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This question seems to be a close parallel of this one, but it's about a different design by a different author.

I read a news article about a "helical engine" design by NASA engineer David Burns. The design allegedly exploits relativistic effects to seemingly violate conservation of momentum (as in the earlier question). It involves an oscillating mass which travels faster in one direction than the other, thus having higher relativistic mass on one side and imparting unbalanced momentum to its container.

But, Burns asks, what if the ring’s mass is much greater when it slides in one direction than the other? Then it would give the box a greater kick at one end than the other. Action would exceed reaction and the box would accelerate forwards (see video below).

Read more: https://www.newscientist.com/article/2218685-nasa-engineers-helical-engine-may-violate-the-laws-of-physics/#ixzz62dnGnjzh

Now, I'm not a physicist, but I'm just smart enough to have gotten the sense that this is almost certainly nonsense. As soon as I read "may violate the laws of physics" in the headline, in fact. Is it nonsense, and if so, what is unaccounted for in this model that would balance the momentum transfer at each side of each oscillation? How does the math play out?

TypeIA
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  • @Qmechanic As I said in the first sentence, this appears to be a different design by a different author than the one referenced in your linked question. Or am I mistaken on that? – TypeIA Oct 17 '19 at 19:26
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    This notion amounts to a claim that relativistic dynamics as it is usually understood is wrong. The mainstream understanding of special relativity is one that conserves momentum without question. – dmckee --- ex-moderator kitten Oct 17 '19 at 19:26
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    Yup, it's all wrong. – knzhou Oct 17 '19 at 19:32
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    Relativistic dynamics is different from Newtonian dynamics in a lot of ways. Force isn't parallel to acceleration anymore. Acceleration gets harder the faster an object moves. You can mimic some of these changes by just pretending that you still have Newtonian dynamics but the mass can change with velocity, but we don't use this language in physics because it's misleading -- there are many effects this idea can't capture. (For example, the fact that force is not parallel to acceleration.) It is a very fragile analogy that causes a lot of conceptual mistakes. – knzhou Oct 17 '19 at 19:32
  • It's easy to prove that Newtonian mechanics conserves momentum. It's also easy to prove that relativistic mechanics conserves momentum. A weird hybrid of Newtonian mechanics plus a magically changing "relativistic mass" does not conserve momentum, but nobody has ever thought this was a correct description of nature. It's the result of taking a helpful simplifying analogy too far. Any reactionless drive formulated using this Frankenstein theory, such as this one, doesn't work. – knzhou Oct 17 '19 at 19:33
  • Related: https://physics.stackexchange.com/q/185437/2451 , https://physics.stackexchange.com/q/129566/2451 – Qmechanic Oct 17 '19 at 19:33
  • If you just bounce an object back and forth, in either Newtonian mechanics or relativistic mechanics, you won't get anywhere. If you change the rest mass of the object to be different going right than left, you will go somewhere, but it will cost momentum to do this. So we haven't gotten thrust for free. – knzhou Oct 17 '19 at 19:33
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    Hi TypeIA: The question appears to be about non-mainstream physics, which is off-topic. – Qmechanic Oct 17 '19 at 19:38
  • @Qmechanic A published design by a NASA engineer isn't mainstream? I agree it's almost certainly bunk, but the question is about exactly why. Also, it's marked as a duplicate (rather than off topic) and it is not a duplicate. – TypeIA Oct 17 '19 at 19:40
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    Published in which journal? Peer-reviewed? – Qmechanic Oct 17 '19 at 19:42
  • @Qmechanic To my knowledge not peer reviewed. Is that a criterion for on-topic questions here? Also, again, it is not a duplicate, at least not of the linked questions. – TypeIA Oct 17 '19 at 19:50
  • I think that the question of how [physics.se] should respond to something-that-has-recently-made-a-big-splash-in-the-breathless-popsci-media-and-social-networks is probably better on meta then in the comments of a single question – dmckee --- ex-moderator kitten Oct 17 '19 at 20:12
  • NASA apparently has no peer review, minimal peer review, or really bad peer review, so engineers who want to play physicist can write whatever nonsense they want and post it on NASA servers. They seem unembarrassable. – G. Smith Oct 17 '19 at 23:34
  • @TypeIA see this answer. The idea was meant to be vetted in a meeting, as a simple form of peer review, in order to find the mistakes. The popular media turned it into clickbait. NASA's not at fault, it's The New Scientist who has done the disservice. They used to be good... – uhoh Oct 23 '19 at 03:16

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