6 ms·

I'm happy to discuss this stuff, but I think the conversation will be more pleasant if you avoid the snark, and maybe if you're a bit more willing to entertain

I'm happy to discuss this stuff, but I think the conversation will be more pleasant if you avoid the snark, and maybe if you're a bit more willing to entertain the possibility that someone who's spent their career studying a subject might have spotted things about it that you haven't recognized yet. In this particular case, my impression is that (like the paper's author) you haven't understood the core of the objection: that the non-zero field outside of the gap between the plates is important. In your derivation, you only address the exterior field long enough to say that it "quickly drops to zero". But we know that the electric potential far from the (finite!) plates approaches zero, even though it is high at the positive plate. So it's clear that even if the field outside is relatively small, its integral is not negligible: it adds up to exactly enough to cancel out all of the speed gained by an ion that started out exactly between the plates. If you're really determined on this, I can type up a sketch of the full calculation for a pair of circular plates. But the energy considerations ought to be enough to show what the ultimate result will be. (And, to be clear, I'm not even looking at pairs of charged particles with different masses: it doesn't even require that level of analysis. This is true for any one charged particle considered on its own.)

T-A 7y ago
> the conversation will be more pleasant if you avoid the snark After you. > maybe if you're a bit more willing to entertain the possibility that someone who's spent their career studying a subject You mean someone like Zubrin [1], who's spent decades designing space propulsion systems? Does it really seem plausible to you that he wouldn't understand conservation of momentum, as used e.g. to derive the rocket equation [2]? > In this particular case, my impression is that (like the paper's author) you haven't understood the core of the objection: that the non-zero field outside of the gap between the plates is important. That was the subject of the entire subthread which you just dismissed by instead incorrectly invoking conservation of momentum. > If you're really determined on this, I can type up a sketch of the full calculation for a pair of circular plates. Not really, because once you graduate from basic E&M, or just read the work which you decided to criticize without first bothering to read it, you learn about things like plasma screening [3], which (as also pointed out in the subthread which you dismissed) implies that you should not integrate out to infinity. [1] https://en.wikipedia.org/wiki/Robert_Zubrin https://en.wikipedia.org/wiki/Robert_Zubrin [2] https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation#Derivation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation#De... [3] https://en.wikipedia.org/wiki/Electric-field_screening https://en.wikipedia.org/wiki/Electric-field_screening
- Steuard 7y ago
  I'm very open to the possibility that I'm missing something important in all this. But I still don't feel like you've made any clear argument about why my energy reasoning about the fields outside of the parallel plates fails. Your initial response to me here didn't address the exterior field at all: your linked calculations (which you called "the full derivation") were explicitly just about fields between the plates, and in fact assumed the plates were infinite. You also claimed there that I had "failed to apply conservation of momentum correctly in the general case of two different masses", even though my objection had nothing to do with the mass difference. I do agree that the non-zero field outside of the gap was the subject of the previously linked subthread. But the discussion there was about "ballenarosada"'s (flawed) estimate of the exterior field: my overall claim (and theirs!) is that the specific functional form of that field can't matter, since we know from fundamental principles that its integral (the electric potential) must go to zero at infinity. You claimed there that if one neglects plasma screening, "you get a finite contribution which can be made arbitrarily small relative to the work done inside the capacitor, where the force is constant, simply by increasing the size of the capacitor." This claim is incorrect. Again, the easy way to prove that is just to note that the electric potential goes to zero at infinity. But if you want to actually consider the electric field due to a finite parallel plate capacitor, you can compute that easily enough along the axis of a pair of circular plates. As it must be, the integral from the midpoint to the positive plate is exactly equal and opposite to the integral from the positive plate to infinity (independent of the plate radius or the separation). Your final claim is that plasma screening means that integrating all the way to infinity isn't necessary. I'll admit that the details of that are well outside my area of expertise, and I'd appreciate hearing more about it. But my strong sense is that if the number of charged particles is finite, the fundamental result from basic electromagnetism must remain true regardless of screening effects: the electric potential at infinity must be zero. (Right??) So as I understand it, screening effects can make significant changes to the shape of the potential curve, but regardless of those details its values at the plate and at infinity remain unchanged. And that's all we need to conclude that there is no net thrust for an ion that doesn't change direction (like the ones that the author claims will produce thrust). In particular, if we imagine a positively charged plate in a plasma, I can easily imagine a negative screening charge accumulating right next to the plate, which would in a very short distance bring the exterior field to zero. But at the same time, that nearby accumulation of negative charge would strengthen the field right next to the plate, almost like a second parallel plate capacitor. The effect would be that positive ions outside of the positive plate would be strongly repelled, and would in turn push the device in exactly the opposite of the claimed direction of thrust. I'd love to learn more about the plasma physics of this situation. But I'm less interested in the specific details there than I am in understanding anything that would interfere with the basic energy analysis that I've outlined here and above.
ithkuil 7y ago
This exchange seems interesting. I'd really like to learn more
shoyer 7y ago
A very similar exchange with T-A happened back in the comment thread from last year: https://news.ycombinator.com/item?id=17432975 https://news.ycombinator.com/item?id=17432975 As you and several others pointed out last year, the energy considerations here are good enough to invalidate the scheme.
- T-A 7y ago
  > As you and several others pointed out last year, the energy considerations here are good enough to invalidate the scheme. The only "energy consideration" in that thread is this, from "ballenarosada": "The potential is 0 at infinity, positive at the first plate and negative at the second. An incoming ion starts at 0 potential, climbs a big hill to get through the first plate, then falls down below 0. Then on the way out it has to climb back to 0 potential at infinity. So the ions gain energy inside the plates but lose it all back on either side." In order to believe this to be correct, you must fail to understand that the device is electrically neutral. As you move away from it, the distance between the plates becomes negligible compared to your distance from it, and all you see is a neutral object exerting a negligible attraction on the ion. This is, of course, the principle behind multipole expansions [1]. "balenarosada" thought he or she could just use the dipole term of the expansion to model the capacitor, maybe misled by the name "dipole drive". In hindsight, maybe Zubrin should have gone with "capacitor drive" to prevent this misunderstanding. [1] https://en.wikipedia.org/wiki/Multipole_expansion https://en.wikipedia.org/wiki/Multipole_expansion

T-A 7y ago

> the conversation will be more pleasant if you avoid the snark After you. > maybe if you're a bit more willing to entertain the possibility that someone who's spent their career studying a subject You mean someone like Zubrin [1], who's spent decades designing space propulsion systems? Does it really seem plausible to you that he wouldn't understand conservation of momentum, as used e.g. to derive the rocket equation [2]? > In this particular case, my impression is that (like the paper's author) you haven't understood the core of the objection: that the non-zero field outside of the gap between the plates is important. That was the subject of the entire subthread which you just dismissed by instead incorrectly invoking conservation of momentum. > If you're really determined on this, I can type up a sketch of the full calculation for a pair of circular plates. Not really, because once you graduate from basic E&M, or just read the work which you decided to criticize without first bothering to read it, you learn about things like plasma screening [3], which (as also pointed out in the subthread which you dismissed) implies that you should not integrate out to infinity. [1] https://en.wikipedia.org/wiki/Robert_Zubrin https://en.wikipedia.org/wiki/Robert_Zubrin [2] https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation#Derivation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation#De... [3] https://en.wikipedia.org/wiki/Electric-field_screening https://en.wikipedia.org/wiki/Electric-field_screening

Steuard 7y ago
I'm very open to the possibility that I'm missing something important in all this. But I still don't feel like you've made any clear argument about why my energy reasoning about the fields outside of the parallel plates fails. Your initial response to me here didn't address the exterior field at all: your linked calculations (which you called "the full derivation") were explicitly just about fields between the plates, and in fact assumed the plates were infinite. You also claimed there that I had "failed to apply conservation of momentum correctly in the general case of two different masses", even though my objection had nothing to do with the mass difference. I do agree that the non-zero field outside of the gap was the subject of the previously linked subthread. But the discussion there was about "ballenarosada"'s (flawed) estimate of the exterior field: my overall claim (and theirs!) is that the specific functional form of that field can't matter, since we know from fundamental principles that its integral (the electric potential) must go to zero at infinity. You claimed there that if one neglects plasma screening, "you get a finite contribution which can be made arbitrarily small relative to the work done inside the capacitor, where the force is constant, simply by increasing the size of the capacitor." This claim is incorrect. Again, the easy way to prove that is just to note that the electric potential goes to zero at infinity. But if you want to actually consider the electric field due to a finite parallel plate capacitor, you can compute that easily enough along the axis of a pair of circular plates. As it must be, the integral from the midpoint to the positive plate is exactly equal and opposite to the integral from the positive plate to infinity (independent of the plate radius or the separation). Your final claim is that plasma screening means that integrating all the way to infinity isn't necessary. I'll admit that the details of that are well outside my area of expertise, and I'd appreciate hearing more about it. But my strong sense is that if the number of charged particles is finite, the fundamental result from basic electromagnetism must remain true regardless of screening effects: the electric potential at infinity must be zero. (Right??) So as I understand it, screening effects can make significant changes to the shape of the potential curve, but regardless of those details its values at the plate and at infinity remain unchanged. And that's all we need to conclude that there is no net thrust for an ion that doesn't change direction (like the ones that the author claims will produce thrust). In particular, if we imagine a positively charged plate in a plasma, I can easily imagine a negative screening charge accumulating right next to the plate, which would in a very short distance bring the exterior field to zero. But at the same time, that nearby accumulation of negative charge would strengthen the field right next to the plate, almost like a second parallel plate capacitor. The effect would be that positive ions outside of the positive plate would be strongly repelled, and would in turn push the device in exactly the opposite of the claimed direction of thrust. I'd love to learn more about the plasma physics of this situation. But I'm less interested in the specific details there than I am in understanding anything that would interfere with the basic energy analysis that I've outlined here and above.

ithkuil 7y ago

This exchange seems interesting. I'd really like to learn more

shoyer 7y ago

A very similar exchange with T-A happened back in the comment thread from last year: https://news.ycombinator.com/item?id=17432975 https://news.ycombinator.com/item?id=17432975 As you and several others pointed out last year, the energy considerations here are good enough to invalidate the scheme.

T-A 7y ago
> As you and several others pointed out last year, the energy considerations here are good enough to invalidate the scheme. The only "energy consideration" in that thread is this, from "ballenarosada": "The potential is 0 at infinity, positive at the first plate and negative at the second. An incoming ion starts at 0 potential, climbs a big hill to get through the first plate, then falls down below 0. Then on the way out it has to climb back to 0 potential at infinity. So the ions gain energy inside the plates but lose it all back on either side." In order to believe this to be correct, you must fail to understand that the device is electrically neutral. As you move away from it, the distance between the plates becomes negligible compared to your distance from it, and all you see is a neutral object exerting a negligible attraction on the ion. This is, of course, the principle behind multipole expansions [1]. "balenarosada" thought he or she could just use the dipole term of the expansion to model the capacitor, maybe misled by the name "dipole drive". In hindsight, maybe Zubrin should have gone with "capacitor drive" to prevent this misunderstanding. [1] https://en.wikipedia.org/wiki/Multipole_expansion https://en.wikipedia.org/wiki/Multipole_expansion