The vacuum state in the instant form defined at fixed t is acausal and infinitely complicated. For example, in quantum electrodynamics, bubble graphs of all orders, starting with the e + e − − γ γ intermediate state, appear in the ground state vacuum; however, as shown by Weinberg, such vacuum graphs are frame-dependent and formally vanish by powers of 1 / P 2 as the observer moves at P → → ∞ ∞ . Thus, one can again match the instant form to the front-form formulation where such vacuum loop diagrams do not appear in the QED ground state. This is because the + momentum of each constituent is positive, but must sum to zero in the vacuum state since the + momenta are conserved. However, unlike the instant form, no dynamical boosts are required, and the front form formulation is causal and frame-independent. The infinite momentum frame formalism is useful as an intuitive tool; however, the limit P → → ∞ ∞ is not a rigorous limit, and the need to boost the instant-form wave function introduces complexities.