[talanum1 (OP)]

We have that the Electromagnetic field has an Energy. Energy = ForcexDistance, thus there are forces in an Electromagnetic field. We know the energy density of an EM field = ExH. Then it is just dimensional analysis and the inspiration of Maxwell's Equations to arrive at the formula for force in an EM field:

F₂₁ = ,

where S is left arbritary and a cross-product fits between E and H and a dot product is assumed with dA, E, H being vector functions and A being a vector. x_1 is a coordinate of space. t is time, c is the speed of light, E is the electric field and H is the magnetic field. Quanta 2 is located at x_1+cdelta t and quanta 1 is at x_1.

The force could be on quanta of spacetime or on photons. Since there are forces we have dp/dt != 0 so the photon or spacetime quanta must have non-constant momentum, thus non-constant Energy.

[talanum1 (OP)]

Actually, the formula should read:

F₂₁ =

since the integrated has dimensions of energy per volume.

[Bored chemist]

Since there are forces we have dp/dt != 0 so the photon or spacetime quanta must have non-constant momentum, thus non-constant Energy.

Newton worked out that, if we have forces, we have a non constant momentum.
You seem to be a few centuries late. (And have probably come up with a circular argument)