In QFT, the n-point Green's functions, or correlation functions, are de ned by this quantity for a time-ordered product of  elds, which itself is related to the scattering amplitude by the LSZ reduction formula.
In QFT, the n-point Green's functions, or correlation functions, are de ned by this quantity for a time-ordered product of  elds, which itself is related to the scattering amplitude by the LSZ reduction formula. The time derivative of this quantity for a time-independent operator is "i over h-bar" times this quantity for the operator's commutator with the Hamiltonian, a more general case of Ehrenfest's theorem. This value equals the trace of the density matrix, rho, times an operator. It can be de ned as the bra of a state times an operator times the ket of a state, or as the integral over all space of the product of the conjugate wavefunction, an operator, and the wavefunction. This value for an operator squared minus the square of this value for an operator is a de nition of uncertainty. For 10 points, what quantity is the mean value of an observable in quantum mechanics?
In QFT, the n-point Green's functions, or correlation functions, are de ned by this quantity for a time-ordered product of  elds, which itself is related to the scattering amplitude by the LSZ reduction formula.
vacuum expectation value [or VEV; or expected value; or condensate; accept mean value before "mean"; prompt on average value]