
Sun’s Puzzling Plasma Recreated in a Laboratory - joeyespo
https://www.quantamagazine.org/suns-puzzling-plasma-recreated-in-a-laboratory-20190729/
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ars
I've never understood how the sun's magnetic fields are described as entities
in their own right. Isn't a magnetic field a result of a moving charge, and
not a thing that can exist on its own?

Or is that simply a shorthand way of saying the plasma creating the magnetic
field is moving which causes reconnection, etc.?

But the plasma is also described as moving _because_ of the magnetic field
moving!

So which is it? Anyone able to explain?

~~~
Sniffnoy
> Isn't a magnetic field a result of a moving charge, and not a thing that can
> exist on its own?

No. Even in the absence of any charge at all, moving or otherwise, Maxwell's
equations still have a nonzero solution. This is known as "light".

~~~
stefco_
To elaborate on this a bit: you can talk about electric and magnetic fields as
properties of space itself, and they have a fully-established physical reality
to them. Moving charges appear as terms in Maxwell's equations making
contributions to the field, but the field is the ground truth.

Magnetic fields from moving charges are pretty cool because you can show that
they are equivalent to electric charges with relativistic effects due to
charges' motions factored in. But thinking about relativistic motion equations
for a huge number of particles is a tough starting point conceptually if
you're just trying to find how a particle at a particular point in space will
behave. So pragmatically, magnetic fields are a great conceptual tool. They
also hold up straightforwardly once you bring in special relativity.

One thing that's very nice about electric/magnetic fields is that they are
local. You can calculate the effect of magnetic forces on solar ejecta by just
calculating the local field. It's much simpler conceptually to talk about how
a particle will respond to a local magnetic field than it is to talk about how
it will respond to a tremendous number of moving charges. It also ends up
being very mathematically elegant; Maxwell's equations are very beautiful and
conceptually simple, and their common applications are likewise beautiful and
simple considering the seeming complexity of EM phenomena.

Furthermore, in some cases, like those involving light (EM waves propagating
through space and time), you don't even have the luxury of knowing where the
charges are that produced the field/how they moved: if a light wave is
striking a piece of material from some distant light source, _the only thing
you know about /can measure is the field itself_; it's therefore really nice
to be able to do all of your calculations using the field.

But EM fields are more than just frameworks for understanding or calculating
forces. They are very physical things. EM fields can literally carry
linear/angular momentum and energy away from them. Let's say you have a
satellite shaped like a long bar with two lasers at opposite ends pointing in
opposite directions orthogonal to the main axis of the craft:

    
    
                           ^
      beam 1 points up ->  |
                           ==============
                                        | <- beam 2 points down
                                        v
    

Firing these lasers at very high power is like firing very weak thrusters, and
_your satellite will start to rotate_ ; its angular momentum will change! Even
though the light waves from those lasers are not interacting with intervening
matter (at least in the classical picture), it's _still carrying carrying away
momentum and energy_ ; from the center-of-mass frame, the EM field now has
clockwise angular momentum (from the view of the above diagram), while the
satellite has an equal and opposite counterclockwise angular momentum.

This comes up again in general relativity, where EM fields energy and momentum
contribute to the curvature of spacetime in _precisely_ the same way as mass
does. In other words, if you had a huge amount of light passing through a
region of space, it would be just as gravitationally attractive as an
equivalent amount of mass (after converting units using E = mc^2).

When you think about it, conservation of momentum + the speed of light + EM
fields' ability to move objects (i.e. change their mass and momenta)
_necessitates_ that they have this sort of physical reality. In the bar-
satellite example, the laser beams are pushing back on the ship; since light
doesn't travel infinitely fast, there is no equal-and-opposite reaction from
some regular matter that the EM fields would mediate instantaneously, i.e. no
regular matter that's pushing against the satellite to get it spinning (as
would be the case with a regular thruster used for stationkeeping). The
momentum has to go _somewhere_ ; as it turns out, it goes into the field
itself in an extremely well-defined way.

So, again, EM fields are the real deal; moving charges _contribute_ to fields,
but the fields themselves have their own well-measured, well-described
physical reality in classical physics.

