[FRIAM] Least Action

[Pieter Steenekamp]

Exactly, Dave! Modern physics is like that one weird friend who always gets
everything right but refuses to explain why. It’s undeniably the most
accurate model for predicting how known things interact with other known
things in the universe—because, well, it just *works*. But if you're
feeling brave and insist on asking *why*, you're free to philosophize,
speculate, or even perform interpretive dance about it. Just don’t expect
any satisfying, intuitive answers—none have RSVP’d yet.

On Wed, 12 Mar 2025 at 21:46, Prof David West <profwest at fastmail.fm> wrote:

> everyone knows that balls can read minds—especially that of the thrower.
> So it knows the intended end point. Any deviance from that is just willful
> disobedience. Least action is just the ball's laziness.
>
> davew
>
>
> On Wed, Mar 12, 2025, at 10:44 AM, Pieter Steenekamp wrote:
>
> There's a *"nice"* layman’s explanation of the principle of *least action*
> (https://www.youtube.com/watch?v=qJZ1Ez28C-A)—though I don’t quite agree
> with it. (It does, however, include a rather neat explanation of quantum
> mechanics that I find useful—but that’s another discussion.)
>
> Back in engineering school, when calculating trajectories, we relied
> entirely on Newtonian mechanics, applying it so relentlessly in
> problem-solving that it became second nature. Later, I encountered the
> principle of *least action* and its claim to be more fundamental than
> Newton’s laws.
>
> A common example used to illustrate this principle goes like this:
> If someone throws a ball from point A to point B, the ball *evaluates* all
> possible paths and then follows the one of least action.
>
> This framing presents a problem. Here’s my perspective:
> If a person throws a ball from point A and it *happens* to land at point
> B, a post-mortem analysis will confirm that it followed the path of least
> action. But that’s an observation, not a mechanism.
>
> The distinction is subtle but important. In both cases, when the ball
> leaves the thrower’s hand, it has no knowledge of where it will land. Throw
> a thousand balls with slightly different angles and velocities, and they’ll
> land in a distribution around B. Yet the layman’s explanation suggests that
> each ball somehow *knows* its endpoint in advance and selects the
> least-action trajectory accordingly.
>
> I don’t buy that.
>
> My view (and I welcome correction) is that the ball simply follows
> Newton’s laws (or the least action laws) step by step. It doesn’t *choose*
> a trajectory—it merely responds to the local forces acting on it at every
> instant. Once it reaches its final position, we can look back and confirm
> that it followed the least-action path, but that’s a retrospective
> conclusion, not a guiding principle.
>
> Ultimately, in this context, Newton’s laws and the least-action principle
> are equivalent descriptions of the same physics—neither requires the system
> to "know" its endpoint in advance.
>
>
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