What does the identity operator look like in QFT, written in momentum eigenstates?
106506
How to derive completeness relation in quantum field theory with a Lorentz invariant measure?
128276
Quantization of orbital angular momentum
59004
Why does time evolution operator have the form $U(t) = e^{-itH}$?
89295
The interpretation of mass in quantum field theories
101171
Why do all fields in a QFT transform like *irreducible* representations of some group?
29952
Time-ordering vs normal-ordering and the two-point function/propagator
14999
How does wave function collapsing fit into QFT?
24072
Intuitive explanation of why $\dim\operatorname{Im} T + \dim\operatorname{Ker} T = \dim V$
103624
What is the Physical Meaning of Commutation of Two Operators?
143157
Perturbation theory: justifying expansion in terms of eigenstates of the basis Hamiltonian
44843
What is the difference between $|0\rangle $ and $0$?
103579
Why are the zeroth order terms in degenerate perturbation theory the eigenstates of the perturbing Hamiltonian?
130016
How to decompose a unitary transform into two-level unitary matrices?
75365
How are string vibration modes related to particle identity?
131906
"An operator is hermitian". Implications?
14203
Finding the creation/annihilation operators
49759
Proving that $i\hbar\frac{\partial}{\partial \mathbf{p}}$ is the operator of $\mathbf{x}$ in momentum space