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The equation for general relativity E^2= M^2c^4 + P^2c^2 is usually reduced in special relativity using the argument:

If the mass is at rest, momentum is zero, therefore

E^2 = 0 + M^2c^4

Therefore

E = Mc2.

Similarly if M is zero,

E^2 = p^2c^2 + 0

And therefore E = pc

The problem with this is that pc is only = to mc^2 if m is at rest.

If M is in motion there is an additional kinetic energy component E= 1/2mv^2, therefore the total energy is more than pc.

As a consequence I think that the intrinsic momentum, as represented by the massless photon, has to be considered separately from inertial momentum of a particle.

Comments please.