Does our universe contain individual magnetic fields ?
For example two different magnets, one here on earth and one on mars. Do both of them have their own magnetic field? Or is there only one single field that stretches across the entire universe, but have different strength on different locations? For example around a magnet ?
Answer
Clear answer: yes, there is only one electromagnetic field for the whole universe.
Ontological answer: You can go to any spot in the universe and take a measurement. Even if you were to find a large chunk of space with "0" as your result (and for this part of the answer we do not care if this is physically possible or not), the field itself would still be there, displaying those "0"'s. Oh, let's ignore black holes as that's obviously not the idea you had in mind.
Physical answer: If people talk about the "field of an electron", making it sound as if that were some kind of localized thing, then that is just a shortcut to reduce incredibly complex maths to something they can handle. None of the interactions have a hard border or a defined cut-off radius. That quite literally means that every single charged particle in the universe influences/interacts with every single other charged particle in the universe, no matter how far away it is. If you wiggle your finger here on earth you will definitely change the EM field on Mars. In practice this influence is so utterly, unimaginably small that it obviously is neither measurable nor anything you need to keep track off. But it is still there.
Regarding the comment on this: Finger-Mars is obviously a half-joking example. The point is not the light cone, event horizon or the magnitude of the influence, but that it *is* the same *field*; the Finger-wiggling does propagate (at least in our theories), but obviously with such absurdly small numbers as to be totally irrelevant.
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