The quest for a reactionless spacecraft thruster


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Typo: Ernst Mach, 1838 - 1916.

I just assumed he really knew his relativity!


Interesting. Sounds a lot more plausible than the emdrive thingee that’s been hyped so much lately–just waiting for that one’s supposed effects to be attributed to some uncaught environmental influence since it violates physics otherwise–this sounds completely within reason though. Surprised this effect of varying mass hasn’t been explored before–it seems sort of obvious once explained.

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Mind = blown. Very interesting theory, and an extremely well-written article.


Fascinating. Especially the part where becoming an academic physicist was an obstacle rather than a realistic step. I am very interested in scientific higher education for a variety of reasons, and I’ve found from personal experience involving friends (and the stats bear this out) that people drop out of the sciences like flies in undergrad and grad school, in large part because the pedagogical process sucks the enthusiasm right out of you.


Also, principal in place of principle. Are we pedants for even noticing? Or just people who care about the details?


A very intriguing article. One question regarding Mach’s though experiment though.

What if there was only one object in the universe? Mach argued that
it could not have a velocity, because according to the theory of
relativity, you need at least two objects before you can measure their
velocity relative to each other.
Taking this thought experiment a step further, if an object was
alone in the universe, and it had no velocity, it could not have a
measurable mass, because mass varies with velocity.

How can it be that the singular object has neither mass nor velocity? I can see how it has no measurable mass or velocity, as in order to take a measurement it has to be relatavistic, but the motion of atoms within (or even if the object was a single atom) would give it mass.
It would just be unknowable surely, not intrinsic.


It’s all in the details. Any grammar errors on a scientific-type piece render it worthy of pedantic scrutiny.
I’m surprised you picked that up though, and missed the intro sentence;
“For many decodes…”

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If you think of physics as a theory that’s supposed to give you ways to turn observations into predictions, then something completely unobservable has no place in it. This is more a question of terminology than anything else; but for many working physicists at least, unknowable quantities aren’t part of physics.
(This was a reply to pixieshifter above, not a comment on the article as a whole)

Tangent: It’s a problem that’s almost as old as the internet itself - reading pedantic grammar posts interrupts discussion about the topic at hand, but at the same time, those corrections cannot be dismissed entirely. I’m surprised that there hasn’t really been a technological solution to grammatical mistakes in blog posts, I’m not talking about auto-correct, I’m talking about a way of marking a comment as a grammatical correction (or a tangential comment, for that matter) and making it visible only to the mod, author and other people who care about those kinds of things. And as someone who has a real problem ignoring misplaced apostrophes, I can totally understand the need to say something about a typo. Maybe we should move this whole discussion to BBS suggestion box…


ah, but that would negate the thrill of being a pedant


Also, about 2/3 of the way down, there’s this: He has, after all, been pursuing this project without any outside help for a couple of decades.

It p’bly should read “without any outside help for a couple of decodes,” to remain consistent with the lede 'graph, “For many decodes, a fantasy among space enthusiasts has been to invent a device that produces a net thrust in one direction…”

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I don’t know about that, phase in quantum mechanics seems unavoidable but yet is not observable (for a single particle) as all phases (for a single particle) are gauge equivalent.


This is pretty fascinating, though it seems so simple (relatively) that one wonders why places like Lockheed Skunkworks (or let’s face it, Google) aren’t playing with it. Sort of a “what do they know that Woodward isn’t saying?” vibe.

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I don’t know the details of the experiments, but with a high-frequency reciprocating device, it’s probably important to rule the other effects that made the Dean drive seem to create reactionless thrust without, you know, actually creating reactionless thrust.

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What is with posts showing up once, and then, after returning, disappearing from the discussion, with part or all appearing in the writing box?

Except that this ability to vary mass in situ is non-trivial, and it seems (from the article) like it was almost unknown until Woodward showed that it could be done.

As far as I can tell, the whole question of whether it works is exactly tied to that single question – can mass really be changed back-and-forth in situ? If it can, the drive definitely works, as shown eloquently by the boxes-and-crank thought experiment.


Also, if the single object has no mass, would it suddenly gain mass if it split into two objects?

I think that’s a great idea. It could be like the Medium system where comments can be added to individual paragraphs, but they could just be for the mods/authors.

The question will then quickly be answered: to people point out grammatical errors because they want to see the article corrected, or because they want to tell other people that they spotted an error?