The EmDrive: the cold fusion of the 21st century?

June 16, 2013

A writer for Wired asked for my comments a new Chinese paper on the EmDrive, which seems to refuse to die even after all these years. My response was too long for him to include, so here it is in full:

The most important thing that your readers need to understand is that the EmDrive is a perpetual motion machine: it violates the most fundamental law of physics, namely, the conservation of energy–momentum. If it were true, then all of physics—from Galileo and Newton through to Einstein, from nuclear and particle physics through to astrophysics and cosmology—would be overthrown. It would be far bigger news than the discovery of the Higgs boson—indeed, that discovery would be rendered invalid, because it relies crucially at each and every step on the conservation of energy–momentum.

Could such a world-changing claim be true? I’m tempted to invoke Monty Python; but for the benefit of readers on this side of the Atlantic, I think that it is arguably an understatement to say that it is “rather unlikely”.

The magnitude of the claim makes it both easier and more difficult to refute at the same time, both theoretically and experimentally.

On the theoretical side, a simple observation is that all of physics is based on the conservation of energy–momentum. Any calculation using the current laws of physics that violates this law must contain an error. It’s like someone starting with two slices of bread and a piece of pork, and then ending up with a chicken sandwich. You don’t need to follow each step of the process to know that something is not kosher.

Unless they argued that they had invented a way to transmute pork into chicken. Similarly, EmDrive proponents must explain how they are replacing the laws of physics with new laws that violate the conservation of energy–momentum.

Unfortunately, both Shawyer and the new Yang Juan et al. paper claim to use nothing more than the standard laws of electrodynamics—so we already know that each of their theoretical results must contain an error.

Finding the exact error, however, can be as difficult as finding a needle in a haystack. In the case of Shawyer’s original theoretical calculation, his argument was relatively clear and clean, and his error was consequently relatively simple to find: he neglected some of the forces on the device. When they are included, the claimed thrust disappears.

The Yang Juan et al. paper, however, is more akin to that haystack. They quote many valid equations of electrodynamics, but then stitch them together with numerous assumptions, and then use numerical simulation to compute a result. Without having a spare year to dig through their calculations and simulations, it’s impossible to know where they made their mistake. (Now you know why the Patent Office refuses to accept any more applications for perpetual motion machines.) I recommend that they submit their paper, and simulation code, to a reputable physics journal like the Physical Review, who might be able to find a graduate student with nothing better to do than debunk their submission.

A possible source of their error is their Fig. 1. In diagram (a) they show an open system, where microwaves are thrust into outer space. Such a system would indeed show a tiny amount of thrust: the microwave photons are the propellant. But they reject diagram (a) because the microwaves leak out (obviously), which prevents a standing wave (Shawyer’s claimed mechanism for getting amplification of the tiny thrust) from being maintained. They then replace this with diagram (b), which has placed on the exhaust a “matched load used to absorb the heat transferred from reflected microwaves”. This statement makes no sense at all: reflected microwaves would not transfer heat—only momentum, namely, the force that would prevent the system from getting any net thrust. If something more sophisticated is meant, then it is not explained, and certainly not modeled in their equations. It is possible that neglect of the momentum transfer to this “matched load” is the missing force in their calculations.

The experimental side of things is even more complicated. To provide a simple proof that a closed system violates the conservation of energy–momentum, one really needs to demonstrate the effect in space: if a completely closed EmDrive were to start accelerating in a particular direction, then that would be astonishing evidence that the laws of physics were done for.

It is almost impossible to measure and account for every force for an Earth-bound experimental arrangement: not only must the device be sitting on something (to stop it falling down to the center of the Earth), but it will (usually) be surrounded by air. Chinese experimenters may claim to have measured every single force and torque on the system, but it would carry far more weight if someone like NASA or an aerospace company were to confirm experimentally that the laws of physics had really been overturned.

If the history to date of perpetual motion machines is any guide, one must assume that such an outcome would, again, be “rather unlikely”.

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