NASA measured ‘impossible’ EM thrust – but is it really a big deal?
The copper bucket that supposedly breaks physics has finally passed peer-review. But, Cathal O'Connell writes, don't start packing your bags for Proxima b just yet.
The 'copper bucket' – or EM drive prototype – in a vacuum chamber.
NASA
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.