Yep, EM is inverse sq., if you want to know if the two ar connected that’s beyond the knowledge of current physics, but I’m sure a theorist in superstring, M-theory or LQG would be able to give you some interesting ideas about this.
In four spatial dimensions, gravity would obey an inverse cube law. In general, for n spatial dimensions, gravity is as to 1/d[sup]n-1[/sup]. I assume EM works the same way.
Well, the B-field from a magnetic dipole obeys a rough inverse-cube law. That’s probably what you’re thinking of, since there are no magnetic monopoles and all that. The same is true of electric dipoles. If you could get something with negative mass, the same would be true of gravitational dipoles. However, if you look at Maxwell’s equations, you’ll see that electric and magnetic fields have a very satisfying symmetry.
Now, you’re trying to draw a relation between Newton’s Law of Gravitation and the field law of a magnetic dipole. You can’t. It’s not a simple matter of changing a 2 to a 3 in the equation. Look at Equation 2. Yes, there is a 1/r[sup]3[/sup] there, but there’s also some junk with theta in it. Compare the Law of Gravitation and you’ll see there’s not a simple parallel to be drawn.