## The Eternal Question of Inertia

The general theory of relativity states gravity is not a force, but a curvature of spacetime. The geodesic equations predict what the paths of objects will be as they follow their natural geodesic paths in either flat or curved spacetime. And they do not need a force to constrain them in these paths for they are conforming to a generalized inertia path. But as soon as some agent forces the object into geodesic deviation, inertial force is manifested in the object. In the case of a gravitational field this is called weight. What causes weight or inertial force?
 Momentum

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 Quote by e2m2a The general theory of relativity states gravity is not a force, but a curvature of spacetime. The geodesic equations predict what the paths of objects will be as they follow their natural geodesic paths in either flat or curved spacetime. And they do not need a force to constrain them in these paths for they are conforming to a generalized inertia path. But as soon as some agent forces the object into geodesic deviation, inertial force is manifested in the object. In the case of a gravitational field this is called weight. What causes weight or inertial force?
For an actual answer (but not within General Relativity) see Dennis Sciama's beautiful 1953 paper "On the Origin of Inertia".

He shows that if you extend a simple Newtonian model of gravity to the whole universe, using concepts modelled on the electromagnetic field, then inertia arises naturally as a force opposing acceleration relative to the vector potential of the universe (and similarly angular momentum is relative to the rotation of the universe). This model also fully satisfies a form of Mach's principle.

This model is so neat and seems to be such a plausible explanation of how it all works that I find it quite disturbing that GR is totally incompatible with it.

## The Eternal Question of Inertia

Thanks, I will look into it.