Gravity Is Not Like the Other Forces
Imagine some kind of alien life form that arose and evolved in intergalactic space. They evolve intelligence and start developing physics. Their physics knows about electromagnetism and maybe the strong and weak nuclear forces, but they have never seen any sufficient concentrations of mass to notice gravity.<p>They would still know about mass but for them it would just be a property of matter that determines how strongly it reacts to force.<p>Their model of how force works would be that for each kind of force there is some kind of charge. How much of that kind of force you have between two bodies is determined by how much of that kind of charge each has and how far apart they are.<p>How each body responds to that force is determined by its mass.<p>Mass and charge are independent for all of their known forces. You can have two bodies with the same mass but different charge for a given kind force, or two bodies with the same charge for that force but different mass.<p>Eventually they would discover gravity and I think it would immediately stand out as fundamentally different from the other forces because its charge is not independent of mass. You double the gravitational charge on a body you also double its mass. You double its mass you also double its gravitational charge.