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China Claims Tests of 'Reactionless' EM Drive Were Successful (popsci.com)
An anonymous reader quotes a report from Popular Science: The "reactionless" Electromagnetic Drive, or EmDrive for short, is an engine propelled solely by electromagnetic radiation confined in a microwave cavity. Such an engine would violate the law of conservation of momentum by generating mechanical action without exchanging matter. But since 2010, both the United States and China have been pouring serious resources into these seemingly impossible engines. And now China claims its made a key breakthrough. Dr. Chen Yue, Director of Commercial Satellite Technology for the China Academy of Space Technology (CAST) announced on December 10, 2016 that not only has China successfully tested EmDrives technology in its laboratories, but that a proof-of-concept is currently undergoing zero-g testing in orbit (according to the International Business Times, this test is taking place on the Tiangong 2 space station). If China is able to install EmDrives on its satellites for orbital maneuvering and altitude control, they would become cheaper and longer lasting. Li Feng, lead CAST designer for commercial satellites, states that the current EmDrive has only a thrust of single digit millinewtons, for orbital adjustment; a medium sized satellite needs 0.1-1 Newtons. A functional EmDrive would also open up new possibilities for long range Chinese interplanetary probes beyond the Asteroid belt, as well freeing up the mass taken up by fuel in manned spacecraft for other supplies and equipment to build lunar and Martian bases. On the military side of things, EmDrives could also be used to create stealthier, longer lasting Chinese surveillance satellites.
Oh hey, random Youtuber dismissed the claims because they don't match what he learned in high school physics or something, so screw empirical testing from NASA and CAST.
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It seems that every test of EM drives by credible scientific organizations so far has been successful.
I'm pretty sure that's not correct. Last time I read up on it there were reports that it worked, reports that it didn't, reports that it "worked" but didn't always thrust in the same direction, and one peer-reviewed paper reporting that it worked had to be retracted after the authors discovered errors in their analysis.
I hope it does work, and that it turns the laws of physics on their collective head. But it really sounds like they're just measuring noise.
Don't forget the report of FTL data transmission coming out of CERN a couple of years ago.
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Not good enough for the spaceship, though. Adding a substantial amount of mass to gain a couple of millinewtons of trust isn't too helpful.
Those millinewtons can be applied over a very long time though, allowing significant speeds to be achieved. Moreover, missions to far-away objects would no longer have their lifetimes limited by running out of fuel.
If it works how some scientists think it works, that would indeed rewrite the laws of physics as we know them -- and this is scary
Why is it scary? The physical models we have now are good enough for all of the machines that we've built (indeed, many of them are fine with models a few centuries old). Stuff isn't going to break as a result of this, but stuff that we'd previously thought was impossible now might turn out not to be and physicists have a lot more work to do to create models that explain them. That's a pretty exciting, but not very scary.
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Building something better generally requires both some experimentation and some theory. Even if you don't know why it works, reality demands you know basically how it works.
Ironically QM is the "classic" example of this. We don't know why superconductors work, but as we created better working theories on how it worked, we were able to discover better materials
I would say that perpetual motion machines are kind of scary.
It's a perpetual motion machine.
If you have an engine that produces one newton per 10W of input power, then move it 20 meters a second, you can extract 20W from this.
At 200m/s, 200W. Leaving 10 (or 190W) of free energy output after you subtract the first.
A major, major problem with the reports is that the results vary so much, and their error bars do not overlap.
If someone says "we have conclusively measured the height of Madonna, and she's 1.8m high +-0.1m" - that is one thing. If further investigation says "1.85+-.1" - then that's great, and is a confirmation.
If the next person to measure her comes out with 47m+-10m - then they have not meaningfully replicated the measurement, and disparity of measurements by various groups is a hallmark of something being wrong.
This, combined with the fact that some people don't get it to work at all leads to it being plausible that in fact nothing is happening.
The thrusts reported don't overlap.
It doesn't, it just changes the numbers at which breakeven occurs to ones not easy to achieve on earth.
Unless you get to 1N/300000000W (in which case it is a well understood photon drive)
Photons have no mass but they do have momentum. You could, in theory, make an engine by pointing a torch in the opposite direction to the way you want to go and turning it on. Such an engine would have no reaction mass but would not violate the law of conservation of momentum.
There are three possibilities as I see it:
1. The device doesn't work
2. Something with momentum is being ejected but we just haven't found out what yet
3. The law of conservation of momentum is wrong.
Of the three, I would happily bet my house that it is not the third one.
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Only if the theory that suggests constant thrust is true. One big weakness of all of the proposed models for observed thrust is that they're mostly utter rubbish. If there is thrust, it won't be via a perpetual-motion permitting mechanism.
The grandparent used mixed power and energy units but I think the problem is easier to grasp with work and energy:
If the reactionless drive is assumed to produce trust based on the power input only, the force produced F is some constant efficiency constant, C, times the power. Let's say C=0.000005 and F=0.000005 P.
Assume we run this engine for a time period of t. Then the total energy E spent is P.t.
The work done by the engine is force it produces F times displacement s. Note that there are no time units in the calculation. W=F.s , the same amount of work is done with a given F and s, regardless of how much time displacement s takes.
You can convert work into energy and vice versa, so if you get more work than the energy you put in, you have a perpetual motion machine. That is if Net Work=F.s-P.t is greater than 0.
The problem is the energy we put in to system directly depends on time, while the work we extract from it does not. As the engine goes faster P.t drops proportionally, while F.s stays constant. The required speed for Net Work being positive is v=P/F, that is v=(P.C)/P and that is v=1/C. With the efficiency figure you have given the breakeven speed is 200000m/s. That is a a high but physically possible speed. The efficiency could have been a thousand times less, and it would still be possible to make a perpetual motion machine.
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Actually we do, for reasons that have nothing whatsoever to do with the EMdrive. For one: General relativity and quantum mechanics are fundamentally incompatible - something about requiring different vacuum energy levels if I recall correctly. Which implies that there's a flaw in one or both of those theories. And there are other such situations as well. And then there's the various observational anomalies that we can't yet explain, which strongly suggest that either something in our current theories is wrong, or that there are other forces at work that we haven't yet discovered.
All of which is perfectly normal - such incompatibilities and unexplained anomalies are the guideposts that we use to probe deeper into the rules governing the universe, and improve our theories so that they better reflect reality. If the EmDrive does indeed work, which is looking increasingly likely, then it's simply one more unexplained anomaly. It's noteworthy only because (A) at first glance it seems to violate conservation of momentum, something that has gone basically unchallenged since Newton first formulated it, and (B) it has clear immediate applications.
And perhaps also because, given (B), we may for a time find ourselves in the situation of building and optimizing increasingly powerful and expensive engines without having any solid theory of the physics that governs their operation. Which has been extremely rare in recent centuries, though you could argue that much of modern pharmacology falls in that category. For example, we have only vague and overlapping theories as to how, exactly, hormonal birth control prevents pregnancy, and much the same can be said of many other medications - proving that they do work is far simpler than understanding exactly how they're interacting with the body's incredibly complex bio-chemical systems to achieve their results. On a more "technological" front gunpowder is the most recent example that springs to mind - it was used and optimized for centuries before chemistry even existed as a formal field of study, to say nothing of having a reasonably solid theoretical basis.
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A few reference points:
The big reason the EM Drive is attracting any interest at all is that it seems to be generating thrust per watt ratios at least an order of magnitude greater than theoretically possible with a photon rocket (i.e. shining a laser/flashlight/microwave/radio source out the back end), so there's no accepted theoretical basis for the thrust it's generating, unless maybe it's actually vaporizing itself and leaking pressurized metal gas out the back end
It would also seem to break conservation of momentum, which has gone basically unchallenged since Newton first formulated the laws of motion and kick-started physics as a mathematical science. If the engine thrust imparts momentum in one direction, then something else needs to gain an equal amount of momentum in the opposite direction (e.g. the exhaust from a rocket), and within our current understanding, that doesn't seem to be happening.
Finally, conservation of energy (Ein=Eout + heat) could also be broken by a reactionless thruster as it's heavily dependent on speed. If the EmDrive generates a constant thrust for a constant energy input as it accelerates (completely untested), then its kinetic energy will increase at a steadily increasing rate, since energy increases with the square of velocity. At some point the incremental increase in kinetic energy will be larger than the incremental consumption of electrical energy, at which point you could theoretically attach it to the rim of a wheel turning a generator to produce more energy than you're consuming. (normal rockets don't face this issue since the exhaust is being slowed down while the rocket accelerates, so the total kinetic energy change remains constant.)
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