This is the first I’ve heard of this and I would like to learn more, partly because of my own harebrained not-quite-reactionless engine idea that I know can’t possibly work really, but I can’t see why yet.
Where should I look?
A really excellent, conceptually clear exposition of relativity is given in chapters 11 and 12 of Morin's Mechanics. The modified version of F=ma is covered in section 12.5.
N
lim [1 -φ/Ν]^
N→∞ [-φ/Ν 1]
That key basic insight is not just the resolution to the twin paradox but the basis for event horizons near black holes and such. (There is a surface at coordinate -c²/a where time stands still.)
This includes the relativistic version of Newton's second law, which shows that F is generally not parallel to a.
[1] https://iopscience.iop.org/article/10.1088/1742-6596/1141/1/...
It's a complicated way if saying almost everything is possible so don't believe old dogs who have given up on optimism. https://www.brainyquote.com/quotes/arthur_c_clarke_100793
> Burns has worked on his design in private, without any sponsorship from NASA
Seems like they didn't, though as usual the article completely buries that information.
I kinda wish NASA had a policy like Disney, where you can't use your affiliation with them in association with personal projects (though for a different reason.)
My only complaint is that publicity is disproportionately allocated to the studies least likely to be right. While paper isn't limited, room in the newspaper is.
The vacuum part was an emphasis on requiring a counterpart to change momentum.
In an atmosphere you could flap your wings, or use electric fans, and argue what it means to ride a swing, but it would be besides the point, as in an atmosphere there is no need for propellant-less engines from the article.
As a definition, no. But no one has been able to figure out reactionless propulsion; as far as anyone can tell, the only way to accelerate in space is to throw something in the opposite direction (matter or light).
But, if you provide some momentum to an object like a hammer and release it, you will move. That goes back to "you need to leave something behind in order to go somewhere."
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.
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 doesn't work.
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, as explained in gus_massa's comments. So we haven't gotten thrust for free.