Formally, there is a simple connection between the Hamiltonian formulation of quantum field theories quantized at fixed time t (the "instant form") where the observer is moving at infinite momentum and light-front Hamiltonian theory quantized at fixed light-front time τ τ = t + z / c (the "front form"). A typical energy denominator in the instant-form is 1 / [ E i n i t i a l − − E i n t e r m e d i a t e + i ϵ ϵ ] where E i n t e r m e d i a t e = ∑ ∑ j E j = ∑ ∑ j m 2 + k → → j 2 is the sum of energies of the particles in the intermediate state. In the IMF, where the observer moves at high momentum P in the negative z ^ ^ direction, the leading terms in P cancel, and the energy denominator becomes 2 P / [ M 2 − − ∑ ∑ j [ k ⊥ ⊥ 2 + m 2 x i ] j + i ϵ ϵ ] where M 2 is invariant mass squared of the initial state. Thus, by keeping the terms in 1 P in the instant form, one recovers the energy denominator which appears in light-front Hamiltonian theory. This correspondence has a physical meaning: measurements made by an observer moving at infinite momentum is analogous to making observations approaching the speed of light—thus matching to the front form where measurements are made along the front of a light wave. An example of an application to quantum electrodynamics can be found in the work of Brodsky, Roskies and Suaya.