Abstract: In well behaved quantum field theories, all states are supposed to “look like the vacuum at short distances.” Making this statement precise, however, has been a serious challenge. One important reason to understand the universal short-distance structure in quantum field theory comes from the semiclassical approach to quantum gravity, where black hole “area operators” universally introduce a vacuum-like entanglement defect in the semiclassical Hilbert space. From this perspective, finding universal short-distance structure in quantum field theory would help to confirm the old notion that gravity smooths out naive divergences in the entanglement of quantum fields. In this talk I will explain the “boost conjecture” for modular flow, which is a sharp form of the universality of ultraviolet entanglement, then I will partially resolve the boost conjecture in free field theory by showing that states with non-vacuum-like modular flows cannot be constructed in the vacuum Hilbert space. Based on work in progress with Gautam Satishchandran.