minus side. In this case it is possible to add a negative electric field even if the direction of the electric field is different. The thrust can be reversed by changing the polarity of the bias.