Mass-energy equivalence:
https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalenc... E=mc^2
E=(m)(c^2)
E=(0)(c^2)
E=0
But that's for at-rest inertia. Photons are only very rarely if ever "at rest".
(Are photons "at relative rest" in Lagrangian points, accretion discs, superfluids, 0 Kelvin, and/or when created from just n-body gravity, etc?)
Energy-Momentum relation: https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relati... :
> [Energy-Momentum relation] is the extension of mass–energy equivalence for bodies or systems with non-zero momentum. It can be written as the following equation:
E^2 = (p^2)(c^2)+(m^2)(c^4)
E^2 = ppcc+(0)(c^4)
E = sqrt(ppcc)
E = sqrt((p^2)(c^2))
c is the speed of light in a vacuum; without superfluidity (which occurs in helium in deep space for example); and without nonlocal entanglement; and without loophole-free solutions to spacetime (quantum teleportation).
Anyways, further from "Energy-Momentum relation":
> The Dirac sea model, which was used to predict the existence of antimatter, is closely related to the energy–momentum relation
From "Dirac Sea" https://en.wikipedia.org/wiki/Dirac_sea :
> The Dirac sea interpretation and the modern QFT interpretation are related by what may be thought of as a very simple Bogoliubov transformation, an identification between the creation and annihilation operators of two different free field theories.
> [...] Dirac sea theory has been displaced by quantum field theory, though they are mathematically compatible
SQG Superfluid Quantum Gravity says there needn't be antimatter; there is pressure and there are phases of matter in a varyingly Superfluidic cosmos. The Dirac wikipedia article also mentions SQG.
And further from "Energy-Momentum relation":
> The quantities E, p, E′, p′ are all related by a Lorentz transformation. The relation allows one to sidestep Lorentz transformations when determining only the magnitudes of the energy and momenta by equating the relations in the different frames