December 1, 2021

I, Science

The science magazine of Imperial College

Peter Shatwell sifts the fact and the fiction in support of the EmDrive - a proposed space propulsion system that appears to contradict a fundamental law of physics

A new study [1] from the Dresden University of Technology in Germany has shown results in favour of the working operation of the highly controversial “EmDrive”. The EmDrive is a supposed ‘propellantless’ spacecraft propulsion drive, first proposed by Roger Shawyer in 1999. In 2001, Shawyer got a grant from the government to pursue the idea and founded the company “Satellite Propulsion Research Ltd.” to work on it. He has been persistently promoting his idea since.

NASA JPL test of an ion thruster as means of spacecraft propulsion. Here, the momentum exchange is achieved by the acceleration of xenon ions.

A New Kind of Propulsion

All current spacecraft propulsion systems rely on an exchange of momentum, either by the ejection of some propellant on board or by pushing against some external field (e.g. Earth’s magnetic field). However, the EmDrive is claimed not to rely on the expulsion of any propellant, and is able to convert electrical energy directly into forward thrust. It is a new type of electromagnetic thruster called a ‘resonant cavity thruster’ that apparently works by microwave resonances within an asymmetric metal cavity.

Many physicists have stated that this is impossible as it violates the conservation of momentum. As such, Shawyer’s work has been met with ridicule and little attention has been given into researching the drive’s operation further. How can you have thrust in one direction and no propellant in the other? It is much like expecting to be able to fly if you grabbed the sides of your chair and pulled upwards hard enough. Indeed, John Costella has written a disparaging paper condemning Shawyer as a charlatan, and debunking Shawyer’s theory about how the EmDrive works.

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The EmDrive

Experimental Support

However, the researchers Professor Martin Tajmar and graduate student Georg Fiedler from Dresden replicated an EmDrive and tested it using a knife-edge balance and a torsion balance in a vacuum chamber. They report to have directly measured thrusts close to predicted calculations and suggest that this warrants further study. Their results were presented at the American Institute for Aeronautics and Astronautics Propulsion and Energy Forum in Orlando at the end of July [1].

This is the most recent independent study in favour of the EmDrive’s operation, following on from earlier efforts by Chinese researchers at Northwestern Polytechnical University (NWPU) in 2010, and scientists at NASA’s Advanced Propulsion Physics Laboratory (Eagleworks) in 2014 and early 2015.

The team from NWPU led by Juan Yang in 2010 built a demonstration version of the drive and tested it with different cavity shapes. They measured a thrust of 720mN following an input power of 2.5kW [2]. In January 2015, Paul March from the group at NASA’s Eagleworks made new results public claiming positive thrust measurements from an EmDrive in a hard vacuum. The thrust measured, under 50W of input power, was 50μN. Unfortunately this is a thousandth of the force that Shawyer claimed to see in his experiments, however the result still lies well outside the error range of the torsion pendulum used so it is at least significant.

How does the EmDrive work?

Conservation of momentum

If the EmDrive truly were completely reactionless, it would violate the conservation of momentum. This is a fundamental law of physics, and so it is much more likely that Shawyer’s theory is wrong rather than to throw well-understood physics out of the window. Shawyer maintains that microwaves bouncing around inside a conical cavity produce the thrust. He claims that microwave radiation at the narrow end of the cavity has less momentum and that the resulting force at the larger end of the cavity is higher. This results in a net force that drives it forward. Unfortunately, this theory is just wrong. But we have still measured thrusts in the laboratory, so what is going on?

Harold White at NASA proposed a different theory claiming that the drive could work by reacting against virtual particles produced by quantum vacuum fluctuations. This type of spacecraft thruster is dubbed a ‘quantum vacuum plasma thruster’ or ‘Q-thruster’. In this model, a Q-thruster would not be propellantless as is claimed for the EmDrive, but it would use quantum vacuum fluctuations as its ‘propellant’. This theory has also been met with criticism. Cosmologist Sean Carroll is quoted as saying that quantum vacuum virtual plasma is just “made-up technobabble”, and that the conservation of momentum is no more possible in quantum mechanics than it is in classical mechanics [3].

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An artist’s illustration of quantum vacuum fluctuations

What Next?

The EmDrive is certainly an interesting oddity in the scientific world. If it violates the conservation of momentum, then the drive should be impossible. However, repeated experimental verification of the supposed ‘impossible’ drive has muddied the issue. Without a working theory, who knows what’s going on? It should be noted that although this experimental work has been published in university journals and presented at conference proceedings, it has not yet been published in a peer-reviewed journal and is unlikely to, seeming as it is very much ‘fringe science’ at the moment.

If the EmDrive does work as it’s supposed to, it could make a huge difference to space exploration. It’s been claimed that a craft powered by an EmDrive could get to Pluto in less than 18 months: a significant reduction in time compared to the New Horizons mission that took over nine years [4]. Of course, many still affirm that the drive is impossible. It is indeed the right attitude to remain sceptical; “Extraordinary claims require extraordinary evidence” as was popularised by Carl Sagan. But, if we do find extraordinary evidence from further research, it will certainly be a big day for physics to find that the conservation of momentum could be violated.

Peter Shatwell is a year 4 student studying for an MSci in Physics 

Images: NASA JPL test of an ion thruster (public domain); EmDrive; Pool balls (Wikimedia Commons); Quantum foam by Alex Sukontsev (Flickr); Featured image: science fiction imagining of spaceship entering a wormhole (Ryan Somma, Flickr)


[1] Tajmar, Martin; Fiedler, Georg (July 2015). Direct Thrust Measurements of an EM Drive and Evaluation of Possible Side-Effects 51st AIAA/SAE/ASEE Joint Propulsion Conference

[2] Yang, Juan; Wang, Yu-Quan; Li, Peng-Fei; Wang, Yang; Wang, Yun-Min; Ma, Yan-Jie (2012). “Net thrust measurement of propellantless microwave thrusters” Acta Physica Sinica (in Chinese) (Chinese Physical Society) 61 (11)