A proton is made of interacting quarks and gluons described by a relativistic quantum field theory, quantum chromodynamics (QCD). The proton structure has mostly been studied through high-energy scattering such as at LHC, and also future Electron-Ion Collider in the US, in which it travels at near the speed of light relative to experimental probes (observers). By approximating the near-light travels by exactly light-travel, Feynman introduced a tremendously useful as well as beautifully simple language of partons. However, solving the quantum structure of a light-travelling system in QCD partons has posed serious challenges in non-perturbative quantum field theory. I will describe a recent attempt of tackling this problem through a field theoretical rendition of Feynman’s original conception of partons, large-momentum effective field theory, along with large-scale numerical simulations of QCD on Euclidean lattices.