ROCKETS are spectacular examples of Isaac Newton’s third law of motion: that to every action there is an equal and opposite reaction. Throwing hot gas out of its engines at high speed (the action) thrusts a rocket off its launch pad and into space (the reaction). But having to carry the propellants needed to create the gas (the reaction mass) is a pain, for at any given moment during a flight the action has to propel not only the rocket itself, but also all of the remaining, unburnt propellant. Most of the effort expended in a rocket launch is therefore directed towards lifting propellant rather than payload. As a result, even the most modern rockets start off with a mass that is more than 90% propellant.

The fantasy of rocket scientists is therefore an engine that needs no propellant. And that is what Roger Shawyer, a British aerospace engineer, claims to have invented. In his view, his EMDrive (the “EM” stands for “electromagnetic”) converts electrical energy straight into thrust, with no need for reaction mass. The only trouble is, that should be impossible.

An EMDrive (see picture) is a conical metal cavity into which microwaves are fed, and inside of which they bounce around. Electromagnetic radiation has no mass, but it does carry momentum (this is the principle by which solar sails work, using the pressure of sunlight to produce thrust). Dr Shawyer argues that the EMDrive’s conical shape results in different levels of radiation pressure at each of the cavity’s ends, and therefore a net thrust in the direction of the thin end. Every physicist who has studied the idea says this is impossible. Because nothing is emitted from an EMDrive, it cannot generate thrust, any more than the crew of a spaceship could fly to Mars by pushing on the walls. Dr Shawyer nevertheless says he can measure this apparently impossible thrust.

Exotic claims of antigravity devices, perpetual-motion machines and the like are hardly unusual (The Economist once received detailed plans for a faster-than-light spaceship in the post). But the EMDrive stands out, for it transpires that Dr Shawyer is not the only person who has detected thrust coming from it. Harold White and his team at the Eagleworks laboratory in Houston, Texas, have done so too—and they are scientists employed by NASA, America’s space agency. The Eagleworks, which is part of the Johnson Space Centre, is a place where the agency tests fringe ideas. And, as Dr White and his colleagues report in a paper just published in the Journal of Propulsion and Power, when they put an EMDrive that they had built themselves onto their test bench, they measured a small but persistent thrust.

So what is going on? The romantic explanation is that the EMDrive is a technological breakthrough which works by harnessing exotic new physics, and that by scaling it up people will be able to conquer the solar system. More likely, it is experimental error. Much has been made, among EMDrive fans, of the fact that Dr White’s paper was peer-reviewed. Although that is important, peer review means only that the experiment was competently executed, not that its conclusions are true. Dr White and his team admit that they have not accounted for every possible source of error. The likeliest explanation is that some overlooked factor is producing the illusion of thrust when, in fact, there is none.

Just occasionally, such mysteries do lead to a revolution. Astronomers in the 19th century had difficulty explaining details of the trajectory of Mercury. To do so, it turned out, you had to throw away classical physics and replace it with the theory of relativity. More often, though, the status quo holds. In 2011 a respected physics laboratory in Italy reported curious results that seemed to show subatomic particles travelling faster than light. They turned out to be caused by faulty wiring in the experiment. And physicists were puzzled for decades by the fate of the Pioneer space probes, whose trajectories through the solar system were not quite what they should have been. Radiation pressure from their internal heat was eventually fingered as the culprit. In physics as in the rest of life, if it sounds too good to be true, it almost always is.