The incredible shrinking force

Around 2000, a guy named Roger Shawyer claimed he could bounce microwaves inside a fancy-shaped can and get them to  push the can forwards, without anything leaving the can. 

This would violate conservation of momentum.  It's like sitting inside a car and making it roll forwards by pushing on the steering wheel.  Standard physics doesn't allow this.  He didn't claim to be using anything other than standard physics. 

So: ho hum, just another guy with a really bad idea.  I get emails like this all the time.

But in 2001, his company got a £45,000 grant from the British government to study this idea.  He built his machine and claimed that with 850 watts of power he could get a force of 0.016 newtons.   That's a bit less than the force of gravity from a penny pushing down on your hand.  It could easily be an experimental error.

Why would people want a machine that uses lots of power to create a pathetically feeble force?   Because - here's the great piece of salesmanship - if it existed, you could use it to build a reactionless drive!  If you had a spaceship with huge amounts of power to spare - like, say, a nuclear reactor - you could use this gizmo to push your spaceship forwards without anything spewing out the back end. 

Again, this is about as plausible as powering a spaceship by having the crew push on it from the inside.   But if you don't know physics, it sounds very exciting. 

The story goes on.  And on.  And on.  It won't die.  In 2012, some Chinese physicists claimed they could get a force of 0.720 newtons from a power of 2,500 watts using some version of Shawyer's device. 

And now NASA is studying it!

They're claiming to see a force one thousandth as big as the Chinese - probably because they are doing the experiment one thousand times more accurately.  And still, some people are excited about this. 

The new device comes with new improved mumbo-jumbo.  Shawyer claimed that thanks to special relativity, classical electromagnetism can violate conservation of momentum.  I took those courses in college, I know that's baloney.  Now the NASA scientists say:

"Test results indicate that the RF resonant cavity thruster design, which is unique as an electric propulsion device, is producing a force that is not attributable to any classical electromagnetic phenomenon and therefore is potentially demonstrating an interaction with the quantum vacuum virtual plasma."

This is baloney too - but now it's graduate-level baloney.  "Quantum vacuum virtual plasma" is something you'd say if you failed a course in quantum field theory and then smoked too much weed.  There's no such thing as "virtual plasma".   If you want to report experimental results that seem to violate the known laws of physics, fine.  But it doesn't help your credibility to make up goofy pseudo-explanations.

I expect that in 10 years the device will be using quantum gravity and producing even less force. 

For an article written by a severely optimistic blogger, see:

For the NASA report see:

• David Brady, Harold White, Paul March, James Lawrence and Frank Davies, Anomalous thrust production from an RF test device measured on a low-thrust torsion pendulum, 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference,  Free version available at

Unfortunately only the abstract is free in the official version.

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