The Dome Paradox: A Loophole in Newton's Laws
(youtube.com)10 points by ta8645 20 hours ago | 11 comments
10 points by ta8645 20 hours ago | 11 comments
jeffwass 19 hours ago | prev | next |
Is there a way to get the key points of this interesting topic without having to watch a 23 minute video?
gerikson 19 hours ago | root | parent |
I think this is the same concept
dventimi 19 hours ago | root | parent |
From the second paragraph:
A mass sits on a dome in a gravitational field. After remaining unchanged for an arbitrary time, it spontaneously moves in an arbitrary direction, with these indeterministic motions compatible with Newtonian mechanics
Well, no. "It" does not spontaneously move in an arbitrary direction. It remains in place forever.PeterWhittaker 18 hours ago | root | parent |
Not on Norton's Dome: it is the classic example of indeterminism in classical mechanics.
While QM is sufficient for indeterminism, it is not necessary, as this example shows.
Even in classical mechanics, physics is weirder than our intuition allows.
dventimi 17 hours ago | root | parent |
> Not on Norton's Dome
Really? Are you sure about that? Have you tried it? Where is this "Newton's Dome" so that the experiment can be replicated?
PeterWhittaker 11 hours ago | root | parent | next |
Not Newton's Dome, Norton's: https://en.m.wikipedia.org/wiki/Norton%27s_dome
mrkeen 12 hours ago | root | parent | prev |
> Well, no. "It" does not spontaneously move in an arbitrary direction. It remains in place forever.
Which experiments have you run forever?
dventimi 12 hours ago | root | parent |
I'll take that as a "no".
19 hours ago | prev |
dventimi 15 hours ago | next |
There are many good treatments of this supposed loophole. I happen to like this one:
https://blog.gruffdavies.com/2017/12/24/newtonian-physics-is...
It points out many flaws in Norton's reasoning, some fatal to his argument, some not. Putting it as simply as I can, Norton seems to claim that "Newton's Laws" are non-deterministic. That's not quite right. Rather, they are non-complete. I.e. they are incomplete. They're incomplete insofar as Newton's First Law ("An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force") establishes first-order and second-order derivatives (momentum and acceleration) as state variables but places no constraints on higher-order derivatives. However, higher-order derivatives are (as many as are needed) among a system's state variables. In many (but far from all), higher-order derivatives are zero and human experience with them is rare, so they're easy to overlook. Norton's (unphysical) Dome is a specific example of a general class of systems where higher-order derivatives are not zero. Given that, the two branches of Norton's equation of motion (for the stable and unstable trajectories) cannot both describe the same system (or the same particle) with the same set of state variables. That's the sleight-of-hand.
Again, all credit to Gareth Davies for working this out. I am absolutely not trying to pass off his work for my own. Just reporting it and trying to summarize it.