I’d really love it to be true, and dream about the possibility, but alas no, the EM drive will not be taking us to the stars.
The concept has been bouncing around for some time now. The EM Drive, or to give it the more correct term, the radio frequency (RF) resonant cavity thruster, is supposedly a reactionless drive that will create thrust without consuming any propellant.
If it sounds crazy then you are right. Imagine sitting inside your car and then trying to move it by pressing against the steering wheel. If you think to yourself, “hey that’s stupid, that won’t work“, then get it, because that is exactly what this claims to be able to do. I should add that sometimes the crazy ideas are right, but usually they are wrong … and yet, and yet and yet … when it comes to this …. OK, let’s explore the topic a bit.
The term EM Drive was coined by a chap called Roger Shawyer who has been banging away at this idea for some time. He founded a company back in 2000 named “Satellite Propulsion Research Ltd” in order to work on it.
- In 2002 he claimed that he had a working prototype, but that it failed to continue operating after overheating
So far it sounds “interesting” and worth keeping an eye on.
2006 saw Mr Sawyer pop back up with a new water cooled prototype that was apparently more efficient. New Scientist got on board, had a picture on its front cover, and ran a story claiming that this looks feasible. [Here is a link to his theory paper via the New Scientist website]
Now this is getting exciting, New Scientist likes it so it must all be true … right?
Well, lets just say that the response to that was rather a lot of “You have got to be fracking kidding me, this is BS” emails going back to New Scientist. Terms such as “gobsmacked by the level of scientific illiteracy” floated about as some pointed out that it used “meaningless double-talk” to obfuscate the problem of conservation of momentum. (Remember that pushing against the steering wheel image, well that is why many choked on this)
New Scientist editor, Jeremy Webb, Editor, needed to respond …
It is a fair criticism that New Scientist did not make clear enough how controversial Roger Shawyer’s engine is. We should have made more explicit where it apparently contravenes the laws of nature and reported that several physicists declined to comment on the device because they thought it too contentious.
… The great thing is that Shawyer’s ideas are testable. If he succeeds in getting his machine flown in space, we will know soon enough if it is ground-breaking device or a mere flight of fancy.
“The article about Roger Shawyer implies that EADS Astrium suppressed this miracle drive for nefarious business reasons (9 September, p 30). The truth (sorry, conspiracy theorists!) is rather different. As the then technical director of Astrium, I reviewed Roger’s work and concluded that both theory and experiment were fatally flawed. Roger was advised that the company had no interest in the device, did not wish to seek patent coverage, and in fact did not wish to be associated with it in any way. The letters you have published point out some of the issues (7 October, p 24).”
The article on “flying by light” describes a machine that uses microwaves to generate thrust. As I read it, I, like the thousands of other physicists who will have read it, immediately realised that this was impossible as described (9 September, p 30). Physicists are trained to use certain fundamental principles to analyse a problem and this claim clearly flouted one of them.
To understand how, consider this. A “Shawyer drive” is installed in a spacecraft floating in deep space far from any other object. Let us say that it got there using nuclear power, since it cannot use sunlight. Switch on the Shawyer drive and the craft begins to accelerate. The craft changes speed and in so doing it changes its momentum without any other external change. Except it doesn’t, because this is impossible. Momentum, according to one of our basic principles, is conserved and cannot be created or destroyed. The craft is breaking this rule.
In a conventional rocket, thrust is achieved without breaking the rule because the combined momentum of the craft and the exhaust gas from the rocket cancel each other out as they move in exactly opposite directions. The principle of conservation of momentum is every bit as true in the world of relativity and quantum mechanics as it was when set down by Newton. The Shawyer drive is as impossible as perpetual motion.
Relativistic conservation of momentum has been understood for a century, and dictates that if nothing emerges from Shawyer’s device then its centre of mass will not accelerate.
This statement holds true in all reference frames. It is likely that Shawyer has used an approximation somewhere in his calculations that would have been reasonable if he hadn’t then multiplied the result by 50,000. The reason physicists value principles such as conservation of momentum is that they act as a reality check against errors of this kind.
What happens next?
From about 2007, Mr Sawyer began reaching out to the US. Boeing expressed an interest. It went as far as Mr Sawyer shipping a prototype to Boing in 2010. In 2012 Boeing was asked to comment on this for an article in aviation week on new forms of space propulsion. They confirmed that they had looked into the idea, but that they were no longer working with Mr Sawyer on it.
Side note, yes that article is basically a claim that the Chinese have a working prototype, but that was written four years ago and so far nothing has been demonstrated. The Chinese “success” turned out to not be real, they eventually discovered that they had a dodgy power cable. They repeated their experiment in early 2016 and the measured thrust disappeared.
Meanwhile, Mr Sawyer plods relentlessly on. 2014 saw him present it all to the annual International Astronautical Congress. A paper based on that presentation was later published within Acta Astronautica in 2015.
Last October, just a month ago, Mr Sawyer filed a patent for a new superconducting EmDrive, and also started up a new company called Universal Propulsion Ltd. with the vision there being flying cars.
There is a real Peer-reeviewed paper
A real Peer-reviewed paper entitled “Measurement of Impulsive Thrust from a Closed Radio-Frequency Cavity in Vacuum” has been published (Nov 17). The author is not Mr Sawyer, but instead is a team from NASA.
Oh … so does this mean that this is all real?
I’d hold that thought for a moment.
The abstract reads …
A vacuum test campaign evaluating the impulsive thrust performance of a tapered radio-frequency test article excited in the transverse magnitude 212 mode at 1937 MHz has been completed. The test campaign consisted of a forward thrust phase and reverse thrust phase at less than 8×10−6torr8×10−6 torr vacuum with power scans at 40, 60, and 80 W. The test campaign included a null thrust test effort to identify any mundane sources of impulsive thrust; however, none were identified. Thrust data from forward, reverse, and null suggested that the system was consistently performing with a thrust-to-power ratio of 1.2±0.1mN/kW1.2±0.1 mN/kW.
They got a result, it did measure an effect … 1.2 millinewtons per kilowatt.
Let me put it this way. If you take a couple of gains of rice and then place then in the palm of your hand, the downward force they generate is about equivalent to this.
That’s tiny … very very tiny. So tiny that you can’t rule out some other effect such as thermal expansion. They do point out within the paper that further tests would be needed to rule that out, so no this is not definitive proof that this effect actually exists. They setup the rig, place it in a vacuum, recorded what they measured, then discussed what might explain it, and do make it clear that they cannot rule out something mundane such as thermal expansion. A more refined set of tests would be needed to eliminate other possibilities, and they do offer a few suggestions in the paper. The EM drive is such an extraordinary claim, (remember, moving the car by pressing the steering wheel while sitting inside it), that it warrants some truly robust independent testing before you can conclude that the effect is real.
In other words … “extraordinary claims require extraordinary evidence”.
Often things we can’t explain do pop up, and sometimes it does yield something interesting, but more often it turns out to be something far more mundane. Remember that faster-than-light claim at CERN? The chaps working on that did not believe their measured results, so they announced it in the hope that somebody could advise them on what the problem was. It eventually turned out to be fibre optic cable not being connected correctly. Usually that is how things work out when faced with physics breaking claims.
Just a bit more time
Let’s place all this in a wider context.
There is an on-going trend of media stories about a lone maverick popping up with some claimed device that appears to break the laws of physics and holds out the promise of something truly amazing, usually free energy, and this is an example of exactly this type of claim. It is no surprise to find most of the physics community rolling their eyes when faced with stuff like this, because they have seen how it all usually plays out.
Often the hype is that it just needs a bit more time or a bit more investment and the promised goal is just a few more years away.
I don’t doubt the sincerity of Mr Sawyer. What I do doubt is that he has something that actually works, and also breaks the basic laws of physics. That itself should, and does, provoke an appropriate degree of skepticism about it all.
Nevertheless, we should still remain open to possibilities. Such openness does also warrant considerable caution, especially when laced with potential physics-breaking implications. In other words, it is wholly appropriate for others to explore such concepts and test them, but an appropriate degree of rigour is also warranted.
So will we be flying to the stars one day using an EM Drive?
Sadly nope, so far there is no robust definitive evidence that it actually works.