Re: Fields, mechanical analogies, urban legends, etc.

["John S. Denker" <jsd@MONMOUTH.COM>]

Hi Folks --

I think this is how urban legends get started.  I am willing to bet a
rather large amount of money that the passage cited below is *not* a quote
from _The Feynman Lectures on Physics_ volume II chapter 27.

At 11:19 AM 2/15/99 -0700, Jim Green wrote:
> Let me start with William's Feynman quote:
>
> |  A reference: FIELD ENERGY AND FIELD MOMENTUM, The Feynman Lectures on
> |  Physics, ch 27, vol. II
> |
> |Same with grass-seeds which align when placed between the plates of a
> |capacitor with high P.D. between its plates: Something is there in empty
> |space, although it does not refect light and hence is not visible.  We
> |might mentally-model this "Something" as a population of (virtual?)
> |photons, or as e-fields and/or b-fields.  But just because the "Something"
> |is invisible, or because there are more than one model used to describe
> |it, this does not force us to declare that it is nothing but an abstract
> |concept.
>
> This seems to be offered sort of like a scriptural quote. -- just because
> the Great Feynman said something, it must be true.

To be fair to everybody:
 *) WB's original note did not explicitly claim that was a quote.
 *) OTOH the context was sufficiently ambiguous that reasonable people
could draw the wrong inference.

Moving forward:  This mailing-list has carried lots of discussion of how to
visualize the electrodynamic and/or fluid-dynamic fields.  Feynman gets
cited a lot, sometimes properly, sometimes not.
 *) It's true that Feynman often paints vivid, intuitive pictures of what's
going on.
 *) On the other hand, the vivid pictures are only one third of the story,
and it seems the other two thirds are getting trampled.

A good physics teacher uses a three-step process:
  1) You check the literature.  You do the calculations.  You do the
experiments.  You make darn sure you know what the right answer is.
  2) You describe the answer in terms the students can understand.
  3) You make sure they know the limits to the validity of the analogies
and approximations you used.

Students appreciate item (2) in the short run.  They will appreciate (1)
and (3) in the long run.

If you skip (1) and (3), you are fooling yourself and trifling with your
students.

Making intuitive leaps is easy.  Making *correct* intuitive leaps is much
harder.  Feynman, for one, knew the difference.  He explicitly discussed
the limitations of mechanical models of fields in:
  _The Feynman Lectures on Physics_ volume II chapter 1
   (section 1-5) "What are the Fields?"

Cheers --- jsd