My undergraduate advisor (David Chuss) at Villanova and some of his students recently put together a really nice visualization of the magnetic fields at the center of our galaxy.
Last I talked to him about it, he said that the results were surprising. I think he said no one really knew yet how to explain the vertical yellow streaks representing hot plasma.
A galaxy is so large (e.g. the milky way is ~ 1 billion billions km in diameter!), and bodies are so far from each other that it is far from intuitive that there should even be a magnetic field, and it's even neatly aligned along the arms!
It's that 'spooky action at a distance' thing. It's still never been clear to me but what's the speed of Electromagnetism (i.e. if you drop a highly magnetic object into an area - how quickly is the force impacting the distant surroundings)?
The propagation of the change in the magnetic field travels at the speed of light.
The act of moving a magnet and thus changing the magnetic field at one point also induces a change in the electric field and vice versa, that's why you'll always hear talking about "electromagnetic" waves (which light, radio waves, etc are an instance of).
To make things even more interesting there is also this phenomenon: the magnetic field is created by moving electric changes, relative to the observer. If an observer moves at the same velocity as the electric charge it will not observe a magnetic field. It turns out that when you apply special relativity to the effects of electrical charges (coulomb 's law) you get a theory of electro-magnetism (Maxwell's equations). Magnetism this "follows" from the behavior of electrical fields and special relativity. That doesn't make magnetism less "real" but it hints at the intimate connection that the two forces have and what we now talk about them as "electromagnetism"
Isn’t that distinction more philosophical than not?
Like the Feynman diagram is in any case that a photon is transmitted between two charged objects, whether that photon is “real” or “virtual”.
I know there are physicists who interpret this as there being two distinct things, either particle-like excitations (photons) and more general excitations (not photons). These usually don’t think of Feynman diagrams as representing anything real.
But I also know there are some physicists who consider virtual photons “real”. And in that picture of reality, all EM interaction is “light”.
To add to other comments here: think about what happens when a heavy object (e.g. a planet or a moon) moves, how quickly does the change to the gravitational field propagate to affect the distant surroundings?
This might be a dumb question, but how does this interact with dark matter observations? Do magnetic fields in any way help explain higher than expected galactic orbital rates?
Sure. Can we use it to accelerate a smaller object faster than the whole solar system? Maybe. Will the difference in acceleration be at all noteworthy? Unlikely.
I would be surprised if there would be literally nothing in their centres, ie like some ultra complex version of binary / more complex set of stars circling around each other, with center of rotation being just empty space.
While SMBH at the center are common, it is believed that there are plenty of galaxies that don't have such a black hole including spiral galaxies. For example, M33 [1].
The upper limit on its central black hole is ~1500x of Sol and it's widely believed right now to just not have one. By comparison, the Milky Way’s black hole is 4 million Sol masses and our galaxies SMBH is quite tiny by comparison to many galaxies.
Since them, much more detailed magnetic fields maps have been made of the Milk Way: https://www.forbes.com/sites/startswithabang/2019/09/23/this...
Also for other galaxies: https://www3.mpifr-bonn.mpg.de/staff/rbeck/PSSS18.pdf