# Were passive and active gravitational mass actually ever considered to be different? If yes, why?

Mass produces gravity and is subject to it. It seems impossible to separate the two phenomena and the equivalence of the two forces was stated centuries ago.

Wiki considers measuring two different gravitational masses - active and passive. It also states that:

Although some theorists have speculated that some of these phenomena could be independent of each other.

So did physicists actually ever consider the possibility of different values for active and passive gravitational masses? If so, why might have they expected them to be different and caused by independent phenomena?

• Maybe in your second case you're referring to test particles? Often when treating gravity we consider a configuration of masses and the trajectory of a "test particle" which we consider to have a non zero mass but small enough that its influence on the other masses is negligible, i.e. we consider as if the test particle feels the gravitational field but doesn't produce one of its own (it produces one, but it's too small to matter). Though it's just a convenient approximation to see trajectories, not something physical – user2723984 Oct 12 '18 at 6:14
• @JohnRennie, is there another way to explain the separation between the two phenomena? That is basically my question. – user157860 Oct 12 '18 at 6:14
• Yes, there was an empirical reason for equating gravitational and inertial masses: bodies fall at the same rate of acceleration irrespective of their masses/composition. See my answer here: physics.stackexchange.com/a/431222/133418 – Avantgarde Oct 12 '18 at 6:25
• Theoretical speaking, you can't separate the two things. In fact, geometrically, gravity is nothing but a deformation of the spacetime, and every particle with or without mass will move according to the geometry of the spacetime (This is General Relativity). – Kevin De Notariis Oct 12 '18 at 6:31
• @Avantgarde, the problem here is not equivalence between inertial and gravitational mass, but the necessary equivalence betweenpassiv and active gravitational mass – user157860 Oct 12 '18 at 6:31