Abstract: What the world looks like if one travels at the speed of light? We began to seriously answer this kind of question when probing the internal structure of the proton in high-energy scattering, following the pioneering work of R.P. Feynman. After half a century, we are about to get precise images of a light-traveling proton through an electron-ion collider and checking them against the fundamental theory of strong interactions through a novel theoretical method: large-momentum effective theory (LaMET) in lattice quantum chromodynamics.