On online slide notes, it is mentioned that:
Fermi Golden Rule: $$P_{if}=\frac{2\pi}{\hbar}|M_{if}|^2\rho_f$$ where $\rho_f$ is density of final sates --number of quantum states per unit volume - states in a cubical box wih side $l$ wih periodic boundary conditions - $\overrightarrow{p}=2\pi \overrightarrow{n}/l$
$$dp(p)=\frac{nbofstates}{l^3}=\frac{d^3p}{(2\pi )^3}$$
Then he wrote: $$\frac{d^3p}{2E(2\pi )^3}=\theta(p^0)\delta(p^2) \frac{d^4p}{(2\pi)^3}$$
and said conventional to make this relativistic.
I did not understand how can this be conventional? That is I did not understand also how did this derive from the Non-Relativistic equation above.
