201 Appendix D: Formation Time and Brick Estimates For the same momenta, Eq. (D.4), the leading order lightcone coordinate representation, yields exactly the same formation time, Eq. (D.6), but with p$_{z}$ replaced by E $^{+}$. p ≃ [(1 − x ) E $^{+}$, M 2 + k 2 (1 − x ) E $_{+}$, − k ] (D.8) k ≃ [ xE $^{+}$, M 2 + k 2 xE + , − k ] (D.9) τ$_{f}$ ∼ λ$_{g}$, (D.10) λ$_{g}$ = 1 σρ ∼ 1 ( α 2 µ 2 $^{)}$T 3 (D.11) ∼ 1 αT . (D.12) D.1.2 Bethe-Heitler to LPM The “speed limit” at which incoherent Bethe-Heitler turns over to coherent LPM, γ$_{c}$ , is found when the formation length for the produced radiation is on the order of the radiation’s mean free path: where τ$_{f}$ is given by Eq. (D.2) and