Thunderbolts of the Gods

Zeuzzz said:
I couldn't find the post you are referring to. Is it in this thread? who posted it?
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Zig did. Find it yourself.

[latex]$\bf{B} = a x \hat y + b y \hat x$[/latex]

The magnetic field lines cross/reconnect at x=y=0. Start with a>b; increase b until b>a, and many lines have reconnected.

My total ignorance of the well established properties of magnetism and electromagnetics.
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Fixed that for you.
 
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The simple reason for that is that things that dont exist in the real world cant reconnect. A field line is a locus that is defined by a vector field, it has no substance and so can have no properties. Field lines can be used to map out fields, but the actual lines do not exist.

sol invictus said:
I have no idea what you're talking about.
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I gathered.

Do you understand the difference between a conceptual construct created as convenient tools for visualizing a process and the physical process itself? You do realize that magnetic fields are not actually 'made of' of field lines, don't you?

The field lines are useful tools that we put in to help model fields. As such, they can not do anything 'physical' like reconnect or merge, because they really do not exist. You might say that the field lines are what cause the particle to move, but that is completely wrong, its the magnetic field that causes the particle to move, the field lines are just put in by us to help understand the movement.

The concept of field lines exists only in one’s mind. It does not exist in three-dimensional space.
 
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Zeuzzz said:
I couldn't find the post you are referring to. Is it in this thread? who posted it?
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The posts are here and here.

I used slight different notation than Sol, but it's the same field.

Magnetic recconection is also based on some assumptions that are not 100% accurate.
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As most complex physics models do, and as pretty much any numerical simulation does. So?

It relies on ideal magnetohydrodynamics where magnetic field lines are 'frozen' into the plasma, making it infinitely conductive, so magnetic fields get “frozen into” it.
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This is obviously false. In an ideal conductor, the magnetic field cannot change at all, under any circumstances. Magnetic reconnection therefore obviously requires that the plasma not be an ideal conductor.
 
robinson said:
I have to take issue with your contention that field lines "do not exist". I believe they do, based on my own experiments. As to these lines "breaking, and reconnecting", I agree that it certainly violates known physics.
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I think the argument is that they exist only as an arbitrary construct, like pressure lines on a weather map, or topographical lines on a relief map. Richard Feynman notes in his Feynman Lectures on Physics (Vol.2, 1-2):

Richard Feynam (Fair Use extract) said:
There have been various inventions to help the mind visualize the behavior of fields. The most correct is also the most abstract: we simply consider the fields as mathematical functions of position and time. We can also attempt to get a mental picture of the field by drawing vectors at many points in space, each of which gives the field strength and direction at that point. Such a representation is shown in Fig. I–I. We can go further, however, and draw lines which are everywhere tangent to the vectors — which, so to speak, follow the arrows and keep track of the direction of the field. When we do this we lose track of the lengths of the vectors, but we can keep track of the strength of the field by drawing the lines far apart when the field is weak and close together when it is strong. We adopt the convention that the number of lines per unit area at right angles to the lines is proportional to the field strength. This is, of course, only an approximation, and it will require, in general, that new lines sometimes start up in order to keep the number up to the strength of the field. The field of Fig. 1-1 is represented by field lines in Fig. 1-2. (emphasis in original)
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Although his diagrams are not reproduced here, it is interesting that it appears to show open field lines, which occur due to the approximation inherent in using field lines.
 
robinson said:
I have to take issue with your contention that field lines "do not exist". I believe they do, based on my own experiments. As to these lines "breaking, and reconnecting", I agree that it certainly violates known physics.
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Can you expand on what those experiments are?

I like iantresman's analogy with topographical lines (which really map a scalar rather than vector field, but the point about abstraction comes through nicely).
 
I can expand, but time is short today, and the time to play here is done for now.

We need an entire topic about magnetism. Or rather, ferromagnetism, paramagnetism, diamagnetism and electromagnetism.
 
Ziggurat said:
The posts are
This is obviously false. In an ideal conductor, the magnetic field cannot change at all, under any circumstances. Magnetic reconnection therefore obviously requires that the plasma not be an ideal conductor.
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Yes, in a way, but the frozen in property does play a pivotal role in the actual 'magnetic reconnection' process as it is currently understood. Infact, the actual process is not very well understood, even the scientists that study that field (if you could call it that) seem to disagree on the many different magnetic reconnection models.

http://mrx.pppl.gov/Publications/uzdensky06pop.pdf
Ever since it was established that the classical Sweet-
Parker reconnection model2–4 with Spitzer resistivity is too
slow and that the Petschek5 fast-reconnection mechanism
cannot be realized in resistive magnetohydrodynamic
MHD with uniform resistivity,6–10 theoretical studies of fast
reconnection have proceeded as a competition between two
schools of thought. The first one invokes the idea of enabling
the Petschek mechanism by a strongly localized anomalous
resistivity due to plasma microinstabilities triggered when a
certain current threshold is exceeded.9–12 The second, commonly
referred to as the Hall reconnection mechanism, relies
on the two-fluid effects that become important when the reconnection
layer becomes so thin that ion and electron motions
decouple from each other.
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It seems that some of the models proposed for 'magnetic reconnection' really do rely on magnetic field lines being 'frozen in' to a plasma due to its (apparently) infinite conductance. heres one of those papers from princeton;

http://mrx.pppl.gov/Publications/inomoto06prl.pdf
Magnetic reconnection [1], the breaking of the frozen-in condition of magnetic field lines in electrically conducting plasmas, plays an important role in global magnetic selforganization phenomena in laboratory and space plasmas. [...]

Within the electron inertial region, the electron frozen-in condition is broken by electron inertia or the off-diagonal components of the pressure tensor
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The exact process that makes this reconnection occur is still open to debate even amougst the people who study it. The chances are that it is not anything to do with 'reconnection' or 'open' magnetic fields.
 
edd said:
Can you expand on what those experiments are?
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Doesn't the old "iron filings / magnet" experiment appear to show field lines? Do they always appear in the same position, or can they appear anywhere parallel to the magnetic fields? Can an iron-filing field-line be "added" in between two existing field lines? What controls the spacing between iron-filing field-lines? Sorry, I digress.
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edd said:
I like iantresman's analogy with topographical lines (which really map a scalar rather than vector field, but the point about abstraction comes through nicely).
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Can we change the topography of a surface, eg a hilly area, such that the topographical lines appear to reconnect, and do they really?
 
iantresman said:
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Doesn't the old "iron filings / magnet" experiment appear to show field lines? Do they always appear in the same position, or can they appear anywhere parallel to the magnetic fields? Can an iron-filing field-line be "added" in between two existing field lines? What controls the spacing between iron-filing field-lines? Sorry, I digress.
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The iron filings is misleading. It's never really made clear but I certainly came away from lessons where we did that at school thinking that magnetic fields had some kind of onion skin structure thanks to those, and I bet a lot of other people did too.

But the problem is that the iron filings themselves influence the field locally. The filings become small magnets themselves, and it's therefore not surprising they bunch up in lines. They're influencing the field they're supposed to be outlining.
 
edd said:
The funding is largely controlled by the same people that use the funding and I don't see how you can criticise this as they're both the best qualified people to make such decisions
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Priceless. :rolleyes:

edd said:
and this is exactly the thing you keep demanding of criticisms to all your hogwash - namely that they be peer reviewed.
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Again, priceless. :D
 
BeAChooser - what I mean is that while high level decisions about funding so much money to look at such and such an area can be taken without expert knowledge, you can't expect say, the US Congress, to specify the exact details of how research should be done and who by. It's only logical to have people that understand the field do that, right?
And yes, to a lesser extent it's the same group of people sitting on telescope time allocation committees to determine who gets the valuable telescope time for what research, although broad decisions might be taken at a higher level by less involved experts.

How exactly would you make funding decisions into research? Please specify what sort of people would have the decisions, not what decisions should be made, and why this would lead to better value for money in research?
 
Zeuzzz said:
As such, they can not do anything 'physical' like reconnect or merge, because they really do not exist.
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The field lines are a way of representing the field, which (if you insist) is in turn a way of representing the forces a charged test particle feels, which is ultimately what we measure. Those forces change with time in general, which means the fields change, which means the lines move and change. One of the things they can do is reconnect.

Deal with it.
 
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Zeuzzz said:
The exact process that makes this reconnection occur is still open to debate even amougst the people who study it. The chances are that it is not anything to do with 'reconnection' or 'open' magnetic fields.
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Well, it's got nothing to do with fields which have a nonzero divergence (which is what Gauss's law for magnetism really states), that's true. As for the important mechanisms, well, considering that the vocal critics from the EU side have had trouble even recognizing that there's actually no magnetic field divergence involved, I'll have to pass on your evaluation of what the "chances" are.
 
iantresman said:
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Doesn't the old "iron filings / magnet" experiment appear to show field lines?
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Sort of. See below.

Do they always appear in the same position,
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No.

or can they appear anywhere parallel to the magnetic fields?
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Yes.

Can an iron-filing field-line be "added" in between two existing field lines?
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Yes.

What controls the spacing between iron-filing field-lines?
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In a real iron filing experiment, the iron filings themselves can change the field. It is therefore not actually a nonperturbative measurement method. That perturbation often bunches up the field lines so that the field actually decreases outside the lines of iron filings, and leads to the clumping that's usually seen, but that's got nothing to do with what the field in a non-perturbed case is, and where exactly the clumping takes place depends upon the random initial pisitioning of the iron filings, not simply the field.
 
iantresman said:
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Can we change the topography of a surface, eg a hilly area, such that the topographical lines appear to reconnect, and do they really?
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Yes.

Consider two hills in a plain. Between the hills there's a mountain "pass" - a saddle point. Now draw the contour lines, focusing on the saddle. Notice that the contour lines form an X at the saddle.

Now pick two points symmetrically on either side of the saddle, partway up the slopes of the hills. Those are not connected by a contour line. But now raise the ground at the saddle until it becomes the highest point (higher than the hills, if necessary). Now those two points are connected by a contour line - they have "reconnected".
 
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sol invictus said:
Yes.

Consider two hills in a plain. Between the hills there's a mountain "pass" - a saddle point. Now draw the contour lines, focusing on the saddle. Notice that the contour lines form an X at the saddle.

Now pick two points symmetrically on either side of the saddle, partway up the slopes of the hills. Those are not connected by a contour line. But now raise the ground at the saddle until it becomes the highest point (higher than the hills, if necessary). Now those two points are connected by a contour line - they have "reconnected".
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Like this? Or should he pass be a ridge above the plain. I can't seem to get the X. Or is there another pass at ridge at right-angles to the pass?
 

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iantresman said:
Can we change the topography of a surface, eg a hilly area, such that the topographical lines appear to reconnect, and do they really?
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To expand on what Sol said, you can even make a topological map which has equal-elevation lines of exactly the same form as the magnetic field lines I gave before: namely, Elevation = ax2-by2. This isn't exactly two hills on a plain, thought the shape near the saddle point will be the same. But your drawing doesn't actually do it: you need the feet of the hills to sort of overlap, so that along one axis (say, east-west) the land rises on both sides, and in the other direction (say, north-south), it falls away on both side. Otherwise it's not a saddle point.
 
Ziggurat said:
To expand on what Sol said, you can even make a topological map which has equal-elevation lines of exactly the same form as the magnetic field lines I gave before: namely, Elevation = ax2-by2. This isn't exactly two hills on a plain, thought the shape near the saddle point will be the same. But your drawing doesn't actually do it: you need the feet of the hills to sort of overlap, so that along one axis (say, east-west) the land rises on both sides, and in the other direction (say, north-south), it falls away on both side. Otherwise it's not a saddle point.
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Although it's beyond my CorelDraw skills, this is from the Wikipedia article on the "Saddle point"
 

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And a contour map from the Wiki article on Morse theory. If only there was an animated version around.
 

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iantresman said:
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Although it's beyond my CorelDraw skills, this is from the Wikipedia article on the "Saddle point"
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Yeah, that's a good picture.

So pick two points on either side of the center of the X, each slightly up its respective hill. Now imagine raising a little hill up right on top of the X by piling up a mount of dirt, and you'll see that when it gets high enough, those two points suddenly get connected by a contour. Right when it happens they are each at the center of their own X - that's how it's possible for the line to "break" and "reconnect".

But it's really a smooth continuous process, as you can see.

Incidentally a process like this is responsible for many phenomena in physics, from first-order phase transitions to rainbows.
 
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Note that in the saddle point, when the lines reconnect, nothing actually happens top the medium they are representing (the hill). I was under the impression that when metaphysical field lines 'reconnect' or 'break' there was supposed to be a huge amount of energy released?

Well, I certainly haven't seen any exploding hills recently :)
 
Zeuzzz said:
Interesting, i had never heard of this before, do you have a source for that?
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Yes.

Me.

Zeuzzz said:
Note that in the saddle point, when the lines reconnect, nothing actually happens top the medium they are representing (the hill). I was under the impression that when metaphysical field lines 'reconnect' or 'break' there was supposed to be a huge amount of energy released?

Well, I certainly haven't seen any exploding hills recently :)
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That's because contour lines don't obey Maxwell's equations.
 
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