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So far I've failed to grok the many arguments for and against this drive's "impossibility". I'd love to find an ELI5 for it. I think at this point, I'm resigne

by jchung 10y ago

So far I've failed to grok the many arguments for and against this drive's "impossibility". I'd love to find an ELI5 for it. I think at this point, I'm resigned to waiting for an emdrive cubesat to just prove it once and for all: http://www.popularmechanics.com/science/energy/a22678/em-drive-cannae-cubesat-reactionless/ http://www.popularmechanics.com/science/energy/a22678/em-dri...

cjensen 10y ago
Against is pretty simple Argument #1: it violates Newton's Third Law of Motion. Argument #2: every time they improve the test apparatus, the measured force declines and is "coincidentally" close to the measurement limits of the apparatus. Argument #3: It was originally designed based on a misunderstanding certain principles of Physics. Now they are trying to find a new explanation for why it works. What are the odds that you misunderstand physics, design something based on that misunderstanding, and that it then works in a barely-detectable way due to "unknown mechanism"?
- danielmorozoff 10y ago
  Aren't new physics drawn from violations of old laws?
  - mikeash 10y ago
    Yes, which is why violating a known law isn't automatically disqualifying. But it is highly suggestive, and requires extremely good evidence to surmount. So far, such evidence has not been forthcoming, so this remains a good reason to doubt. Every so often, something comes along that overturns an established law, like the photoelectric effect or the Michelson–Morley experiment. But the vast majority of the time, the laws remain intact and it's the claim which is somehow flawed. (See the Pioneer anomaly for a recent example where new physics was long considered as a potential explanation, but it ended up being a completely mundane effect.)
    - danielmorozoff 10y ago
      This is not the first experiment undertaken by Nasa/Pentagon/Boeing or the DoD. As well as similar measurements confirmed by other research teams (including foreign teams). I totally agree it requires drastic evidence, but it could be explained by another physical action not necessarily the violation of Newton's laws or effects that supersede those laws like quantum effects https://drive.google.com/file/d/0B7kgKijo-p0idV9tcmVIVzZrdTQ/view https://drive.google.com/file/d/0B7kgKijo-p0idV9tcmVIVzZrdTQ... http://www.ibtimes.co.uk/emdrive-exclusive-roger-shawyer-confirms-mod-dod-interested-controversial-space-propulsion-tech-1586392 http://www.ibtimes.co.uk/emdrive-exclusive-roger-shawyer-con... http://www.slideshare.net/KurtZeller/investigation-of-a-partially-loaded-resonant-cavityzellerkraft http://www.slideshare.net/KurtZeller/investigation-of-a-part...
      - mikeash 10y ago
        If it's another physical action within known physical laws, then nothing interesting is happening and it's a failure of the experimental setup. Yes, many experiments have been done, and none have conclusively disprove it, but at the same time none of them have conclusively shown that something is going on either. The detected effect remains well within the realm of potential error, so until that uncertainty is resolved, "error" remains a much better bet than "new physics."
        achileas 10y ago
        >If it's another physical action within known physical laws, then nothing interesting is happening and it's a failure of the experimental setup I'm not sure what makes you think this. There's no failure in science when an effect is observed and it's explainable within the frame of current theory. If that was the case every experiment I've done would have been a 'failure.'
        mikeash 10y ago
        With the experiments that showed a result, that result is not explainable within the frame of current theory. The whole purpose of the experimental setup is to isolate the test article from any possible interactions that could exert force on it, other than the one being tested. If the results seen so far are due to mundane interactions, then that isolation didn't work. Is that not a failure of the experimental setup?
  - Florin_Andrei 10y ago
    You have to look at it in terms of a barrier. It's not impossible to break it, but it requires a lot of "oompf". The greater the violation, the greater the "oompf" required. So, because this thing would change basically all Physics books, there's a huge burden on them to prove it beyond any shade of doubt. Forget about flirting with detection error; it should be as clear as the sun rising in the morning. Please note that relativity, big as is was, was only at a low-to-medium level of "oompf". It didn't change quite everything, far from it.
    - danielmorozoff 10y ago
      Though I totally understand your point. I would argue relativity was a tectonic shift in how we understand physics. These ideas existed before Einstein in mathematics/ geometry (Grassmann & Reimann). But they were just ideas and not a full-fledged theory.
- russdill 10y ago
  And don't forget, violating Newton's Third Law of Motion also violates the conservation of energy, linear momentum, angular momentum, and thermodynamics. https://www.lhup.edu/~dsimanek/museum/themes/NewtonsThird.htm https://www.lhup.edu/~dsimanek/museum/themes/NewtonsThird.ht...
DennisP 10y ago
There's a claim that it's just basic physics of microwaves bouncing around. That one's nonsense; from the article, "Woodward likened explaining the results seen at NASA purely in terms of microwave pressure to arguing that you can accelerate a car by getting in the driver’s seat and pushing on the windshield." As that suggests, the drive would violate conservation of momentum as we currently understand it. It would also violate conservation of energy. Because of relativity, there's no such thing as absolute velocity, so the thrust can't vary by velocity. Since kinetic energy is proportional to the square of the velocity, but the acceleration is constant for a constant energy input, there's some velocity at which you're gaining more energy than you're putting in. So if it does work, it'd be radically new physics. But there are various ideas for how it might work, like Woodward's, which is derived from general relativity and involves an explanation of inertia; he's been working for decades on reactionless drive experiments based on that idea. We don't currently have a generally-accepted explanation of inertia, so who knows.
- jwatte 10y ago
  I rather like the "everything is quantized and this is a result of accumulated truncation errors" theory.
Steuard 10y ago
The arguments against boil down to "Any reactionless drive must by definition violate conservation of momentum." We shouldn't reject any scientific conclusion as unquestionable gospel, of course, but off the top of my head I'm not sure that I can think of any fact about the laws of nature that I consider more firmly established than "momentum is locally conserved". (I'm a physics professor, for the record.) Conservation of momentum is part of the bedrock of everything from relativity to quantum field theory, so if you throw it out, you'll need to come up with entirely different (seriously: entirely different) replacements for those theories that nevertheless manage to reproduce their hundreds of enormously successful predictions (to just as many decimal points). Again, it could be true! Keep an open mind! But if it is, then literally every discovery in (say) particle physics in your lifetime has been a fluke: just dumb luck. You can decide how to balance that likelihood against the evidence presented thus far in favor of the drive. I'm really not sure what the argument against impossibility is, apart from a fervent longing for something of the sort to be real. (A longing that I share, for what it's worth.) They claim to have some data showing that something really is generating thrust, but as I recall some experiments have shown comparably large results in both the "actual test" and "control" conditions, so I'm not sure what to make of that evidence.
- wahern 10y ago
  But if it is, then literally every discovery in (say) particle physics in your lifetime has been a fluke: just dumb luck. Why couldn't conservation of momentum, as we understand it, be incomplete, like Newton's Laws, rather than wrong or coincidental? Note that I'm not grasping at straws wrt the EmDrive. The distinctions between incompleteness and error is far more interesting to me, especially in the context of science. And I'm curious if your statement was more than a rhetorical assertion.
- AnimalMuppet 10y ago
  > I'm really not sure what the argument against impossibility is... It's humility. It's admitting that there are things we don't know, and that some of them could give exceptions to the things that we do know. I think that "willing to be proven wrong, but requiring proof" is the correct stance.
dTal 10y ago
If you have an em-drive cubesat, just putting that (battery-powered, closed system) on a balance beam / torsion pendulum in a vacuum chamber is a more rigorous experiment than anything that's been done so far.

by jchung 10y ago

cjensen 10y ago

Against is pretty simple Argument #1: it violates Newton's Third Law of Motion. Argument #2: every time they improve the test apparatus, the measured force declines and is "coincidentally" close to the measurement limits of the apparatus. Argument #3: It was originally designed based on a misunderstanding certain principles of Physics. Now they are trying to find a new explanation for why it works. What are the odds that you misunderstand physics, design something based on that misunderstanding, and that it then works in a barely-detectable way due to "unknown mechanism"?

danielmorozoff 10y ago
Aren't new physics drawn from violations of old laws?
- mikeash 10y ago
  Yes, which is why violating a known law isn't automatically disqualifying. But it is highly suggestive, and requires extremely good evidence to surmount. So far, such evidence has not been forthcoming, so this remains a good reason to doubt. Every so often, something comes along that overturns an established law, like the photoelectric effect or the Michelson–Morley experiment. But the vast majority of the time, the laws remain intact and it's the claim which is somehow flawed. (See the Pioneer anomaly for a recent example where new physics was long considered as a potential explanation, but it ended up being a completely mundane effect.)
  - danielmorozoff 10y ago
    This is not the first experiment undertaken by Nasa/Pentagon/Boeing or the DoD. As well as similar measurements confirmed by other research teams (including foreign teams). I totally agree it requires drastic evidence, but it could be explained by another physical action not necessarily the violation of Newton's laws or effects that supersede those laws like quantum effects https://drive.google.com/file/d/0B7kgKijo-p0idV9tcmVIVzZrdTQ/view https://drive.google.com/file/d/0B7kgKijo-p0idV9tcmVIVzZrdTQ... http://www.ibtimes.co.uk/emdrive-exclusive-roger-shawyer-confirms-mod-dod-interested-controversial-space-propulsion-tech-1586392 http://www.ibtimes.co.uk/emdrive-exclusive-roger-shawyer-con... http://www.slideshare.net/KurtZeller/investigation-of-a-partially-loaded-resonant-cavityzellerkraft http://www.slideshare.net/KurtZeller/investigation-of-a-part...
    - mikeash 10y ago
      If it's another physical action within known physical laws, then nothing interesting is happening and it's a failure of the experimental setup. Yes, many experiments have been done, and none have conclusively disprove it, but at the same time none of them have conclusively shown that something is going on either. The detected effect remains well within the realm of potential error, so until that uncertainty is resolved, "error" remains a much better bet than "new physics."
      - achileas 10y ago
        >If it's another physical action within known physical laws, then nothing interesting is happening and it's a failure of the experimental setup I'm not sure what makes you think this. There's no failure in science when an effect is observed and it's explainable within the frame of current theory. If that was the case every experiment I've done would have been a 'failure.'
        mikeash 10y ago
        With the experiments that showed a result, that result is not explainable within the frame of current theory. The whole purpose of the experimental setup is to isolate the test article from any possible interactions that could exert force on it, other than the one being tested. If the results seen so far are due to mundane interactions, then that isolation didn't work. Is that not a failure of the experimental setup?
- Florin_Andrei 10y ago
  You have to look at it in terms of a barrier. It's not impossible to break it, but it requires a lot of "oompf". The greater the violation, the greater the "oompf" required. So, because this thing would change basically all Physics books, there's a huge burden on them to prove it beyond any shade of doubt. Forget about flirting with detection error; it should be as clear as the sun rising in the morning. Please note that relativity, big as is was, was only at a low-to-medium level of "oompf". It didn't change quite everything, far from it.
  - danielmorozoff 10y ago
    Though I totally understand your point. I would argue relativity was a tectonic shift in how we understand physics. These ideas existed before Einstein in mathematics/ geometry (Grassmann & Reimann). But they were just ideas and not a full-fledged theory.
russdill 10y ago
And don't forget, violating Newton's Third Law of Motion also violates the conservation of energy, linear momentum, angular momentum, and thermodynamics. https://www.lhup.edu/~dsimanek/museum/themes/NewtonsThird.htm https://www.lhup.edu/~dsimanek/museum/themes/NewtonsThird.ht...

DennisP 10y ago

There's a claim that it's just basic physics of microwaves bouncing around. That one's nonsense; from the article, "Woodward likened explaining the results seen at NASA purely in terms of microwave pressure to arguing that you can accelerate a car by getting in the driver’s seat and pushing on the windshield." As that suggests, the drive would violate conservation of momentum as we currently understand it. It would also violate conservation of energy. Because of relativity, there's no such thing as absolute velocity, so the thrust can't vary by velocity. Since kinetic energy is proportional to the square of the velocity, but the acceleration is constant for a constant energy input, there's some velocity at which you're gaining more energy than you're putting in. So if it does work, it'd be radically new physics. But there are various ideas for how it might work, like Woodward's, which is derived from general relativity and involves an explanation of inertia; he's been working for decades on reactionless drive experiments based on that idea. We don't currently have a generally-accepted explanation of inertia, so who knows.

jwatte 10y ago
I rather like the "everything is quantized and this is a result of accumulated truncation errors" theory.

Steuard 10y ago

The arguments against boil down to "Any reactionless drive must by definition violate conservation of momentum." We shouldn't reject any scientific conclusion as unquestionable gospel, of course, but off the top of my head I'm not sure that I can think of any fact about the laws of nature that I consider more firmly established than "momentum is locally conserved". (I'm a physics professor, for the record.) Conservation of momentum is part of the bedrock of everything from relativity to quantum field theory, so if you throw it out, you'll need to come up with entirely different (seriously: entirely different) replacements for those theories that nevertheless manage to reproduce their hundreds of enormously successful predictions (to just as many decimal points). Again, it could be true! Keep an open mind! But if it is, then literally every discovery in (say) particle physics in your lifetime has been a fluke: just dumb luck. You can decide how to balance that likelihood against the evidence presented thus far in favor of the drive. I'm really not sure what the argument against impossibility is, apart from a fervent longing for something of the sort to be real. (A longing that I share, for what it's worth.) They claim to have some data showing that something really is generating thrust, but as I recall some experiments have shown comparably large results in both the "actual test" and "control" conditions, so I'm not sure what to make of that evidence.

wahern 10y ago
But if it is, then literally every discovery in (say) particle physics in your lifetime has been a fluke: just dumb luck. Why couldn't conservation of momentum, as we understand it, be incomplete, like Newton's Laws, rather than wrong or coincidental? Note that I'm not grasping at straws wrt the EmDrive. The distinctions between incompleteness and error is far more interesting to me, especially in the context of science. And I'm curious if your statement was more than a rhetorical assertion.
AnimalMuppet 10y ago
> I'm really not sure what the argument against impossibility is... It's humility. It's admitting that there are things we don't know, and that some of them could give exceptions to the things that we do know. I think that "willing to be proven wrong, but requiring proof" is the correct stance.

dTal 10y ago

If you have an em-drive cubesat, just putting that (battery-powered, closed system) on a balance beam / torsion pendulum in a vacuum chamber is a more rigorous experiment than anything that's been done so far.