I had a bit of an epiphany recently while explaining the weightlessness of free fall to my son - a way to see it using horizons. The insight that Einstein had in 1907 that a falling man would not feel his own weight was apparently the happiest thought of his life, and although I admire Einstein, I've always been wary about this evidence-less thought. It is almost as if Einstein was trying to convince himself. The insight proved to him that inertial mass (the resistance to acceleration) was equivalent to the gravitational mass (the attraction to other matter) and so they cancelled out. Lovely and symmetrical, but 100% true?
This is called the equivalence principle and it has been tested many times by experiments that are far more accurate versions of Galileo's dropping of two heavy balls from the tower of Pisa (I often had the amusing thought that he was aiming for one of his many critics). The balls hit the ground at the same time, thus proving the equivalence of inertial and gravitational mass. Or does it?
There is a loop hole. The change of inertial mass in quantised inertia is such that the effect is independent of the mass. The acceleration changes to: a = GM/r^2 + 2c^2/Theta. This has a constant second term, which means that both balls would still fall together, but a little faster. This means the experiments done so far (based on the two balls) will be blind to QI. They need to look at speed of fall instead.
The epiphany I had was imagining the spacecraft we were watching on screen (in Independence Day), and getting rid of all fields and only thinking of horizons. As the craft accelerates towards Earth there is a single Rindler horizon above it which damps the Unruh radiation above it pulling it up (inertia), and many little horizons caused by atoms/matter in the Earth below it damping the fields there and pulling it down (gravity). Whereas in general relativity the path of the craft is along an abstract vector in space-time (a thing that can never be tested for directly), in QI the balance is caused by horizons and their damping of the Unruh field, something that can be tested for (Unruh radiation has now been detected, see reference).
Modern theoretical physics disdains the idea of testability, but I do not, and it has been found that the best theories are always the testable ones, almost as if the cosmos gives us a reward for sticking our necks out. "Well, the Lord hates a coward." - Jim Malone.
Lynch et al., 2021. Experimental Observation of Acceleration-Induced Thermality. Phys. Rev. D 104, 025015. https://arxiv.org/abs/1903.00043