Gravity and electromagnetism both obey inverse square laws, but electromagnetism is negligible on astronomical scales because positive and negative charges cancel so astronomical objects are electrically neutral (or close enough to it that electromagnetic effects are negligible).
The strong and weak interactions are only significant on very short distance scales; from the standpoint of "force laws" you can think of them as having an exponential decay with distance that makes them drop off to essentially nothing by the time you get to distances much larger than the size of an atomic nucleus. (That's an oversimplification, but it's enough to see why you can ignore them on astronomical scales.)
The electric force cancels over short distances. Magnetic force may act over multi-light-year distances, as may be easily seen in the turbulent flow of luminous plasma visible near supernova remnants, supermassive black holes, and in star-forming regions. Turbulent flow of neutral gases at such pressure and scale is impossible because collisions are too infrequent.
> Magnetic force may act over multi-light-year distances, as may be easily seen in the turbulent flow of luminous plasma
The plasma itself is magnetic, so the magnetic force itself is not acting over multi-light-year distances; the material carrying the magnetism is traveling over those distances and bringing its magnetism with it.
Also, this magnetism doesn't affect the orbits of planets or stars; my original statement was focused on what affects those things.
I agree with GP, the measurable effects of the magnetic field are not strong, it's just that they're close because charged particles carried them close.
Maybe a better electromagnetic example would be radio signals from pulsars, which are enormous and travel great distances and still provide measurable signals even if they are light years away. We cannot on earth measure the gravity of these objects, nor the atomic forces, but we can readily meausure their electromagnetic sprayings.
Yet, nearly all of the extended structure of luminous interstellar gas we see in the best pics on APOD is the plasma fluid interaction that astrronomers can't be bothered about. SOFIA could have revealed much, if not grounded. Are any of the new orbital telescopes about to come online equipped to record polarization?
Astronomers, as a rule, detest all discussion of EM, often going so far as to label people who insist on discussing it cranks. There are exceptions, but also cranks. It can be hard for the public to tell them apart. Astronomers do not assist, perhaps for fear of being labeled cranks themselves.
I know from talks that filaments[0], galactic lobes[1], structure of arms[2], etc relate to EM — but finding anything at the layperson level is basically impossible. Just short blog posts, but no real explanation of how this all relates or what drives it.
The fact of E-M fields affecting the motion of mass, in preference to or in addition to gravitation, is anathema.
To hint at any role of E-M in the large-scale evolution of the universe is a good way to eliminate any possibility of a research grant. Probably this is just because the people deciding don't want to be obliged to learn how to evaluate whether the research program makes any sense. So, plasma fluid dynamics work normally is studied only at the scale of an individual planet or star, or at most a galaxy.
SOFIA, the telescope carried in a Boeing 747, lost its funding in part because mainstream astronomers found its unique capability of mapping magnetic polarization uninteresting.
> As I understand it, planetary magnetic fields influence solar winds; stellar ones structures in the galaxy; etc.
Yes, I was oversimplifying somewhat. Magnetic fields don't affect things like the orbits of planets or stars. But they do have other effects on astronomical scales.
The strong and weak interactions are only significant on very short distance scales; from the standpoint of "force laws" you can think of them as having an exponential decay with distance that makes them drop off to essentially nothing by the time you get to distances much larger than the size of an atomic nucleus. (That's an oversimplification, but it's enough to see why you can ignore them on astronomical scales.)