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Q & A: magnetism and gravity in the cosmos

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Most recent answer: 02/26/2018
Q:
I hope this question hasn't been asked on here before. I've been banging my head on the wall trying to answer this question and learn more about this subject. My question is this.Why does astronomy, cosmology and physics primarily only do calculations of the universe using gravity. Magnetism is a far more powerful attractive force, as well, accretion is also another force of attraction in the universe, although I'm unsure how strong a force it is. Should not the attraction between objects in the universe be calculated using the sum of all attractive forces, not just Gravity?I have a hunch that dark energy, and dark matter can be explained by the power of magnetism, and electromagnetism of Super-massive black holes in the hearts of Galaxies. Going back to the statement of magnetism being far stronger than gravity. If you take a magnetic object, and drop it, it falls to earth due to earths gravity. (And possibly some insubstantial amount of magnetism going on in the core [but ignore that for now].) Now if you take another magnetic object, and interact with the object you just dropped, and say... lift it off of the ground? You just used the magnetism of 2 relatively small objects to counter and overpower an entire celestial body's worth of gravity. Why am I hearing almost nothing about magnetism in the calculations of Galaxies or the universe itself. The only notable information about magnetism I've heard is that Black holes are almost unimaginably magnetic. And that the arms of galaxies follow along the magnetic fields of black holes, which makes sense, given how strong magnetism is compared to gravity. Anyway, I could go on for hours about this subject, just please try to give me some idea as to why there seems to be an avoidance of magnetism in Astronomy.
- Andre Gillingham (age 26)
Trenton, Ontario, Canada
A:

Magnetism is a big deal in the behavior of some objects, e.g. neutron stars. For the very large-scale cosmic phenomena, however, magnetism plays a negligible role. How can that be? 

At long distances the gravitational field from a star or even a galaxy falls off as the square of the distance. The magnetic field falls of as the cube of the distance.  

Also: On one hand, the total gravitational field of (for example) a galaxy is simply the sum of the gravity of its parts,  That is, there is no cancellation. On the other hand, the contributions to the total magnetic field,  produced by many small magnetic objects (eg, spinning stars) tend to cancel out.

So at large distances you can forget about the magnetic field, even when some of the objects involved have strong fields locally.  This analysis is borne out by direct measurements of large-scale magnetic fields, which are very small.

Mike W.


(published on 02/26/2018)

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