NASA measured ‘impossible’ EM thrust – but is it really a big deal?

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The ‘copper bucket’ – or EM drive prototype – in a vacuum chamber.

Two years ago, NASA scientists announced they measured something that should not be possible – thrust from an engine that does not use propellant. 

Now, after a year-long review process, a paper describing the work has been published in the Journal of Propulsion and Power. It describes how the NASA team measured a thrust of about 1.2 millinewtons per kilowatt from the controversial electromagnetic or EM drive, though they are at a loss to explain how.  

The new work will add fresh impetus to the technology which has been touted by some as the future of space travel and by others as pseudoscientific hokum. But it is far from conclusive proof that the EM drive really works.  

What is an EM drive?

Down on Earth, we get around with the help of friction. A car’s wheels pushing on the road make it move forwards (or backwards if you’re in reverse). 

The problem in space is there’s nothing to push off – that’s why you need a propellant. According to Newton’s third law, where every action has an equal and opposite reaction, ejecting material in one direction will push the spacecraft in the other. In a rocket, the propellant is the exhaust gas fired out through thrusters at tremendous speed. 

The EM drive is a proposed new way of propelling without propellant – making spacecraft much lighter, cheaper and faster, because they don’t need to lug propellant around.

While only a handful of physics groups take the EM drive idea seriously, popular media has buoyed global interest in EM drive with reports so breathless they must have been written in a vacuum. 

If it works, the EM drive could take us to Mars in just 10 weeks (as opposed to about six months). Meanwhile, a trip to Alpha Centauri, the closest star system to our solar system and home to the possibly Earthlike planet Proxima-b, would take 92 years (as opposed to millennia using conventional thrusters).

Oh, and its inventor, the British electrical engineer Roger Shawyer, also claims it will give us flying cars, unlimited energy and solve the global warming crisis. His company, Satellite Propulsion Research Ltd, is currently seeking investment to commercialise the technology.

How is it supposed to work?

The basis of EM drive is a metal drum, empty but for the microwave photons bouncing around inside it millions of times a second. 

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As the photons bounce back and forth, they generate a small pushing force on each end of the drum like ping pong balls hitting a wall. 

So far, this is nothing controversial. We know photons carry momentum – that’s how sunlight can propel spacecraft equipped with a solar sail.

If the drum is a cylinder, the forces on each end cancel out and nothing happens. But Shawyer’s idea, proposed around 2001, is to use a tapered drum, with one end wider than the other. 

Because of relativistic effects, he said, this shape would create an imbalance in the bouncing force and push the whole drum from the inside. This “pushing from within” is what breaks Newton’s law.

As John Baez, a mathematical physicist at the University of California, Riverside, puts it, “it’s like sitting inside a car and making it roll forwards by pushing on the steering wheel”.  

Has the EM drive been tested before?

Yes, but not conclusively. 

In 2012, a team of Chinese scientists from Northwestern Polytechnical University in Xi’an reported measuring a large net thrust (720 millinewtons at 2,500 watts of input power). 

Later they realised that this measurement was an error stemming from a dodgy a power cable. They repeated their experiment in early 2016 and the measured thrust disappeared.  

Meanwhile, in July 2015, a German group led by Martin Tajmar at the University of Dresden also tested an EM drive device in a vacuum. 

They did measure a thrust, but in several directions besides the intended one. Tajmar concluded this to be a “null result”. 

So where does NASA come in?

Back in 2013, a small team at NASA’s Advanced Propulsion Physics Laboratory started to tinker around with the EM drive and another related technology called Cannae drive. 

The team is headed by Harold “Sonny” White, the scientist trying to warp spacetime using electromagnetic fields

In early EM drive work, White’s team did detect a net thrust, though they were criticised at first for performing the work only in air. 

That meant they couldn’t rule out the role of air molecules, kicked away from the heated device during operation, as the source of the anomalous thrust. 

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Now, the same team has published a paper describing their latest vacuum tests of their EM drive. The results seem to dispel the role of air because the team measures the same thrust in normal atmosphere and in a vacuum.

They also found that this force increased in rough proportion to the increased microwave power, which would be expected if microwaves were the source. 

In the experiment, White’s team sat their EM drive on the end of a torsion balance – an instrument that measures tiny forces. 

They measured a thrust of 1.2 millinewtons per kilowatt. That’s the force you feel if you place seven grains of rice on your palm. A kilowatt is what a power-hungry household appliance, such as a vacuum cleaner, might use. 

The measured force is tiny but still more than 100 times stronger than that generated by a solar sail. If generated continuously, even one millinewton could be enough to propel a spacecraft to tremendous speeds, given enough time. 

The team noted nine possible sources of anomalous thrust in their set-up and accounted for as many as possible. 

For instance, to account for any systematic bias in their measuring instrument, they mounted the device on the swinging arm in both directions. In both cases, the thrust was towards the narrow end of the device. 

Puzzlingly, the original theory proposed by Shawyer seems to predict the thrust should go in the opposite direction than the NASA results show. Instead, White’s team suggests the device may work instead by pushing off the “quantum vacuum”, meaning the sea of virtual particles that fill even empty space. 

Does this mean NASA has ‘validated’ the EM drive? 

No. Getting a paper through peer-review is not conclusive proof that the EM drive really works. It just means that a bunch of independent scientists have pored over the methods and found nothing obviously wrong. 

Nothing in standard physics can explain how the EM drive might work. That is not necessarily a reason to reject the result out of hand – we know physics is not complete – but it is a reason to maintain scepticism. As Carl Sagan said, “extraordinary claims require extraordinary evidence”.

We can only learn from “anomalies” of the past. The faster-than-light neutrinos detected by CERN in 2011 turned out to be the fault of an improperly connected fibre optic cable. The source of the EM drive anomaly is most likely something just as mundane.

Caltech physicist Sean Carroll tweeted back in 2014: “The eagerness with which folks embrace sketchy claims about impossible space drives would make astrology fans blush.”

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