Photon-photon pair production
= Photon-photon pair production
{title2=$E_1E_2(1-\cos\alpha)\geq2m_e^2c^4$}
Two <photons> meeting at angle $\alpha$ can produce an <electron> and a <Positron> when their combined <four-momentum> has sufficient <invariant mass>. For a head-on collision the threshold is $E_1E_2=m_e^2c^4$. Threshold means zero relative <momentum> in the <centre-of-momentum frame>, not necessarily in the laboratory.