Chinese scientists appear to have validated a propellentless space propulsion technology previously branded as impossible. Based on earlier British research, it is averred that the EmDrive concept provides sustained thrust at low cost and weight, but this has yet to be accepted even as a workable theory by the wider propulsion community.
The EmDrive story started in 2001 when engineer Roger Shawyer set up Satellite Propulsion Research (SPR) to exploit his new concept in electrical propulsion. He was helped by a modest grant from the U.K.'s now defunct Trade and Industry Department.
Space propulsion relies on Newton's laws of motion: propellant is ejected backward at high velocity, and the craft is pushed forward with equal and opposite momentum. Even with high exhaust velocity, such as ion drives ejecting particles at 30 km per second (more than 62,000 mph), the mass of propellant is a limiting factor.
Shawyer's EmDrive does not have any exhaust. It consists of a tuned cavity shaped like a truncated cone into which resonating microwaves are channeled. Like other radiation, these exert a tiny pressure when reflected off a surface. According to Shawyer, the pressure exerted on the large end of the cavity is greater than the pressure on the small end, producing a net thrust.
This appears to be a violation the law of conservation of momentum. However, Shawyer says net thrust occurs because the microwaves have a group velocity (the velocity of a collection of electromagnetic waves) greater in one direction than the other and relativistic effects to modify the Newtonian mechanics. Shawyer compares the EmDrive to a laser gyroscope, which also looks like a closed system but is actually open and works thanks to relativistic effects.
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