Quantised Inertia (QI) is a theory of inertia first proposed in 2007 by physicist Mike McCulloch, from the University of Plymouth in the UK.
The phenomenon of inertial mass is defined in Newton's First Law: "Objects move in straight lines at constant speed unless pushed on" but it has never been explained. Quantised inertia explains inertial mass, for the first time, by combining relativity and quantum mechanics.
According to relativity, an accelerating object will see a Rindler horizon a certain distance away, in the direction opposite to that of its acceleration vector. This is because information travels at the speed of light.
An accelerating object will see itself surrounded by so-called Unruh radiation. The horizon splits virtual particles, so that they become real radiation.
Quantised Inertia combines these two predictions by saying that the horizon damps the Unruh radiation on one side of the object, rather as sea defences damp ocean waves. The resulting gradient in the quantum vacuum pushes the object back against it acceleration, explaining inertia (McCulloch, 2007, 2013).
Quantised Inertia further predicts that for objects with very low accelerations, such as stars at the edges of galaxies, the Rindler horizon moves so far back that it is close to the cosmic horizon. As such, the waves of Unruh radiation are now damped equally all around, and the mechanism of inertia collapses. This explains why stars at the edge of galaxies can orbit faster than expected but still remain bound to the galaxy. As a result, Quantised Inertia predicts galaxy rotation perfectly without the need for dark matter or any adjustment (McCulloch, 2012, 2017).