Why Alfven Was Wrong
From January 20:
It's the physics they don't comprehend. I could never teach these folks a lick about math, but I can definitely teach them physics.
How about a quick show of hands, how many of the following individuals have actually read "Cosmic Plasma" by Hannes Alfven? ... Tim Thompson
You have asked this question about a bazillion times, keep getting the same answers, and then just ask it again like it's the first time. How many times do I have to tell you ... YES ... I have read the book and I have two copies of it in my physics library. I used the book as a reference when I was a graduate student.
So, how about a show of hands from Michael Mozina:
Have you read Magnetic Reconnection: MHD Theory and Practice by Priest & Forbes?
Have you read Nonlinear Magnetohydrodynamics by Deiter Biskamp?
Have you read Fundamentals of Plasma Physics by Paul Bellan?
Have you read The Physics of Plasmas by T.J.M. Boyd & J.J. Sanderson?
Have you read Plasma Physics for Astrophysics by Russell Kulsrud?
Have you read Plasma Astrophysics by Toshiki Tajima & Kazunari Shibata?
Have you read Conversations on Electric and Magnetic Fields in the Cosmos by Eugene Parker?
If you have not read any of these, can you tell us what plasma physics books, other than Alfven, you actually have read?
How many plasma physics classes have you taken?
How many plasma physics laboratory experiments have you performed yourself, or assisted with?
Mozina never responded. Let us be clear on something: it is the
physics that Mozina does not understand at any level. It is the
physics that Mozina constantly gets wrong. We can never teach Mozina physics because he is immune to learning, already quite convinced that his ignorance is in fact knowledge, he is beyond hope. But that need not apply to others reading this thread. So let me do something I said I would not do, since Mozina will never answer the questions above. Let me explore what
Hannes Alfven really says about magnetic reconnection (not surprisingly, Alfven
does not say what Mozina claims he says).
The following passages are transcribed from the book
Cosmic Plasma by Hannes Alfven; D. Reidel Publishing Co., 1981. All of the emphasis (
italics) are Alfven's from the original text, none are mine. First, Alfven addresses boundary conditions.
Page 28, section II.5.1 - Boundary Conditions
"Consider a volume V of space limited by a surface S. The properties of the plasma inside of S depend on the boundary conditions. Thus by changing the current through S, we can change the behavior of the plasma. As was seen in the preceding paragraph, the properties of the plasma depend on the whole circuit in which the current flows. This means that we can describe the plasma inside of S by parameters inside S only if i = 0 everywhere on S.
Hence, even if we know all plasma parameters (like density, temperature and magnetization) at every point inside S, we can describe the plasma properties theoretically only if there is no electric current crossing the surface. Therefore, the boundary problems have to be analysed very carefully."
This is a necessary consequence of Alfven's adoption of the circuit formalism for describing the physics of plasmas. He takes the position that it is not possible to properly describe the physics of plasmas by local variables alone, but that one must include the entire circuit, hence is requirement that no electric current can flow into or out of a given volume, if you want to use only local parameters to describe the plasma.
This leads into Alfven's specific statement about magnetic reconnection ...
Page 29, section II.5.3, - 'Magnetic Merging' Theories
"What we have found means that we can describe plasma phenomena inside a finite volume
only if no electric current crosses the surface. In the terminology of the magnetic field description, this means that we can describe plasma phenomena inside a finite volume only if
the perpendicular component of the curl is zero at every point of the surface."
"All theories of 'magnetic merging' (or 'field line reconnection') which do not satisfy this criterion are misleading or erroneous, and deserve no attention. This does not mean that
all papers in which 'magnetic merging' is used are of no interest, because there exist some good papers (e.g.,
Hill, 1975) in which the term is merely a synonym for "current sheet acceleration".
These two paragraphs reveal a serious weakness in Alfven's method, in the context of modern plasma physics, and reveal precisely why Alfven was in fact wrong to reject magnetic reconnection. His statement above was correct, and as far as I know remains correct to this day. He was right to reject magnetic reconnection in the context of the boundary conditions he laid out, but carefully note what Alfven actually says, in Alfven's own words: "
All theories of 'magnetic merging' (or 'field line reconnection') which do not satisfy this criterion are misleading or erroneous, and deserve no attention." Alfven did not categorically reject the idea of magnetic reconnection, he only rejected it in cases where his boundary conditions apply. The problem, of course, is that his boundary conditions can be quite unrealistic and do not apply in most cases to real plasma in space, and this is where Alfven fails:
Alfven's boundary condition requirements do not necessarily reflect the physical state of real plasmas, either in space, or in the laboratory.
Magnetic reconnection requires oppositely directed magnetic fields. Parallel magnetic fields cannot reconnect. Alfven's boundary conditions establish only parallel magnetic fields and therefore rule out at once all cases where one would expect magnetic reconnection to occur. Hence, Alfven is right to claim that magnetic reconnection will not occur when his boundary conditions are satisfied. But he was wrong to assume that his boundary conditions were physically realistic when applied to real space & astrophysical plasma. He was wrong to require all plasmas to satisfy boundary conditions which automatically rule out magnetic reconnection.
Alfven established faulty boundary conditions because he was guided by a faulty paradigm, namely the circuit paradigm for describing the physics of plasmas. That paradigm has serious limitations. It is still used by plasma physicists today, but has limited application. Alfven's reliance on this circuit formalism may have been good enough to establish the basics of plasma physics and the basics of ideal MHD, but was not good enough to handle the advances that have given us modern plasma physics. None of the modern plasma physics text books I have in my library apply the circuit analogy to plasma physics the way Alfven did. It simply does not apply to realistic space plasma physics in most cases, and we can in fact deal with
all cases today using the local parameters of the plasma without appealing to Alfven's limiting boundary conditions.
Now, let me add the caveat that I am not a specialist in plasma physics (although I actually have coauthored one paper in space plasma physics:
Bolton, et al., 1989). But I do have a master's degree in physics, I can read plasma physics text books, and most importantly of all, I know when to ask for help. So I acknowledge hints from
tusenfem on interpreting Alfven in this case. I don't think I am making any big mistakes here, but feel free to correct any errors (even the little ones).
Finally, once again, I direct the interested reader to the book
Magnetic Reconnection: MHD Theory and Application; Priest & Forbes, Cambridge University Press, 2000. Section 4.2 describes the Sweet-Parker mechanism, and section 4.3 describes the faster Petschek mechanism, both of which are reconnection mechanisms that take place in plasma environments which are not allowed to exist under Alfven's restrictive boundary conditions.
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