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The 'Impossible' EM Drive Being Tested By NASA May Finally Be Explained (technologyreview.com) 532

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.
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The 'Impossible' EM Drive Being Tested By NASA May Finally Be Explained

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  • Quantized inertia? (Score:5, Insightful)

    by Lisandro ( 799651 ) on Wednesday April 20, 2016 @11:39PM (#51953353)

    We'll eventually find out we really live in a simulation...

  • Great summary (Score:3, Insightful)

    by Anonymous Coward on Wednesday April 20, 2016 @11:39PM (#51953355)

    Great job lets not even try to attempt to summarize the article, instead lets post this like its a trailer for the 11 o'clock news!

  • Thanks, Summary (Score:5, Insightful)

    by ScentCone ( 795499 ) on Wednesday April 20, 2016 @11:39PM (#51953357)
    Really appreciate the complete lack of even a whiff of the explanation in the summary.
    • Re:Thanks, Summary (Score:5, Interesting)

      by Narcocide ( 102829 ) on Wednesday April 20, 2016 @11:43PM (#51953367) Homepage

      Short version: photons seem to have inertial mass after all.

      • Re:Thanks, Summary (Score:4, Interesting)

        by guruevi ( 827432 ) <evi AT evcircuits DOT com> on Thursday April 21, 2016 @12:42AM (#51953551) Homepage

        Not at all that. From what I can read from the (equally bad) article, the claimed effect may be due to the (currently unobserved) Unruh effect. The Unruh effect is a hypothetical black body radiation observed by an accelerating observer. Basically, if you were accelerating in reference to a "stationary" observer, you observe yourself heating up (very, very slightly) while the "stationary" observer would not see this heat.

        • Re:Thanks, Summary (Score:5, Informative)

          by Anonymous Coward on Thursday April 21, 2016 @02:35AM (#51953843)

          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.

          • The first isn't very far-fetched. They definitely do have energy-derived mass as per E=mc^2, and this mass supposedly is indistinguishable from other forms of mass.
            The other - we don't have solid proof either way, though the observations currently seem to go "against".

            • The first isn't very far-fetched. They definitely do have energy-derived mass as per E=mc^2, and this mass supposedly is indistinguishable from other forms of mass.

              No, they don't. Energy is not mass. That equation is actually incomplete: the full equation is E^2-p^2c^2=m^2c^4. For photons, E=pc, so their mass is zero.

              Now, they can contribute to the invariant mass of a system, but that's different from the photons themselves being massive, which we're almost sure they aren't, to an extremely high precision.

        • The article outright states (twice) that inertial mass is a required assumption of this theory.

          I actually think the article was unusually good, in that it struck a balance between elementary and oversimplified, and hyper-obscure. However, it did bury the lede a bit -- it also has a variable speed of light within the cone as an assumption.

      • Re:Thanks, Summary (Score:5, Interesting)

        by Anonymous Coward on Thursday April 21, 2016 @12:51AM (#51953587)

        Photons do not have rest mass, but they do carry both energy and momentum.

        A photon imparts a force and thus may transfer momentum onto a reflecting mirror. The Mirror has mass and is thus subject to this quantized acceleration.

        Also. The idea that the speed of light in a vacuum varies in a electromagnetic cavity is largely accepted as part of Quantum Electrodynamics.

        This is closely related the Casimir force that developed between two uncharged metal plates. Empty space is filled with a continuum of virtual electromagnetic modes ( field fluctuation of every size). As the plates come together more and more long wave modes are excluded from the vacuum between the plates... but not "outside" , thus Vacuum between the plates is literally more empty, and the plates experience an attractive force. This is crazy.. but it had been actually measured.
        Now....
        In accelerating reference frames some of the virtual modes of the vacuum are converted into real modes this is the"Unruh radiation"... thus space looks like a heat bath when your are accelerating. it is not so clear how time in a heat bath becomes the generator of momentum, and inertial mass, unless General Relativity makes the Radiation bath An-isotropic ( not the same in all directions). thus the radiation pressure form the thermal radiation retards acceleration, acting exactly like inertial mass..... Hmmmm

        so if you try to impart momentum on a small element embedded withing a larger cavity. The smallest change in momentum allowed is now a function of the size of the cavity , and lowest frequency mode that the cavity will support......

        This is right out of an area in physics call Quantum Cavity Electrodynamics ( Google it).... Cool idea There may be something to it... Hell, Why it does NOT work may also be just as interesting. I like it! It been a long time since the Physics degree.

        • The main trouble with all this talk is it doesn't mention amounts. One may be able to have all kinds of funny effects like Casimir and radiation behind the horizon and what not and it's interesting to figure out how momentum is conserved but to go from a measurable effect to 'let's use it for propulsion' is outright silly. The effect will be in the wrong ballpark and you'll be much better off by just using removing the back and using the actual microwaves for propulsion.

      • Short version: photons seem to have inertial mass after all.

        Slightly longer version: If the guy's model is a complete explanation of the measured thrust, then photons have inertial mass.

      • Very Serious Flaws (Score:5, Interesting)

        by Roger W Moore ( 538166 ) on Thursday April 21, 2016 @04:52AM (#51954179) Journal

        Short version: photons seem to have inertial mass after all.

        ...which raises some very serious questions such as why do we always observe photons as having the same speed regardless of frequency? In addition the proposed mechanism means that the quantization of inertia depends on the size of the universe. If this effect is observable today then shortly after the Big Bang the effect would have been incredibly huge due to the far, far smaller size of the universe. This raises serious questions bout the effect on nucleosynthesis etc. which Big Bang models without this physics appear to get right.

        You cannot just rewrite fundamental physics to fix one issue without also looking at the implications of your theory for other predictions which is it likely to change. Worse it seems that nobody has tested these drives for the emission of charged particles. A far, far simpler explanation is that this drive works by electron emission. There are a variety of way this can work which all work in a vacuum but whic would unfortunately not work in space where you are electrically isolated and would eventually build up a counter charge and cause the thrust to reduce to zero over time. This all uses established fundamental physics so it would be nice to see this ruled out BEFORE coming up with crazy new physics. It might be less exciting but it is better science.

        • Worse it seems that nobody has tested these drives for the emission of charged particles.

          Well, yes but there are many other confounding factors too. Both NASA Eagleworks and Dresden have measured forces in the 10 micronewton range. Problem is that the null test at Dresden gave an even larger force.

          20 micro newtons is TINY, and you somehow have to couple 700W into this thing without accidently coupling forces in magnetically or via thermal effects.

          By way of example, You will feel 20 micronewtons of force is

    • > If you bounced microwaves in a truncated cone, thrust would be produced out the open end. Most scientists have snorted at the idea

      I can see why porcine scientists would snort with approval. After all, if you bounce refrigerators around in a truncated cone (nozzle), they'll produce thrust as they exit the narrow end. If you bounce toasters around, they'll produce thrust out the nozzle. Same with coffee makers. Therefore, if you bounce microwaves around a nozzle ...

  • tl;dr (Score:5, Informative)

    by ArchieBunker ( 132337 ) on Wednesday April 20, 2016 @11:43PM (#51953365) Homepage
    • Re:tl;dr (Score:4, Funny)

      by Anonymous Coward on Thursday April 21, 2016 @02:52AM (#51953869)

      The hypothetical Unruh effect (or sometimes Fulling–Davies–Unruh effect) is the prediction that an accelerating observer will observe black-body radiation where an inertial observer would observe none. In other words, the background appears to be warm from an accelerating reference frame; in layman's terms, a thermometer waved around in empty space, subtracting any other contribution to its temperature, will record a non-zero temperature.

      That just blew my mind. It blew my mind so much that I can actually predict the future. I predict that I'm going to be in a bar one day, pretty snookered, and I'm going to be yelling about how, if you can hypothetically put me in a glass-encased vacuum at 0 degrees with a thermometer and an asbestos glove, I'll be holding the thermometer with the asbestos glove and waving it around like I'm trying to signal a passing ship and I'll look at the thing at it's going to read 0.1 degrees. But I won't know the name of what I'm describing, nor why it works. I'll bet a drink on it, though.

  • by Shawn Willden ( 2914343 ) on Wednesday April 20, 2016 @11:51PM (#51953393)

    I was honestly expecting to find an explanation of some subtle source of experimental error that covered it, not a possible theory explaining why it (maybe) works. I'm really looking forward to experimental testing of the improvements predicted by the theory. Who knows? With a decent explanatory theory, it might even be possible to turn it into a practical thruster. That would be awesome.

    • by meerling ( 1487879 ) on Thursday April 21, 2016 @01:43AM (#51953741)
      Awesome would be a epic understatement if it actually works and can be scaled up.
      Even if it can't be scaled up, it would be fantastic!
      I'm still worried it's a massive screwup that everybody repeated and nobody has found yet, but seems to be less and less likely. Still...
      • by Jeremi ( 14640 )

        There's only one thing to do... pack one of these things onto Elon's next rocket launch and see whether it can move itself around or not. The proof is in the pudding...

      • About a year ago I remember reading that the control experiment also produced thrust....
        http://phys.org/news/2015-07-s... [phys.org]

        Has something changed since then?

  • by dltaylor ( 7510 )

    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.

    • by Shadow99_1 ( 86250 ) <.moc.liamg. .ta. .99wodahseht.> on Thursday April 21, 2016 @12:20AM (#51953489)

      The problem was that even with all you just said the thrust was higher than they expected. Hence the big issue with the drive and why people said it couldn't work. Yet it kept working in real world tests.

    • 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.

      That's not what's theorized is happening here. I could try to summarize, but you'd be better off reading the article.

    • by Boronx ( 228853 )

      I don't think there's a nozzle.

    • by guruevi ( 827432 ) <evi AT evcircuits DOT com> on Thursday April 21, 2016 @12:52AM (#51953601) Homepage

      The summary of the article is very wrong, there is no nozzle. If it had a nozzle it would be easy to explain, anything with a nozzle will operate as a rocket regardless of the wavelength you produce (Newton's law about action/reaction) and laser/microwave drives with nozzles have been built, we already use ion drives after all.

      This 'engine' is completely closed. It's basically a closed cone in which you send microwaves and somehow you get acceleration. In Newtonian physics this would make no sense because it's a closed system, there is no "action" on the outside (basically the sum of all vectors of force generated come out to 0). However there seems to be something happening at the quantum level (the sum of all vectors is not 0 perhaps because at some quantized level there are hypothetically 'rounding errors').

    • by Namarrgon ( 105036 ) on Thursday April 21, 2016 @02:15AM (#51953815) Homepage

      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.

    • That's amazing! That's the same reason why my toast always lands peanut butter-side down!
  • by FreeBillClinton ( 2648747 ) on Thursday April 21, 2016 @12:04AM (#51953431)
    Have they tried analyzing this thing with an E-meter?
  • FTFY summary (Score:5, Informative)

    by Okian Warrior ( 537106 ) on Thursday April 21, 2016 @12:12AM (#51953467) Homepage Journal

    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 [wikipedia.org], 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 [wikipedia.org], 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.

    • Re:FTFY summary (Score:5, Informative)

      by ljw1004 ( 764174 ) on Thursday April 21, 2016 @12:28AM (#51953509)

      The author's paper on the EM-drive is here: http://arxiv.org/abs/1604.0344... [arxiv.org]

      I was interested to read that he claims his theory also explains galactic rotation without the need for dark matter, and it explains cosmic acceleration without the need for dark energy. Neither of those can be experimentally verified, so he's pretty excited to have an actual experiment to test his theories.

      • by mbkennel ( 97636 ) on Thursday April 21, 2016 @03:00AM (#51953889)
        It looks like nonsense because it treats photons as if they were Newtonian particles and with ignorance of Maxwell's equations and relativity.

        Start with section 2. It treats photons as particles with some momentum m*v. I mean, what? That's just wrong. Photons are relativistic p = E/c and quantum mechanical, E = 2\pi hbar f.

        I mean take a look at this:

        "Normally, of course, photons are not supposed to have inertial mass in this way,
        but here this is assumed. It is not clear what the size of this mass is, but it is
        clear for example that light inside a mirrored box produces a kind of inertial mass
        for the box. "

        So in orthodox physics, photons are not supposed to have inertial mass, but also in orthodox physics light makes inertial mass and it's clear that it's so.

        The second statement, about light inside a mirrored box, is so because of relativity and the assertion of the equivalence principle. Electromagnetic fields are part of the stress energy tensor (following Maxwell) which feeds into the source term of general relativity. So yes, there is some sort of inertial contribution, but in fact it can be computed pretty exactly, and it's extraordinarily tiny, and really mostly related to the energy density of the EM field.

        So relativity sometimes, but not other times? WTF?

        And if the non-standard theory that inertia comes from matter interacting with Unruh radiation, how exactly does that work with photons? Photons don't interact with photons. Zero cross section until the point that they are so energetic they can pop out electron/positron pairs from the vacuum, which is so far not an experimentally accessible regime.

        Presumably the idea is that the Unruh radiation inside the cavity is quantized in a particular way different from free space, but wouldn't that mean that inertia of (presumably charged) particles inside that cavity would be altered? But he was talking about the non-sensical 'inertial mass' of the photons themselves. WTF?

        I don't mind non-standard theories and their exploration at all, but it's necessary to be clear which standard axioms are being rejected and which others are preserved, and follow that consistently. I just saw very unclear physics.
        • You can't approach these topics pounding a pulpit of 2nd year college physics.

          Unbound photons might indeed have a rest mass, and not move at C (the speed of propagation of space-time ripples). Physicists are more careful than to say it must be zero by postulate, and so experiments have been done to set upper bound on photon rest mass such as observing the galactic potential vector and galactic plasma (so less than 3E-27 eV/C^2 by the way). Also worth mentioning that photons inside superconductors have n

    • Re:FTFY summary (Score:5, Interesting)

      by myowntrueself ( 607117 ) on Thursday April 21, 2016 @12:47AM (#51953573)

      Second, if you want to appeal to Noether's theorem [wikipedia.org], 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...)

      Theres something about this that reminds me of Zenos paradoxes.

      The Eleatics had the idea that they could logically prove that reality is nothing like anything we can imagine.

      First, if you assume that space or time are discrete, you are led to paradox. So they can't be discrete.
      Second, if you assume that space or time are continuous, you are led to paradox. So they can't be continuous.
      So space and time can be neither continuous nor discrete, nor can they be both continuous and discrete.
      What is left? Nothing we can imagine. There appears to be no other option than continuous or discrete.
      Therefore time and space, reality, must be something unimaginable.

      Yes, there were people thinking like this a very very long time ago.

      • Yes, there were people thinking like this a very very long time ago.

        do you think it's more interesting/impressive/etc that they managed to think of these things without the complex mathematics, or that we're able to think of them in terms of complex math?

      • What paradox do you run into if space and time are discrete?
        • What paradox do you run into if space and time are discrete?

          They go through a whole series of examples testing space being continuous, space being discrete, time being continuous, time being discrete, various combinations. Its really complicated and not something I'd go into detail here.

          I'm just saying they used fairly sophisticated methods of reasoning to arrive at a fairly sophisticated and bewildering conclusion; that the underlying structure of space and time are nothing like anything we can conceive of.

          Whether theres any flaw in the examples they give and the s

    • by guruevi ( 827432 )

      a) The experts have no idea
      b) It's always good for lay people to discuss and perhaps even find out more about this, perhaps they can learn or even educate or become the next Einstein
      c) Most 'experts', especially the ones journalists quote, simply rely on Newtonian physics to explain things which is fine for most things in and around our solar system but not at either end of the scales
      d) There are a number of valid theories these days about physics. There should be nothing "more sophisticated", for a physics

  • Farnsworth (Score:3, Funny)

    by Anonymous Coward on Thursday April 21, 2016 @12:20AM (#51953491)

    The ship stays where it is and the engines move the universe around it

    • The ship stays where it is and the engines move the universe around it

      Scotty: It never occurred to me to think of Space as the thing that was moving!

  • The engine appears to work by utilizing a Maxwell's demon.
  • But the universe won't notice the maintenance downtime to support development of universe 2.0.
    Me.

    We apologize for the inconvenience.
    God.

  • What does this statement mean?

    At very small accelerations, the wavelengths become so large they can no longer fit in the observable universe. When this happens, inertia can take only certain whole-wavelength values and so jumps from one value to the next.

    Reading the reference http://arxivblog.com/?p=207 [arxivblog.com] didn't help me understand.

  • Turtleology (Score:5, Funny)

    by Tablizer ( 95088 ) on Thursday April 21, 2016 @05:22AM (#51954247) Journal

    The lopsided nature of the cone causes the Ether Turtle's shell to become warmer than its belly. This difference is uncomfortable to reptiles and makes it shift around a bit, causing the turtle underneath to adjust to compensate, in turn triggering a similar re-shuffling of the turtle below it, and so on all the way down, causing the universe to shift position relative to the probe.

  • by Bill Hayden ( 649193 ) on Thursday April 21, 2016 @09:47AM (#51955227) Homepage

    What's that you say, an actual News for Nerds story? Bravo, Slashdot!

  • by Pseudonymous Powers ( 4097097 ) on Thursday April 21, 2016 @10:29AM (#51955541)

    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?

  • by mveloso ( 325617 ) on Thursday April 21, 2016 @11:33AM (#51956015)

    This "thing" shows the interesting interaction between the engineering community and the scientific community. This is why you should take scientist statements with a grain of salt.

    Engineers: look, this works!
    Scientists: that violates the laws of science and is impossible.
    Engineers: who gives a sh*t what you think? Here's the data
    Scientists: the data must be wrong
    Engineers: you try it
    Scientists: we have no f*cking idea what's happening, but it's happening
    Engineers: f*cking pinheads
    Scientists: oh, maybe this is what's happening

    If you take scientists too seriously, you never get past step #1.

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