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# Golden Rule

Golden Rule, in the New Testament, saying of Jesus. In the Gospel of Matthew he says, "In everything do to others as you would have them do to you; for this is the law and the prophets." The Gospel of St. Luke has "Do to others as you would have them do to you." It is stated negatively in the Book of Tobit in the Arpocrypha.
In quantum physics, Fermi's golden rule is a way to calculate the transition rate (probability of transition per unit time) from one energy eigenstate of a quantum system into a continuum of energy eigenstates, due to a perturbation.

We consider the system to begin in an eigenstate $| irangle$ of a given Hamiltonian $H_0$. We consider the effect of a (possibly time-dependent) perturbing Hamiltonian $H\text{'}$. If $H\text{'}$ is time-independent, the system goes only into those states in the continuum that have the same energy as the initial state. If $H\text{'}$ is oscillating as a function of time with an angular frequency $omega,$, the transition is into states with energies that differ by $hbaromega$ from the energy of the initial state. In both cases, the one-to-many transition probability per unit of time from the state $| i rangle$ to a set of final states $| frangle$ is given, to first order in the perturbation, by

$T_\left\{i rightarrow f\right\}= frac\left\{2 pi\right\} \left\{hbar\right\} left | langle f|H\text{'}|i rangle right |^\left\{2\right\} rho,$
where $rho$ is the density of final states (number of states per unit of energy) and $langle f|H\text{'}|i rangle$ is the matrix element (in bra-ket notation) of the perturbation $H\text{'}$ between the final and initial states.

Fermi's golden rule is valid when the initial state has not been significantly depleted by scattering into the final states.

The most common way to derive the equation is to start with time-dependent perturbation theory and to take the limit for absorption under the assumption that the time of the measurement is much larger than the time needed for the transition.

Although named after Fermi, most of the work leading to the Golden Rule was done by Dirac who formulated an almost identical equation, including the three components of a constant, the matrix element of the perturbation and an energy difference. It is given its name due to the fact that, being such a useful relation, Fermi himself called it "Golden Rule No. 2.