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Conservation of energy (and momentum) can be expressed as the
divergence of the stress-energy tensor:
0 = ∇i Tij [1]
As always, there is an implicit sum over i=0,1,2,3.
The j=0 piece of equation [1] expresses conservation of energy.
The j=1,2,3 pieces express conservation of the various components
of momentum.
The j=0 piece says that the power is equal to -1 times the 3D
(spacelike) divergence of the flow of energy.