The EM drive, the so-called “impossible” space drive that uses no propellant, has roiled the aerospace world for the past several years, ever since it was proposed by British aerospace engineer Robert Shawyer. In essence, the claim advanced by Shawyer and others is that if you bounced microwaves in a truncated cone, thrust would be produced out the open end. Most scientists have snorted at the idea, noting correctly that such a thing would violate physical laws. However, organizations as prestigious as NASA have replicated the same results, that prototypes of the EM drive produces thrust. How does one reconcile the experimental results with the apparent scientific impossibility? Wednesday, MIT Technology Review suggested a reason why.
“Today we get an answer of sorts thanks to the work of Mike McCulloch at Plymouth University in the U.K. McCulloch’s explanation is based on a new theory of inertia that makes startling predictions about the way objects move under very small accelerations.
“First some background. Inertia is the resistance of all massive objects to changes in motion or accelerations. In modern physics, inertia is treated as a fundamental property of massive objects subjected to an acceleration. Indeed, mass can be thought of as a measure of inertia. But why inertia exists at all has puzzled scientists for centuries.
“McCulloch’s idea is that inertia arises from an effect predicted by general relativity called Unruh radiation. This is the notion that an accelerating object experiences black body radiation. In other words, the universe warms up when you accelerate.
“According to McCulloch, inertia is simply the pressure the Unruh radiation exerts on an accelerating body.”
McCulloch believes that this phenomenon has already been observed with the so-called fly-by anomaly in which space probes experience strange, unexplained jumps in momentum as they fly past Earth on their way to other planets. The same theory explains the thrust being experienced in tests of the EM drive.
“The idea is that if photons have an inertial mass, they must experience inertia when they reflect. But the Unruh radiation in this case is tiny. So small in fact that it can interact with its immediate environment. In the case of the EmDrive, this is the truncated cone.
“The cone allows Unruh radiation of a certain size at the large end but only a smaller wavelength at the other end. So the inertia of photons inside the cavity must change as they bounce back and forth. And to conserve momentum, this must generate a thrust.”
McCulloch claims that he can predict the amount of thrust that an EM drive will produce with a series of complex calculations. He believes that the theory can be tested in two ways. First, placing a dielectric inside the cavity will enhance the thruster’s effectiveness. Second, changing the dimensions of the cavity will reverse the direction of the thrust. “That would happen when the Unruh radiation better matches the size of the narrow end than the large end. Changing the frequency of the photons inside the cavity could achieve a similar effect.”
McCulloch’s theory is hardly a mainstream one-yet. His idea that photons have mass and that the speed of light can change inside the EM drive’s cavity is something that the physics community will readily accept. Still, with experiments still showing thrust, what other explanation is there?
If the EM drive can be made to work, the technology would revolutionize space travel. Much of the mass of any spacecraft, whether it is propelled by chemical rockets or some more exotic technology such as an ion drive or a nuclear engine, is fuel. But all an EM drive would need is a power source, a nuclear reactor or solar panels, and any spacecraft that uses it could tool around the solar system at will.
Mars, for example, would be weeks instead of months away, In theory, an expedition to Saturn would be a three-year round trip, allowing for extensive visits to Titan and Enceladus. The EM drive could be as much if not more of a game changer for space flight as the liquid fuel rocket.