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The 'Impossible' EM Drive Being Tested By NASA May Finally Be Explained (technologyreview.com)
MarkWhittington writes: The EmDrive, the so-called "impossible" space drive that uses no propellant, has roiled the aerospace world for the past several years ever since it was proposed by British aerospace engineer Robert Shawyer. In essence, the claim advanced by Shawyer and others is that if you bounced microwaves in a truncated cone, thrust would be produced out the open end. Most scientists have snorted at the idea, noting correctly that such a thing would violate physical laws. However, organizations as prestigious as NASA have replicated the same results, that prototypes of the EmDrive produces thrust. How does one reconcile the experimental results with the apparent scientific impossibility? MIT Technology Review suggested a reason why.
https://en.wikipedia.org/wiki/...
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If you have multiple emitters into the chamber, angled toward a reflector, each emitter has a vector of momentum parallel to the axis of the motor, and another perpendicular to it. If the emitters are spaced properly, the perpendicular vectors will cancel, and the parallel components, summed, will be less than the momentum of the photons leaving the chamber through the "nozzle", giving a net forward thrust.
Most scientists have snorted at the idea, noting correctly that such a thing would violate observed physical laws.
The EM drive was discussed at length on other sites, and few posts were able to shine any light on the issue. Some items of note:
First, if your understanding of physics does *not* predict the Casimir effect, then you probably shouldn't be blithely dismissing the theory. The EM drive is based on a theory of physics that's more sophisticated than simple "momentum is conserved". It supposes an hypothesis that's different than what is currently accepted, but in a subtle way that is difficult to detect.
It's similar to relativity: most of our tests validate Newtonian physics, but you find relativity when you go looking for it.
Second, if you want to appeal to Noether's theorem, note that the theorem refers to smooth manifolds. If space is quantized, then Noether's theorem doesn't apply (despite being true). It's possible that Noether's theorem will break down at small scales. (If space is smooth, ie *not* quantized, then the true location of any particle is a [mathematical] real number with infinite entropy and it's action is non-computable. Not that having a non-computable universe is a problem, but...)
All in all, I get the impression that everyone commenting on the EM drive should probably keep quiet and let the experts sort it out.
I don't have any comment on either the theory or the experiment, but it's an interesting proposal.
From the Wikipedia page:
This is analyzed by Rothman and Boughn[32] who point out that the standard theory of radiation pressure is more complicated than the simplified analysis suggests.
NASA tested it in vacuum chamber to prevent this issue.
Architectural plans are like computer source code with a couple of differences: You only compile once.
And yet they admit there are some thermal issues in your cited article. "He also admits that there are still traces of contamination caused by thermal expansion in the system". It still worth noting that these are tiny forces being measured and it's on the order of experimental error rather than useful thrust.
Thank Douglas Adams for that quote, not the AC...
The amount of thrust they're seeing, even at microNewtons, is far higher than could be produced by the radiation pressure of simply emitting photons at those energy levels. If it wasn't, there wouldn't be all this fuss.
NASA measured an average of 91 microN with 17 W, or 5.3 microN/W. The Chinese measured 720 milliN at 2500 W - about 300 microN/W. By contrast, expected radiation pressure would be closer to 0.003 microN/W.
Why would anyone engrave "Elbereth"?
Have you read this part of the article?:
McCulloch’s theory [] makes two challenging assumptions. The first is that photons have inertial mass. The second is that the speed of light must change within the cavity.
"If Photons have mass (i.e. not relativistic mass, actual mass) the world becomes a damn site simpler"
No it doesn't.
General relativity would be fundamentally changed.
"...has no fuel..."
Keep in mind that the EM drive DOES require energy (to generate the microwaves). Perhaps from a nuclear reactor, radioisotope thermoelectric generator, or other efficient, long term, source. What it does not require is "propellant". I.e mass to throw out of the vehicle at high speed. It is a "massless" drive. That is what makes it special.
Why would there be one and only one universe?
By the definition of universe. Multiverse theories do exist, but are not germane to the discussion of how many wavelengths fit into the universe.
What is you definition of universe?
Any event that could create one could create two or 3 or N universes.
...this assumes the universe was "created" in an "event", and that the concept of multiple universes is even sensible. Even assuming that, it's an unsupported assertion. It could very much be like saying "anybody who can discover the Theory of General Relativity can discover two or 3 or N Theories of General Relativity."
Indeed it would have to have something very special to make one and then stop. Something that does not exist in other branches of physics.
What?
If there's more than one, then space is continuous and there's no limit on the wavelength of Unruh radiation.
What? The number of universes has nothing to do with whether space is continuous, and as best I can tell, nothing to do with Unruh radiation.
To give a ballpark estimate(yield varies with about a factor 10), with a 5kw system you can lift 100g (one newton) on earth.
You were right the first time. The people testing the EmDrive were clowns, and Mike McCulloch should never have received a PhD. This reddit thread, which he participated in, should be sufficient to destroy his credibility.
I am incensed this made it to the front page. This is worse science than most climate deniers manage; they are usually a little less blatantly unphysical. The firehose needs a way to tag things as complete bullshit, preferably with the ability to submit a rebuttal link. There is no need for Slashdot to run hoax stories, terrible science, or anything else blatantly untrue.
Those who advocate genocide deserve every protection afforded by law, and none afforded by common human decency.
just build a small one, send it to space, turn the power on and see if it moves.
I try, but as a non-physicist/non-mathematician, all I can really get out of this saga is:
1. Some guy builds and runs a funky apparatus in his lab/garage, and gets some strange results. He reports these excitedly to the world at large.
2. He's obviously smart but possibly deranged, since he claims that the apparatus violates the conservation of momentum, which is a classic crackpot move.
3. Any reputable scientists who have these results brought to their attention uniformly and immediately dismiss them as obvious crackpottery.
4. One night, while drunk, a small group of reputable scientists build the apparatus in their own lab, as a joke, and observe the same strange results.
5. Repeat steps 3-4 a bunch of times.
6. ???
7. Space probe to Alpha Centauri in my lifetime?