What is inertia the property of?

Answer 1

matter is the conventional answer to this question. Classical physics would have us believe that inertia is a sort of "kinetic energy" that causes matter to resist changes in it's momentum. Either at rest, or in motion.

To be more accurate, intertia is a property of the interaction between matter, and the space-time around it. A mass that is accelerating through space-time compresses space-time ahead of it, and stretches space-time behind it. When the mass then stops accelerating, and maintains a steady velocity the space-time around it equalizes forming a moving ripple through space-time containing the object, conserving it's angular momentum. Deceleration reverses this by pressing up against the rear end of this ripple slowing it down.(G-forces are a result of space-time resisting this compression and expansion)

We can then see from this the link between "intertia" and "gravity" .

Gravity causes a much weaker rippling through space-time, not caused by the angular momentum of atoms/mollecules/objects through space-time, but through the circular momentum of the electrons circling the atoms of a LARGE body. Electrons carry much less total mass by themselves, so require much much more matter to cause gravitational effects. Gravity can be likened to a succession of moving compression waves through space-time. Each compression wave "pulls" other objects towards it, as the space-time between the two objects is squeezed closer together.

Answer 2

matter