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V

_{g}=pC

^{2}/E=kC

^{2}/ω

where p=momentum, E=rest energy, k=wavenumber and ω=frequency and C the speed of light,

so that inertia is m=E/C

^{2}classically or

i= ω/C

^{2}quantum mechanically

As long as we don't change the rest energy or total momentum (wavenumber or total frequency) of a wavegroup it will continue to propagate in a straight, unaccelerated line. The property of inertia arises from the combination of quantum mechanics and special relativity, no deep mystery there.

And that's the modern view.