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Yet what you seem to be saying is that this "change" in going from "no field" to "some field" is akin to a disturbance in an existing field.

Exactly! Because consider: an observer at rest with respect to an electrical charge will see an electric field and NO magnetic field, while a moving observer (passing arbitrarily close to the first observer's position) will see both an electric and a magnetic field. The two observers will not agree on whether there is a magnetic field or not; they will, however, agree on the dynamics of locally moving charged particles (i.e., the physics works out the same).

So you see, you can practically never say that any region of space is free from a magnetic field, because the magnetic field in that region will be different for different observers. One man's zero field may be another man's strong field. There can't be anything special about turning on a field; it's the same thing as a change to the existing field. The 4-potential is defined everywhere in space. (Homework: look up the terms "Gauge Principle" and "Gauge Invariance".)

[Geek alert 1: The reason that different observers will see different fields is because of special relativity. From a moving frame of reference, time moves more slowly and space is contracted along the direction of motion. If the field is the same for both observers, the motions of a charged particle can't be agreed upon by different observers. The magnetic force is the force that arises that compensates for the difference in motion between the two versions of spacetime. (It's tempting to call the magnetic force an "apparent force", such as the coriolis force, but it is possible--easy, in fact--to construct magnetic fields that can't be zeroed out by a Lorentz boost.)]

[Geek alert 2: Woah! Wait a second! If the relative distortion of spacetime causes an apparent--no, a real--change in the electromagnetic field for relatively moving observers, why doesn't it do the same thing for the gravitational field? It does. It's called the gravitomagnetic effect. Well, doesn't that just prove what Van Flandern is saying? Isn't that just a different formulation of the same effect? No, because first, unlike the "time delay" canard, the gravitomagnetic effect depends on the speed of the observer and not on the distance from the source, and second, because any "time delay" effects (such as the Poynting-Robertson effect, which really does pertain to the light from the sun) would be orders of magnitude greater than the gravitomagnetic effect.]

(I have used the blue font color to denote scientific ignorance, as is customary on FR.)

"an observer at rest with respect to an electrical charge will see an electric field and NO magnetic field, while a moving observer (passing arbitrarily close to the first observer's position) will see both an electric and a magnetic field. The two observers will not agree on whether there is a magnetic field or not; they will, however, agree on the dynamics of locally moving charged particles (i.e., the physics works out the same). So you see, you can practically never say that any region of space is free from a magnetic field, because the magnetic field in that region will be different for different observers. One man's zero field may be another man's strong field. There can't be anything special about turning on a field; it's the same thing as a change to the existing field."

Ahhh, but there *can* be something special about turning on the field (more below).

Your *observers* in your above example depend upon frames of reference, yet you and I both know that if behavior isn't explained consistently in different frames of reference that someone's equation is wrong. One observer might see a magnetic field, and another observer might not, but that's *not* the same thing as turning on a field in the first place. The two observers won't agree on whether there is or is not a magnetic field in your example, this is true, yet we both know that the field does exist! It just isn't *observable* to one of the participants.

Likewise, the frame of reference doesn't determine the magnetic field of our electromagnet. Regardless of the position of the observer, in reality there is NO field when the electromagnet is OFF, while there is a field that covers a sizeable area when our electromagnet is turned ON.

And what is so special about that fact? Well, that fact means that a magetic field has to propagate outwards over an area in a given amount of time (once we switch the electromagnet ON).

Thus, it is a valid question to ask *how fast* does the field cover that area.

Because consider: an observer at rest with respect to an electrical charge will see an electric field and NO magnetic field, while a moving observer (passing arbitrarily close to the first observer's position) will see both an electric and a magnetic field. The two observers will not agree on whether there is a magnetic field or not; they will, however, agree on the dynamics of locally moving charged particles (i.e., the physics works out the same).

And because Maxwell's equations showed that they will both agree the charge propagates at c, a certain patent clerk drew some rather interesting conclusions.