In quantum field theory, with the field ϕ and the momentum π being operators, their time evolution is governed (in the Heisenberg-picture) by the Heisenberg equation:
˙ϕ=iℏ[ˆH,ϕ]˙π=iℏ[ˆH,π].
Now, in case the Hamiltonian operator ˆH=∫d3x ˆH can be written as an integral over the hamiltonian density ˆH, and the fields and the momenta commute at non-equal positions, do the same equations hold as well with the Hamiltonian operator being replaced by it's density? What would the caveats be?
˙ϕ=iℏ[ˆH,ϕ]˙π=iℏ[ˆH,π].
Answer
You have ˆH=∫d3xˆ˜H(x). That implies that canonical Relations will be slightly altered.
For a Quantum field Operator ˆϕ(x′,t) distributed over space x′ and time t, you will have a relation like the following:
[ˆ˜H(x),ˆϕ(x′,t)]=∂∂t^ϕ(x′,t)δ(x−x′).
The Delta function factor ensures not only the commutation of Operators for nonequal space Points; also that after Integration over space, the ordinary commutation Relations are obtained
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