John Baez
2 years agoEditedPublic
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.

Bill McGarvey2 years ago
Tell us how you really  feel, +John Baez ... < chuckling >+27
John Baez2 years ago
Heh. +6
Heikki Arponen2 years ago
First there was the Alcubierre drive stuff, now this... maybe NASA deserves it's budget cut! Goddam Star Trek fanboys/-girls...+3
Frank Nestel2 years ago
Thank you for a clear statement  :-)+2
Asbjørn Heid2 years ago
From the report:

"Testing was performed on a low-thrust torsion pendulum that is capable of detecting force at a single-digit micronewton level, within a stainless steel vacuum chamber with the door closed but at ambient atmospheric pressure."

"Thrust was observed on both test articles, even though one of the test articles was designed with the expectation that it would not produce thrust".

Could simply the cans ("articles") heat up and the air circulation cause the reading? Wonder why they didn't try it in a vaccuum.
Did you find the full version of the NASA technical report +John Baez ? Because as far as I can see, on that link there is only the abstract.+2
Matthew Petersen2 years ago
thank you, this experiment was setting off bs alarms for me just from the wording :l+3
John Baez2 years ago
+Noon Silk - try "Click to view PDF".
+John Baez that just takes me to a pdf version of the abstract!+2
John Baez2 years ago
Oh, sorry.  There's a paper here:

Not free!  Bad: I thought NASA had a policy that it's research is freely available.   It should be!  Can someone download this thing and put it it in a public place?  If my government is spending my money on research, I'd at least like to see it.
+John Baez ah okay; down the bottom there it says "Availability: Abstract Only"; so I can guess that there does actually exist some technical report, but it's not publically available at the moment.+1
John Baez2 years ago
+Noon Silk - read my comment above yours!  We're cross-talking...
Ah, too fast! I see. Oh well; I don't think it's interesting enough to warrant a purchase :) Cheers though; I should've found that myself.+1
To get a real force, you just need to leave one end open. There, I fixed it.+16
John Baez2 years ago
+Mark van Walraven - my god, you've invented the rocket!  :-)

A rocket that gets its thrust by a beam of microwaves is possible... but it sucks: every newton of thrust takes 300 megawatts of power.  This follows from the relation between energy and momentum for photons.  The number 300 million should remind you of the speed of light in meters/second; that's no coincidence.

The idea has a fun history, though:
Todd McKissick2 years ago
Dumb question but is it possible that it's now acting similar to a tuned, coherent light laser and as such, is concentrating the microwaves at a point very near the exit where it heats the air and causes it's expansion to push a little on the can?  If so, placing it in a vacuum would kill all output.

...unless the microwaves were actually expanding the dark quantum gravitrons?
Paolo Scarabelli2 years ago
I'm not sure I got it right. If the device really works, is it supposed to convert 1000 W of energy 0.001 N of force? Isn't radiation pressure more efficient?+2
John Baez2 years ago
+Paolo Scarabelli - if you read my comment two above yours you'll see that using radiation pressure you can convert 1000 watts of power into just 0.0000033 newtons of force: 3.3 micronewtons.  So this device would be vastly more efficient than radiation pressure... if it really worked.+4
Paolo Scarabelli2 years ago
Thanks +John Baez, the comment wasn't visible when I typed mine.+2
John Baez2 years ago
+Todd McKissick wrote: "Dumb question but is it possible that it's now acting similar to a tuned, coherent light laser and as such, is concentrating the microwaves at a point very near the exit where it heats the air and causes it's expansion to push a little on the can?"

Something of this general sort is possible: when you're pumping power into a gizmo, it's not very hard for it to find some way to exert 48 micronewtons of force on its surroundings - that's what they claim to have observed.

"...unless the microwaves were actually expanding the dark quantum gravitrons?"

Now you're talking!  Wikipedia says:

Dr. Harold G. "Sonny" White, who investigates field propulsion at Eagleworks, NASA's Advanced Propulsion Physics Laboratory, notes that such resonant cavities may operate by creating a virtual plasma toroid that would realize net thrust using magnetohydrodynamics upon quantum vacuum fluctuations.

They cite this report, which [cue spooky music here] shows up as a damaged file in my PDF reader:
André Esteves2 years ago
photons are pure momentum... if this drive is impossible wouldn't space sails also not work?+1
If we allow quantum vacuum fluctuations in, then the cavity isn't really closed.
Todd McKissick2 years ago
+John Baez Oh that old research again?  It's so 2000's.  The PDF opens fine for me.  It simply include this video:

It's a real short explanation which shows how you can even create the effect in your kitchen!  Their obvious change in direction dumped old-school propulsion (wasn't interesting enough) for the much newer advanced field of levitation and anti-gravity propulsion.
John Baez2 years ago
+André Esteves - the point is, these folks are claiming the drive works without emitting any microwaves.   The microwaves are supposed to only bounce around the can. That's why they call it a "reactionless drive".  
As noted in my comments above yours, if the device were powered by emitting microwaves to the outside world, it could only get a thrust of 3.3 micronewtons per kilowatt of power.  They're claiming much more thrust... though still a puny amount.
John Baez2 years ago
+Mark van Walraven - quantum vacuum fluctuations, aka virtual particles, aren't a 'thing' of the sort you can use to create thrust.  Unless of course you're claiming to massively rewrite known physics... in which case a bit more experimental confirmation would be called for.+2
John Baez2 years ago
+Todd McKissick - that video is hilarious.   Quotes:

"That's my logo: times divide plus minus in one logo.  That's because I figured out how to do all the functions in math instantly."

"It's not a static stationary aether, it's an inertia aether.  The conventional education and training everyone had is that it's kind of like a loquacious jello."
Isaac Kuo2 years ago
This proves homeopathy! The more diluted the force detected, the greater the effect of making people believe it's true! It even works when the diluted force is also detected in the test subject that was supposed to produce no force at all!+19
+John Baez Damn, I was hoping to radiate the negative-energy particles forwards.... If I call it a "quantum singularity drive," can I still apply for NASA funding?
Matt McIrvin2 years ago
I think that, by now, most people who have some acquaintance with science have an idea that a machine that is claimed to violate conservation of energy is probably nonsense; but they are not as skeptical about claimed violations of conservation of momentum, even though they really should be exactly as skeptical about that.+13
John Baez2 years ago
+Mark van Walraven - go for it!  Try their "Breakthrough Propulsion Physics" project:


"All NASA support to sustain cognizance on these possibilities has been withdrawn as of October 1, 2008."
Matt McIrvin2 years ago
+John Baez One way to think about photon rockets is that rockets, in general, involve a trade-off. The higher the exhaust velocity is, the less reaction mass you need to throw out of the rocket to get a given change in velocity, so it's more efficient in that sense... but it's also less efficient in terms of energy consumed, because most of the power is going into the exhaust, not into the rocket.

The photon rocket is just the extreme case, in which the exhaust velocity is as high as it can possibly be. You need no working material at all aside from the photons, but the thrust you get for a given amount of power is extremely low.
John Baez2 years ago
+Matt McIrvin - want a reactionless drive?   Just take a perpetual motion machine of the first kind and Lorentz boost it! +11
Todd McKissick2 years ago
+Matt McIrvin Don't be so critical!  Haven't you heart about the earth-shattering breakthroughs in thermo?  I can't remember their name but I seem to remember the letters GW, AGW and CC being somehow related.  

We should all keep an open mind to new possibilities.  Otherwise, we might just be killing viable vortex, ZP, magnetic motor and twinkie-pill technologies that the world needs.
Shannon Roy2 years ago
My favorite part so far is how articles like the Wired one say (paraphrasing) "this could be great for space exploration!" as if that's the sum total of what could be achieved if we can routinely break conservation of momentum.+13
Matt McIrvin2 years ago
A while ago I remember people getting excited about a theoretical paper claiming a Casimir-like effect that would allow a device to somehow push against the electromagnetic quantum vacuum, and act as a space drive. Because of the talk of the quantum vacuum and virtual photons, which seem like a kind of "stuff" in empty space, people who would normally have been skeptical figured there might be something to it.

Honestly, I didn't read the paper in enough detail to fully understand what it was on about. But it seems to me that this line of effort is not going to be productive, if the vacuum works according to a relativistic quantum field theory. The vacuum is always a Lorentz-invariant state; it's identical regardless of your (inertial) frame of reference, and you can't "push against" it without also converting the vacuum into a final state that is not Lorentz invariant, and carries some momentum. I have such a device: it's called a flashlight.
Chad MILLER2 years ago
Doesn't the paper's abstract day it's bullshit? They made two. One was designed not to work. They got the same results. Thus, the results are probably an error.+7
John Baez2 years ago
+Shannon Roy - yes.  I also like the whinging at the end of the article:

"In hindsight, it may turn out to be another great British invention that someone else turned into a success."
John Baez2 years ago
+Chad MILLER - great point!  But then they conclude the gizmo is "producing a force that is not attributable to any classical electromagnetic phenomenon and therefore potentially demonstrating an interaction with the quantum vacuum virtual plasma".  Huh?+2
Matt McIrvin2 years ago
The granddaddy of momentum-conservation-violating machines was the mechanical Dean Drive (which seems to have actually worked on a principle similar to an unbalanced washing machine walking across the floor). It gained a huge amount of cred from enthusiastic promotion by science-fiction editor John W. Campbell:

NASA stopped paying attention to variants of the Dean drive some time ago. But I still hear some advocacy of it from people who ought to know better on occasion.
+Richard Lucas Yep, there certainly is, and that is a legitimate mode of locomotion, as are the little cubes that spin up a reaction wheel to fantastic speeds and brake at just the right moment to leap into the air or roll a chosen direction.

Thing is, they work because of gravity and friction. It is gravity that is doing the work of pulling an out of balance object downwards, and gravity and friction together that keep the cube from spinning to allow the reaction wheel to spin up.

Neither is particularly useful in space - in orbit you are already falling with the full force that gravity can supply, and in the absence of an object to push against, spinning up your reaction wheel just makes your vessel spin the opposite direction and no directional force can be thereby produced.
Rajini Rao2 years ago
+John Baez  Thanks for the clear explanation, as always, and for #debunkingjunkscience . We've had the usual breathless headlines about "changing space travel forever" shared to the science community, and I've shared your post there. +2
John Baez2 years ago
+Rajini Rao - thanks!  Greg Egan debunked this idea in 2006:

but like a zombie, it's back.
Rajini Rao2 years ago
An immortal, spontaneously regenerating perpetual motion machine that violates the laws of physics and biology! +7
This is the sort of thing that gives August it's reputation as the crazy news month.+1
Michael Pitman2 years ago
The results don't agree with accepted physics, I think everyone (here) agrees. It's a little odd that they'd agree to test the device/principle, but I think the result should be viewed more along the lines of the published results for the faster than light neutrino experiment at Gran Sasso. "We got an odd result (which in this case would seem to be both expected and unexpected), we've double checked, can't find any issues. You guys have a look, see where we went wrong." You know, the iterative process of scientific experimentation. +4
Michael Pitman2 years ago
Maybe they thought that testing it and showing it to be false would get rid of the cranks, which has now backfired.
Stephan da Silva2 years ago
Sounds like they invented a time machine and traveled to April 1st.+4
James Nordblom2 years ago
Perhaps they intend to power it with cold fusion.+3
James Nordblom2 years ago
+Richard Lucas
Well....for myself...I find the best source to be in  the saliva in my cheek, where my tongue is firmly attached.  :}
Hum, that sounds very weird, but one must first have access to the results, then eventually, other experiments by other team in EU and Russia must be done.
Then, if the results are always here, and show a small force, we're going to have to rethink a part of physics !
By the way, I agree that one must be skeptic and careful.
I must admit that I'd like that there's something true, here because it would be so marvelous if we could achieve an affordable mean to reach space.
John Baez2 years ago
+Michael Pitman - the problem is not that the NASA researchers built a gadget and discovered an unexpected effect.  The problem is that they built 2 copies of the gadget, one of which was disabled, saw a tiny effect in both, and announced that it might be "an interaction with the quantum vacuum virtual plasma", which is not even a thing in standard physics.  There are lots of experiments that show tiny unexpected effects - in fact, most do.  Reaching for fuzzy new buzzwords to explain them is not a mark of a good experimentalist.

"We got an odd result (which in this case would seem to be both expected and unexpected), we've double checked, can't find any issues. You guys have a look, see where we went wrong."

That would be a much wiser approach.
Nat aS2 years ago
Please don't compare this woo to Vacuum Plasma thrusters. They do not violate any laws of anything. The theoretical basis is consistent with known physics, unless you think the Casmir effect is a hoax as well.+1
Matt McIrvin2 years ago
The Casimir effect is not a hoax, but anyone who claims they can use it to extract unlimited amounts of energy from vacuum is wrong. +3
Nat aS2 years ago
Not unlimited.
Gabriel Pascu2 years ago
it's Roger, not Robert Shawyer... not that it matters+1
Michael Tetelman2 years ago
NASA's shrinking thrust is really NASA's shrinking trust.
That's incredible story.
Tuan Nguyen2 years ago
I won't be able to afford this quantum vacuum machine until Walmart sells it.
Tuan Nguyen2 years ago
A good read for further appreciation of... vacuum, be it something they sell at Walmart or beyond.

The least of the benefit for reading this is you get to know a few more phrases such as... RF resonant cavity thruster, quantum vacuum virtual plasma...
Don Denesiuk2 years ago
I would remind you of Clarke's first law.
" When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong. "
That having been said the proprietary aspects of some discoveries do tend to hamper a thorough scientific  evaluation of an idea regardless how potentially rewarding but outlandish it seems.
Take LENR formerly known as "Cold Fusion" for example. Perhaps not exactly as the originators claimed but there's something going on that was previously discounted by traditional science.
After all we don't know everything and previously held scientific truths often fall on the advancement of a new approach.
Then again there's those pesky superluminal  neutrinos.
Michael Pitman2 years ago
Has anyone actually got hold of the paper in question yet? Also, where (if) had it been published? Arxiv is free access but pre-peer-review, right? I'm sure the experimenters were thorough and rigorous, but if they're investigating a claim (currently) explained by dubious physics, I'm sure it's not that easy to create an identical but disabled device.
I just hope any corrections or retractions that may come out as the process works its magic makes the media I'm likely to read, ie G+. If they confirm it, I'm sure it'll cause another hypegasm I'm sure to see. This story isn't quite "faster than light neutrino" level news.
John Baez2 years ago
+Gabriel Pascu - whoops!  Thanks, I'll fix that.
Jacob Jones2 years ago
Really? See what the government is experimenting on? That'll happen. Lmao!
Don Denesiuk2 years ago
Here's a few paragraphs more.
I'd suggest they try it in a vacuum just to rule out any heating effects on the air in the cavity producing thrust
Oops they tried that.
" .  Integration of the two test articles and their supporting equipment was performed in an iterative fashion between the test bench and the vacuum chamber. In other words, the test article was tested on the bench, then moved to the chamber, then moved back as needed to resolve issues. "
John Baez2 years ago
+Michael Pitman wrote: "Also, where (if) had it been published?"

The paper appears to be published here:

• 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,

Unfortunately it doesn't seem to be freely available anywhere.  Eventually someone will liberate it.
Steve Esterly2 years ago
+John Baez, the Wikipedia article on quantum vacuum plasma thruster seems to add to the (my) confusion. Since it is undergoing rapid edits, I guess that is no surprise.
While it clearly refers to the device we are discussing here, it also says it does not violate conservation of momentum. Supposedly it generates plasma from particles arising from quantum vacuum fluctuations and accelerates that plasma, but is not a reactionless drive "because it expels the plasma and thus produces force on the spacecraft in the opposite direction."
This description is of course in conflict with other descriptions of the drive I've seen in the press…
John Baez2 years ago
+Steve Esterly - since the physics described in that article is baloney, I am relieved that you find it confusing.+9
Richard Healy2 years ago
I'm not sure why all the venom.

I commented Yesterday 9:28 AM on this post:

The control was an RF load. NASA claims that there was an effect in both article and null article with respect to the control.

A difference between control, article and null article may indicate experimental errors or a lack of understanding of the phenomena. The first can be rectified, the second can be pursued by the suitably qualified.

If the experiment can be replicated and if no experimental errors were made then there is an effect. I mean, reality is demonstrable right? I'd think that a physicist would be happy for an opportunity to explore and explain a phenomena and encourage scientifically rigorous empirical experimentation to establish whether or not there is something worth pursuing.
John Baez2 years ago
+Richard Healy - the "venom", as you put it, arises when people do incorrect calculations claiming classical electromagnetism violates conservation of momentum (Roger Shawyer) or make up fuzzy phrases like "the quantum vacuum virtual plasma" to "explain" anomalous results, without discussing the possibility of experimental error (the authors of the current paper). +5
John Baez2 years ago
+Peter Zotov - thanks for announcing the existence of a free version of the paper at   I think everyone should read this and think a while.  I'm going to close down comments since moderating them and deleting spam has become a full-time job.+9