Fermi's golden rules

Fermi's golden rules

[¦fer·mēz ¦gōld·ən ′rülz]
(quantum mechanics)
The equations giving the first-order (rule number 2) and second-order (rule number 1) contributions to the transition probability per unit time induced by a perturbation Hamiltonian, in terms of matrix elements of the perturbation Hamiltonian.