A stone on a table won't budge without a kick. Where does this stubbornness come from? The idea that the rest of the Universe is to blame was put forward over a century ago. Now it has been formalized into a rigorous theory.

In the new work, gravity is described not as an attraction between bodies but as a field that sets the scale of distances and time. All matter in the Universe creates this field. A body's inertia is its response to the field created by all matter in the past. The more matter around, the stronger the body resists acceleration.

For ordinary stones and planets, the theory gives the same results as conventional physics. But for very dense objects, like neutron stars, differences appear. These can be tested with observations.

Thus, inertia ceases to be a mysterious property of matter. It becomes the Universe's memory of itself.