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Re: Energy



At 00:31 2001/09/23, Jim Green wrote:

<snip>

My observations -- for what they are worth:

There has been the thought that exact physics is not all that necessary for
most intro classes -- eg "energy flowing" is good enough. What a
frightening thought! It is true that our students have been prepared by
cartoons, Bill Nye, and under-prepared teachers. However, it is not
totally the teachers fault; they learned physics from like teachers; and
they from like teachers, and they...

Now good instructors will step in and try to stem this pitiful momentum

My limited understanding of QED theory is that most or all properties of
electromagnetic radiation, specifically including diffraction and
interference, can be explained by this theory using only the quantum
properties of particles (photons and electrons), without resorting in any
way to wave theories of light.

Thus I am puzzled; should good instructors step in and try to stem the
pitiful teaching of wave theories of light (including Maxwell's wave
equation) in undergraduate courses, and replace it with Feynman's more
exact physics, just because it /is/ more exact? Applying similar logic to
freshman mechanics, should Einstein's curvilinear spacetime supplant
Newton's absolute rectilinear universe, so that neophytes may learn the
whole complex truth all at once, instead of being exposed to it more
gently, in stages?

Such a response seems a bit Draconian to me. Over the past century,
cutting-edge physical theory has become so esoteric and (in many cases)
detached from everyday experience, that it would be difficult if not
impossible to teach introductory courses in a meaningful way without
glossing over some known truths (that is, deviating from the exact), at
least in the beginning.

When I introduce special relativity (the first topic in which older
physical models are revisited with an eye toward revision and enhancement),
I prepare my students by announcing that everything they have learned thus
far is, technically, incorrect, and that they will have to learn new and
deeper theories in order to advance their understanding. Some students are
shocked or dismayed by this declaration, as though I had blasphemed the
gospel; many do not relish the prospect of re-analyzing something they
thought they already knew. To calm them, I also remind them that physics is
not truth, but a model that we use to gain some understanding of truth.
Because it is a model, physics can never be completely exact (in the sense
that it provides a precise description of the universe), just as a model
airplane, no matter how sophisticated and true-to-life, will never embody
all the observed properties of a real airplane.

The unavoidable disconnect between our models and truth should not stop us
from studying our models, nor from building and embracing better models as
our knowledge increases. As long as we are willing to open our minds to new
ideas, our initial model need not, and almost certainly should not, be as
sophisticated and exact as later ones. We don't tell beginning students
that objects fall toward earth because they are following a spacetime
geodesic; they would go complain to the administration if we tried.
Instead, we give them a simpler but less exact explanation, hoping that
they will come back for a better explanation later when they have developed
the capability to absorb and digest it.

Even if the concept of "energy flow" (however one cares to define it) is
conclusively demonstrated to be merely a zero-order approximation to some
more fundamental truth, I see no harm in using it operationally (the same
way we use F = ma) until such time as a more fundamental truth (whatever
that may be; I haven't searched the list archives) can be more readily
introduced to, and its subtleties appreciated by, students.

--MB