# Closed system thrust? **URL:** \ **Category:** Factual Questions\ **Created:** [October 30, 2009, 4:32am UTC](https://boards.straightdope.com/t/closed-system-thrust/515700 "2009-10-30T04:32:59Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [October 30, 2009, 4:32am UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/1 "2009-10-30T04:32:59Z")
Could someone please tell how(or even _if_) [this “EmDrive”](http://emdrive.com/) works? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 30, 2009, 4:37am UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/2 "2009-10-30T04:37:50Z")
There are two possibilities: One, it might shove a bunch of electromagnetic radiation out the back side, producing an utterly negligible, but not exactly zero, thrust via radiation pressure. Two, it might be a lying scam. And of course, if you have to ask which it is, then you should already know the answer. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 30, 2009, 4:41am UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/3 "2009-10-30T04:41:44Z")
> [@Chronos](#): > > There are two possibilities: > One, it might shove a bunch of electromagnetic radiation out the back side, producing an utterly negligible, but not exactly zero, thrust via radiation pressure. > Two, it might be a lying scam. > > And of course, if you have to ask which it is, then you should already know the answer. But you have to admit, it’s a a really cool looking steampunk thruster! ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [October 30, 2009, 4:43am UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/4 "2009-10-30T04:43:38Z")
> [@beowulff](#): > > But you have to admit, it’s a a really cool looking steampunk thruster! More like a steampunk kettle drum. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [October 30, 2009, 1:01pm UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/5 "2009-10-30T13:01:50Z")
A few months ago, Scientific American had an article about the possibility of a reactionless drive. It seems correct, AFAIK. It is inconsistent with Newtonian mechanics, of course, but apparently is quite possible in a gravitational field, thanks to general relativity. One way of thinking about it is to think of how a skater speeds up a spin by pulling her arms in. Well, in curved space you can also get linear motion. Outtasight! ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [October 30, 2009, 1:02pm UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/6 "2009-10-30T13:02:47Z")
> [@Chronos](#): > > it might shove a bunch of electromagnetic radiation out the back side, producing an utterly negligible, but not exactly zero, thrust via radiation pressure. This seems to be more or less what their [Theory page](http://emdrive.com/principle.html) implies. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [October 30, 2009, 9:08pm UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/7 "2009-10-30T21:08:19Z")
> [@Hari\_Seldon](#): > > A few months ago, Scientific American had an article about the possibility of a reactionless drive. From reading this article it sounded like it would be pretty easy to make a reactionless drive out of two wheels mounted so their edges almost touched, and spinning in opposite directions, like two meshed gears. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 31, 2009, 12:45am UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/8 "2009-10-31T00:45:51Z")
I would basically just completely ignore Scientific American as a reputable source nowadays. One of the last issues before I canceled my subscription had a blurb about a device that supposedly would convert gravitational waves to electromagnetic, or vice-versa, at 50% efficiency (if you ignore those pesky factors of m/e), and a few months before that, Milgrom’s model of modified Newtonian dynamics (which isn’t _quite_ crackpot, but is highly implausible, and not taken seriously by pretty much anyone other than Milgrom) made the cover. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [October 31, 2009, 2:01am UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/9 "2009-10-31T02:01:54Z")
> [@Chronos](#): > > There are two possibilities: > One, it might shove a bunch of electromagnetic radiation out the back side, producing an utterly negligible, but not exactly zero, thrust via radiation pressure. > Two, it might be a lying scam. > > And of course, if you have to ask which it is, then you should already know the answer. Here’s an interesting article by physicist John Cramer about the first possibility (the article agrees with Chronos regarding the practicality of the idea) [Alternate View Column AV-82](http://www.npl.washington.edu/AV/altvw82.html) ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [October 31, 2009, 10:57am UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/10 "2009-10-31T10:57:48Z")
> [@Chronos](#): > > I would basically just completely ignore Scientific American as a reputable source nowadays. One of the last issues before I canceled my subscription had a blurb about a device that supposedly would convert gravitational waves to electromagnetic, or vice-versa, at 50% efficiency (if you ignore those pesky factors of m/e), and a few months before that, Milgrom’s model of modified Newtonian dynamics (which isn’t _quite_ crackpot, but is highly implausible, and not taken seriously by pretty much anyone other than Milgrom) made the cover. That may be the case. Milgrom’s piece was presented as a wild speculation, which is fair enough. The piece I mentioned was not and you would have to explain why it is incorrect. It is certainly true that they are in decline. See my thread on MPSIMS: [SciAm ad for a spirit medium - Miscellaneous and Personal Stuff I Must Share - Straight Dope Message Board](http://boards.straightdope.com/sdmb/showthread.php?t=537923) ---
**Author:** ![Nancarrow](https://avatars.discourse-cdn.com/v4/letter/n/c37758/32.png) [@Nancarrow](https://boards.straightdope.com/u/Nancarrow)\ **Post date:** [October 31, 2009, 11:35am UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/11 "2009-10-31T11:35:50Z")
> [@Hari\_Seldon](#): > > A few months ago, Scientific American had an article about the possibility of a reactionless drive. It seems correct, AFAIK. It is inconsistent with Newtonian mechanics, of course, but apparently is quite possible in a gravitational field, thanks to general relativity. Hmm. My (highly rusty) understanding of post-Newtonian physics is that conservation of momentum is one of the few bits of Newton\* that _doesn’t_ get thrown out, by SR, GR, QM or anything else. Is there an explanation up somewhere for how GR can violate CoM? \*essentially because it’s not so much a ‘bit of Newton’, except historically, but rather a consequence of the spatial invariance of the laws of physics. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [October 31, 2009, 12:06pm UTC](https://boards.straightdope.com/t/closed-system-thrust/515700/12 "2009-10-31T12:06:28Z")
It’s utter and complete bullshit. Supposedly, the drive generates thrust because the force exerted by the microwave radiation on the larger end of his cone-shaped resonating chamber exceeds that on the smaller end, while the forces on the walls apparently are supposed to cancel out. In real life, though, they’ll do anything but – in fact, they’ll exactly cancel out the force difference between the larger and smaller end, ensuring a net ‘thrust’ of exactly zero, as Newton would have us expect. This was pointed out by Dr. John P. Costella in [this paper](http://www.assassinationscience.com/johncostella/shawyerfraud.pdf) (PDF), where he specifically points to graphic 2.4 in [this version](http://www.newscientist.com/data/images/ns/av/shawyertheory.pdf) (PDF) of Shawyer’s ‘theory paper’, determining that ‘his [Shawyer’s] arrows are wrong’ – the forces acting on the cone walls, instead of being perpendicular to them as they ought to be, point parallel to the end plates of the cavity. Conspicuously, that graphic and the discussion accompanying it is missing in [this more recent version](http://www.emdrive.com/theorypaper9-4.pdf) (PDF) of Shawyer’s paper. So it’s a bit like filling that cone-shape with gas, and expecting the apparent force difference between front and end wall, generated by the same pressure acting on different areas, to produce motion.