This is certainly the conventional view. But, the question is: Do the students of physics arrive at their views on these topics on their own accord, after digging into the arguments, evidence and history? Or, are certain key details which might alter a student’s educational trajectory simply left out of the picture painted for them? To what extent are physics students told to simply learn how to solve these particular problems, even when “real scientists” have disabused themselves of the methodology? And what impact, over time, might the repeated filtering of this kind of context have upon the students’ ability to question the validity of the things they are memorizing?
A relevant case in point is the story of Hannes Alfven. From David Talbott’s excellent biography of Alfven in Edge Science titled “The Plasma Universe of Hannes Alfven” …
Through much of the 19th and 20th century, most astronomers and
cosmologists had assumed the “vacuum” of space would not permit
electric currents. Later, when it was discovered that all of space is
a sea of electrically conductive plasma, the theorists reversed their
position, asserting that any charge separation would be immediately
neutralized. Here they found what they were looking for in Alfvén’s
frozen-in magnetic fields and in his magnetohydrodynamic equations.
Electric currents could then be viewed as strictly localized and
temporary phenomena—needed just long enough to create a magnetic
field, to magnetize plasma, a virtually “perfect” conductor.The underlying idea was that space could have been magnetized in
primordial times or in early stages of stellar and galactic evolution,
all under the control of higher-order kinetics and gravitational
dynamics. All large scale events in space could still be explained in
terms of disconnected islands, and it would only be necessary to look
inside the “islands” to discover localized electromagnetic events—no
larger electric currents or circuitry required. In this view,
popularly held today, we live in a “magnetic universe” (the title of
several recent books and articles), but not an electric universe. The
point was stated bluntly by the eminent solar physicist Eugene Parker,
“…No significant electric field can arise in the frame of reference of
the moving plasma.”But the critical turn in this story, the part almost never told within
the community of astronomers and astrophysicists, is that Alfvén came
to realize he had been mistaken. Ironically—and to his credit—Alfvén
used the occasion of his acceptance speech for the Nobel Prize to
plead with scientists to ignore his earlier work. Magnetic fields, he
said, are only part of the story. The electric currents that create
magnetic fields must not be overlooked, and attempts to model space
plasma in the absence of electric currents will set astronomy and
astrophysics on a course toward crisis, he said.
Alfven gave his Nobel lecture in 1970. And yet, we still see papers written by people who exhibit no affiliation to the theories under discussion, that criticize the ways in which the MHD equations are being applied.
From “Why Space Physics Needs to Go Beyond the MHD Box” by George K Parks in 2004:
Magnetohydrodynamic (MHD) theory has been used in space physics for
more than forty years, yet many important questions about space
plasmas remain unanswered. We still do not understand how the solar
wind is accelerated, how mass, momentum and energy are transported
into the magnetosphere and what mechanisms initiate substorms.
Questions have been raised from the beginning of the space era whether
MHD theory can describe correctly space plasmas that are collisionless
and rarely in thermal equilibrium. Ideal MHD fluids do not induce
electromotive force, hence they lose the capability to interact
electromagnetically. No currents and magnetic fields are generated,
rendering ideal MHD theory not very useful for space plasmas.
Observations from the plasma sheet are used as examples to show how
collisionless plasmas behave. Interpreting these observations using
MHD and ideal MHD concepts can lead to misleading conclusions.[…]
Serious objections have been raised from the beginning of the space
era about the application of MHD theory to collisionless space plasmas
(Chamberlain, 1960; Lemaire and Scherer, 1973; Heikkila, 1973, 1997;
Alfvén, 1977; Scudder, 1997; Lui, 2001; Song and Lysak, 2001).
Although it is well-known that MHD theory is applicable only to a
restricted class of plasma problems of which collisionless plasmas are
not a part (Krall and Trivelpiece, 1973), MHD and ideal MHD theories
have been used in space without due regard to these restrictions. MHD
theory is useful in the lower ionosphere and lower solar corona where
plasmas are collision dominated. However, plasmas in the solar wind
and magnetosphere are collisionless […] MHD theory will not describe
the physics of these plasmas correctly.Another issue in space physics is treating MHD fluids as ideal
(Parker, 1996). Ideal fluids have infinite conductivity (zero
resistance) and the implicit charge mobility prevents them from
supporting any electric field. The ideal fluid was originally
conceptualized by Alfvén (1953) to study how MHD waves would behave if
conductivity were imagined to be infinite. In such an ideal limit,
magnetic fields would become frozen in the fluid. However, the
frozen-in-field concept requires the strict criterion E · B = 0 which
is not always satisfied in space (Alfvén and Fälthammar, 1963; Alfvén,
1977; Fälthammar, 1989).
It seems to me that the “long-discredited idea” is the ways in which these cosmic plasma models continue to be applied. Unlike the “new things – which real and respectable scientists do all the time”, there is no risk that we might be wasting our time by modeling one of the universe’s fundamental states of matter more accurately, and in accordance with our observations of laboratory plasmas. This is not a gamble, and there’s very little mystery about the implications for some of us.
