Em Drive - Principle of Operation

Principle of Operation

The device exploits an idea first suggested by Allen Cullen in the 1950s, an electrical engineer then at University College London, that involves forces created by reflecting microwaves between opposite walls of a cavity. Cullen published a number of articles on the topic of microwave radiation pressure in cavities during the 1950s. The idea is to try to design a cavity in such a manner that forces on one side are greater than the other.

The drive comprises a resonant cavity flooded with microwave radiation. The radiation exerts radiation pressure on the walls of the cavity, and normal Newtonian mechanics would of course indicate that, no matter what shape the cavity is, the forces exerted upon it from within must balance to zero.

However, Shawyer claims that relativistic effects cause a cavity shaped like a truncated cone to experience a larger force against the large end than the small end, due to the group velocity of the wave changing as the local diameter of the cavity varies.

The increased confinement of the tapered end of the cone leads to a higher effective propagation constant (phase velocity). It also leads to local reflections which account for the apparent force imbalance when considering only the end walls. However, since it is the phase of the light rather than the actual photons bouncing off the walls, each force acts quasi independently from another—much like in a ring laser gyroscope where the beams act as if having an external frame of reference (which they have, since the speed of light is constant). The same principle applies to the EmDrive.

No microwaves or anything else are allowed to leave the device. Since nothing leaves the drive for propulsive purposes an EmDrive can be classed as a reactionless drive. The principle by which the EmDrive is supposed to operate seems to violate conservation of momentum. However, the emdrive website claims that the device is not reactionless because the force is created by a "reaction between the end plates of the waveguide and the Electromagnetic wave propagated within it." It is known that the physics equations describing microwaves, Maxwell's equations, conserve momentum, and this would seem to cast doubt on Shawyer's derivation of a thrust effect. In his paper, Shawyer thus takes the following view: any thrust extracted from the device is directly withdrawn from the energy stored in the cavity (due to the Q reflections an average wave encounters when inserted into the cavity, the energy levels quickly build up). In other words: the apparent force on the wider diameter of the cone seems to lessen. The extent to which that happens perfectly matches the amount predicted by the law of conservation of momentum.