Jury is still out on a fuel-less space engine widely considered to be a hoax.
The EM drive is a proposed space ship propulsion system which produces thrust without fuel and without any form of mass-ejection. The design incorporates a tapered chamber through which microwaves are allowed to bounce around, and is claimed to produce a net positive thrust when the microwaves strike the larger end of the chamber. However, such a design seemingly violates Newton’s third law. Newton’s third law states that every action has an equal and opposite reaction. A conventional rocket engine, for example, pushes the rocket forward, because the spent fuel is ejected backward. With no mass being ejected by the EM drive, it should be impossible for any force to be produced, according to classical mechanics. The EM drive is thus widely regarded by the physics community as a hoax, spread to internet infamy by people who don’t know what they’re talking about.
As it happens, the impossible engine has a few tricks left up its sleeve. The EM drive has been tested by small independent groups before without reliable results, but now the American Institute of Aeronautics and Astronautics have published the results of a test in high vacuum and they claim the propulsion system actually might work. The scientists in charge of the test placed the EM drive on a device called a torsion pendulum and measured how much the device moved during forward, reverse and null thrust. They found that the device consistently produced a 1.2 mN/KW power-to-thrust ratio. The force supposedly produced by the drive is small compared to conventional thrusters, but not insignificant. The force is a factor thousand greater than what might be produced from another potential technology, the solar sail.
The EM drive with its inventor, Roger Shawyer
The findings have been published in a peer reviewed journal and while peer review certainly doesn’t mean that the miraculous findings are legitimate, it means at least a few other scientists have looked at them without finding any glaring issues. Still, these results should be taken with a lot of scrutiny. If the drive does work, nobody seems to quite understand how. The EagleWorks team which tested the device (who are associated with NASA, but have frequently been misreported as being NASA) have proposed that the thrust may originate from the microwaves pushing against a “quantum vacuum virtual plasma”, but Caltech physicist Sean Carroll is quick to point out that quantum vacuums do not produce a plasma.
The team also proposes that the thrust may be owed to nonlocal hidden-variable theory, also known as pilot-wave theory; an interpretation of quantum mechanics which has gained traction recently, but is far from the generally accepted Copenhagen interpretation. Mike McCulloch, a physicist at the University of Plymouth meanwhile, argues that the drive could be evidence of a new theory of inertia involving something called Unruh radiation. This hypothesis however, would require the speed of light to vary in the EM drive, which violates Einstein’s law of relativity.
It seems no matter how you turn it around, some law has to be violated for the EM drive to work. Perhaps the simplest explanation is worthy of some consideration: the results may just be a false positive. The paper cites nine potential error sources, some of which cannot be ruled out as effecting the results. It should also be mentioned that the last time EagleWorks tested the drive, they also found a positive thrust, but that this thrust was present even when the EM drive was disabled.
Incredible breakthroughs and major advances are wonderful, but we should never get ahead of ourselves. Extraordinary claims, such as those proposed by the EM drive, require extraordinary evidence. The lesson here then, seems to be to maintain skepticism until that evidence truly presents itself.