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From Wikipedia,

"The local energy conservation in quantum field theory is ensured by the quantum Noether's theorem for the energy-momentum tensor operator. Thus energy is conserved by the normal unitary evolution of a quantum system."

I know that it is the case in GR that conservation of energy and other conservation laws are relegated to being local only I thought this wasn't the case in quantum field theory.

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  • $\begingroup$ Noether's theorem means that energy is conserved because of time-translation invariance of the Lagrangian. This conservation is expressed locally, but is global. Hope what I said makes sense. $\endgroup$
    – Nikos M.
    Commented Feb 29 at 8:31
  • $\begingroup$ Related/worth checking physics.stackexchange.com/q/143892/226902 $\endgroup$
    – Quillo
    Commented Feb 29 at 8:32
  • $\begingroup$ @NikosM. I'm sorry could you explain what you mean by "expressed locally, but is global" If it's no trouble ofc. $\endgroup$ Commented Feb 29 at 8:34
  • $\begingroup$ @KleinMoretti conservation is global but changes occur locally. That is decrease in one place leads to increase in nearby place. $\endgroup$
    – Nikos M.
    Commented Feb 29 at 8:45
  • $\begingroup$ @NikosM. so you are saying that CE does hold globally and its actually local CE that has has problems? $\endgroup$ Commented Feb 29 at 9:05

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Yes the stress energy tensor operator, also in curved spacetime, satisfies the local conservation equation in a distributional sense $$\nabla_a :\hat{T}^{ab}:_{\omega}(x)=0\:,\tag{1}$$ where the "normal order" is referred to a (vacuum) Gaussian (Hadamard) state $\omega$.

I wrote a work on that many years ago which proves that also at the level of *-algebras of operators (I considered only the KG field, but the procedures can be extended to all types of fields).

Theorem 2.1 (with $V' \equiv 0$) in https://link.springer.com/content/pdf/10.1007/s00220-002-0702-7.pdf

There is also an arXives version.

(1) implies a global conservation law in the presence of a timelike Killing field $K$ $$\int_\Sigma \langle \Psi| :\hat{T}^{ab}:(x) \Psi \rangle K_a n_b d\Sigma(x) = constant$$ where $n$ is the unit vector normal to the Cauchy surface $\Sigma$, for every state $\Psi$ on a suitable domain in the Hilbert space of the GNS representation of $\omega$. It is not obvious that the integral is finite.

Concerning the issue of conservation laws in GR, they exist when Killing vector fields exist. Properly speaking, (1) is not a conservation law as it stands: in the sense of the equivalence principle is, but not in the sense of the Noether theorem.

However it becomes a conservaqtion law in proper sense if there is a Killing vector field. Let us prove it.

A conservation law is of the form $$\nabla_a J^a =0\tag{2}. $$ If this identity holds, the divergence theorem proves that $$\int_\Sigma J^a n_a d\Sigma = constant \tag{2'}$$ on Cauchy surfaces $\Sigma$, where $n$ is the unit vector normal to $\Sigma$. Obviously it is also necessary that the integral converges (Actually, even if it diverges, we may also consider it a global conservation law: the charge is aways infinite. It depends on personal taste.)

Equation $\nabla_a T^{ab}=0$ does not automatically imply (2). However, if there is a Killing field $K$, so that the Killing equation is valid $$\nabla_a K_b + \nabla_b K_a =0 \tag{3}\:,$$
then $J^a:= K_bT^{ba}$ (where $T^{ab}=T^{ba}$) satisfies (2). $$\nabla_a (K_bT^{ba}) = (\nabla_a K_b) T^{ba} + K_b \nabla_aT^{ba} = \frac{1}{2}(\nabla_a K_b + \nabla_b K_a ) \: T^{ba} +0 =0\:.$$

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    – Buzz
    Commented Mar 3 at 17:13