David Hambling London

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.

Shawyer's analysis was challenged after the EmDrive was featured in a science magazine in 2006. John Costella, a researcher in relativistic electrodynamics, described the EmDrive as a fraud and argued that even with relativity there can be no net thrust. Shawyer built demonstration EmDrives to back his claims, including a 7-lb. version he said produced a thrust of 85 millinewtons (mN) with a 300-watt input. Skeptics, convinced of its impossibility, have not even tested the EmDrive.

But the controversy attracted the attention of China's Yang Juan, professor of propulsion theory and engineering of aeronautics and astronautics at the Northwestern Polytechnic University in Xian. Her team set out to explore the EmDrive independently. A 2008 theory paper by Yang and colleagues describes the EmDrive in terms of quantum theory and indicates net thrust is possible. A 2010 follow-up paper calculates a possible thrust of 456 mN from a 1-kw input, and states that the team was getting positive experimental results.