I like Brian's straw man. We have two observers, each driving an
automobile. They are approaching one another. Consider the system
which consists of the two automobiles, less their drivers. In each
driver's frame the other driver's automobile will be the location
of the majority of energy in the system. The two drivers cannot
agree on which automobile has the greater energy. I think that
difficulty offends my sense of reality.
This flowing energy has an explanatory utility, I agree. We are
discussing its corporeal entity, however. It may be useful to
think of energy as flowing without changing magnitude. The end
result is conservation. Pushing this useful idea to the extreme of
imbuing it with reality gave us the very useful theory of caloric.
There have been improvements since, of course, but still there is
no indication that energy exists as a corporeal entity, and there
is no requirement in Nature for its existence.
John Denker has asked if I consider everyone who speaks of energy
density to be in error. Of course I do not; figures of speech and
conventional conveniences are useful. Energy density is a useful
concept. John, please tell me how you view the energy density in a
gravitational field.
There are many conservation laws in Nature. Is linear momentum a
substantial entity? Angular momentum? No, neither is real. When a
gravitational system contains just two bodies we know the familiar
conserved quantities serve well. There are *ten* known scalar
quantities that are conserved in celestial mechanical systems.
Three are linear momentum components, three angular momentum
components, total energy, and three others called "integrals of
area". All are invariant, and none is substantial.