Comment by knzhou | Hacker News Reader

Comment by knzhou | Hacker News Reader

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The description in the article is oversimplified.

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.

> For example, the fact that force is not parallel to acceleration.

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?

Conceptually it's because it gets much harder to speed something up, but not that much harder to change the direction of its velocity. So for a force with both radial (along velocity) and tangential (perpendicular to velocity) components, the acceleration points more tangentially than the force does.

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.

I would also say that I have been at least somewhat successful in teaching people about relativity by ttuncating the Lorentz transform to first order in β, as you can prove that you get the full results back from this as the matrix limit

                   N
    lim   [1  -φ/Ν]^
    N→∞   [-φ/Ν  1]

... basically the physical content of Galileo’s transform is that as you accelerate forwards things start moving backwards. The physical content of Lorentz’s transform adds just one thing to first order: that as you are accelerating, clocks ahead of you appear to tick faster, and clocks behind you slower, proportional to their distance in the direction you are accelerating.

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.)

https://en.wikipedia.org/wiki/Relativistic_mechanics#Force

This includes the relativistic version of Newton's second law, which shows that F is generally not parallel to a.

But note that the 4-force is parallel to the 4-acceleration. However, its spatial component only agrees with classical acceleration in the comoving frame. In general, there's an additional contribution that's however smaller by a factor of (v/c)^2.

"Oversimplified" is one way of putting it. I'd prefer to call it "simply incorrect". The manner in which the ring's rest mass is increased [decreased] is ignored[1]. If you don't ignore it, and apply conservation of (relativistic) angular momentum to the box-ring system, you'll see there's no reason to expect a net thrust at all.

[1] https://iopscience.iop.org/article/10.1088/1742-6596/1141/1/...

How could NASA allow research on such a topic if it is obvious for physicists that this is impossible ?

Even things we think we know very well may be wrong, so having a certain budget for "fringe" subjects which may yield unexpected results is not a bad idea. At the same time it's probably a good idea to assume everything they report is a measurement/test setup error until further proof.

Yup. As it is said "If a physicist says it's possible, then it's possible. If a physicist says it's impossible, then it's undecided."

The actual saying is that everything that an elderly distinguishes scientist has an opinion about is probably possible.

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

Everything in their field, that is. Since some elderly scientists have come up with some pretty nonsensical ideas about things outside their field.

I don't think they have in this case.

> Burns has worked on his design in private, without any sponsorship from NASA

> 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.)

It's always worth trying out new ideas. It's not like we're going to run out of paper or something.

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.

I always tell my kids: If you aren't failing, you aren't trying hard enough.

So it should not be possible to ride a swing?

In a swing, the back-and-forth linear momentum comes from the tension in the wires, while the energy comes from your legs. No conservation laws are broken.

It is not possible to ride an unanchored (and non-couterbalanced) swing in a vacuum.

What special properties does an atmosphere confer to a swing?

You need “downwards” acceleration for a swing. The vacuum bit isn’t important, it’s the gravity. They probably meant 0g rather than vacuum.

It slows you down :)

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.

Maybe they meant in 0g?

none, but centers of gravity do

If you can move levers, it's possible to rate your body. Maybe the inertia can be used as an anchor? Or you could use a sail.

A swing works by moving you back and forth relative to the Earth (and moving the Earth back and forth slightly as well, in the opposite direction, more or less.) The you + Earth system is not actually going anywhere. :-)

So you need to leave something behind in order to go somewhere? But it would only be relative to the thing I left behind? But then there is some "special" relativity that applies everywhere?

> So you need to leave something behind in order to go somewhere?

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).

If you tie a rope to a hammer, then spin it around as fast as you can like a hammer thrower, and drop it. Wouldn't the breaking force propel you in the same direction your let go of the hammer?

If you're in zero g and not on a surface with friction to push against, you're not going to be able to do what a hammer thrower can do standing on Earth.

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."

A swing would need gravity or some other driving forces for momentum.

riding bikes in 0g is also quite difficult, or so i hear.

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