Some of the comparisons between gravity (in the weak field limit) and electromagnetism were really interesting (https://www.physicsforums.com/showthread.php?t=80710 ). Unfortunately, that got locked due to someone presenting their pet theory. The topic seems too good to end, so let's hope we can stay on topic this time. Referring all the way back to: https://www.physicsforums.com/showpost.php?p=667979&postcount=12 Can you give some suggestions how one would go about figuring out how g and H do transform? I don't really understand why it would be different, as shouldn't a "point charge" (rho = m * delta function) be the same in all reference frames? So if the "charge" is invarient, and the equations are already "equivalent" to Maxwell's equations (so they have Lorentz symmetry), what makes these any different at all? I would expect them to transform the same. I would be interested in learning more about this. Thanks for the great introduction! EDIT: Or are we supposed to use relativistic mass density instead of invarient mass density? I guess that would make the "charge" not invarient and then g,H wouldn't transform like E,B.