Similarly, photons can't drop below c, so they gain and lose energy by changing frequency.
Thus moving a piece of matter, which is energy fluctuating in contained space, producers inertia because essentialy the energy contained in matter is moving virtually in all directions simultaneously because of how quickly it's fluctuating in that incredibly small confined area, where it's likely spinning in place as it fluctuates at nearly the speed of light. So it's near always moving in every direction.
So, any attempt to move it threatens to move it faster than the speed of light, even by a small amount. Inertia is the resistance you feel by moving matter and thus grinding it against the speed of light, and the resistance is probably produced by the photons catching onto spacetime itself, the resistance between the two producing high frequencies of fluctuation in that matter-energy as a result of pushing against that inertia.
Conservation of energy is why inertia exists. The light in matter cannot gain in frequency without that energy gain coming from somewhere. So it comes from matter resisting your attempt to change its vector.
The alternative hypothesis is that curvature of space-time and conservation of energy causes inertia. See my post above regarding the best verbal description of how.
Curvature has actually been measured and is already part of every geometry (sometimes it simply 1, in which case it is ignored, or a constant, which gives the two simplest non-euclidean geometries). If the curvature is a function of the position in a smooth differential manifold (space-time), which gives a tensor as output, then you get non-euclidean general relativity. This argument goes W. Clifford 1876 -> K. Pearson (his book of essay) -> A. Einstein 1916, 1922, if you want to look up the references.
By number of independent hypotheses required, general relativity is a better theory with more explanatory power, because it doesn't require any supplementary hypothesis, whereas your theory requires 1 supplementary hypothesis.
It's too bad that Pearson (who mostly communicated Cliffords other unpublished lectures on curvature and intertia) and Einstein were frightful socialists, who lost their brains whenever they argued about economics. Clifford on the other hand wrote a very Mises-like (Theory and History) essay against "empirical" economics and social science, arguing that controlled experiments in social science are not possible, against the German Historical school.
This is why modern physics is a total sham.
Why anarchy fails