July 31, 2015

Despite recent claims, the EmDrive remains long on speculation, short on proof

M31 Andromeda GalaxyA new report from German researchers has made waves by claiming to validate the performance of the controversial EmDrive, but many articles on the topic have vastly oversold the results. Let’s see if we can find some clarity here. To begin with, the EmDrive is what’s known as a resonant cavity thruster. It relies on a magnetron to produce microwaves and is designed to produce thrust towards the narrow end of the cavity.

The problem with the EmDrive (and with all reactionless drives) is that they seem to violate the law of conservation of momentum. That law says that the total linear momentum of a closed system remains constant, regardless of other changes within the system. This is the origin of the phrase “For every action, there is an equal and opposite reaction.” When you take the “reaction” out of one end of the system, it’s difficult to explain how an “opposite” reaction actually gets started.
In the case of the EmDrive, the inventor claims more microwaves push against one side of the resonant cavity than the other, thereby generating slightly (very slightly) more force in one particular direction. There is no known way for this to be true. All validated space engines, whether they are ion thrusters, nuclear thermal drives like NERVA, or conventional chemical rockets, fire a propellant in one direction in order to move the spacecraft along its desired trajectory. The total momentum of the rocket is balanced once factors like air density are accounted for. The EmDrive claims that microwaves can be used to “push” against one particular side of a spacecraft without exerting an identical force on the opposite side.
A few days ago, a pair of German scientists published a paper in which they claim to substantiate the EmDrive’s performance. The scientists claim to have measured thrusts of roughly 20 micro-Newtons, which is in line with what NASA measured last year. There are significant problems with this analysis, however. Eric W. Davis, a Senior Research Fellow at the Institute for Advanced Studies in Austin said to io9: “I noted in [the study’s] conclusion paragraphs that [Tajmar’s] apparatus was producing hundreds of micro-Newtons of thrust when it got very hot, and that his measuring instrumentation is not very accurate when the apparatus becomes hot,” Davis told io9. “He also stated that he was still recording thrust signals even after the electrical power was turned off, which is a huge key clue that his thrust measurements are all systematic artifact false positive thrust signals.”
If the EmDrive is still generating thrust even when the power is turned off, it strongly implies that the measured energy was thermal, and therefore indicative of a false positive reading. This latest project is just one of many that has attempted to determine whether EmDrives or the closely related Cannae Drive can actually function. None of the experiments yet performed have been subjected to rigorous peer review, and many of them were publicized and interpreted by the developers of the drives — not independent scientists.
Proponents of these drives and concepts have argued for various quantum mechanical phenomena that could explain the thrust, including the idea that the microwaves somehow produce thrust by interacting with virtual particles. Most of the proposed explanations have additional problems with the laws of physics, but conservation of momentum is the major sticking point. No theoretical explanation for how the drive works while still conserving momentum has been found to be satisfactory.
It’s easy to see why science journalists and the public want to believe in the possibility of an EmDrive. In theory, this type of engine could cut travel time to Pluto from nine years to 18 months, using an acceleration of 0.4N/kg. That same acceleration would allow us to travel to Mars in less than three months. Such speeds wouldn’t allow for interstellar travel — at 0.00034% of light speed, it would still take 12,852 years to reach Alpha Centauri (give or take), but it would make colonizing the planets in our system much, much easier.
Unfortunately for all involved, wishing really, really hard for a thing to be true doesn’t make it so. Until the EmDrive has undergone rigorous experimental validation from a neutral independent team of scientists with no ties to the inventors, it’s impossible to claim the drive works. The equipment needed to measure the amount of thrust and the experimental controls required to validate it are extensive enough as to be daunting to even well-funded labs. As great as the EmDrive looks on paper, we don’t recommend anyone start packing for their 12-hour Moon excursion anytime soon.

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