James D. Bjorken, a theoretical physicist at the Stanford Linear Accelerator Center (SLAC), predicted in 1967 a very specific mathematical property that data from an experiment not yet performed should exhibit: colliding high-energy electrons against protons far more violently than in conventional elastic scattering — what is known as deep inelastic scattering. Bjorken predicts that the so-called 'structure functions' of the proton, instead of depending on the absolute energy of the experiment, should depend only on a dimensionless combination of kinematic quantities: the property since known as 'Bjorken scaling'. The prediction is counterintuitive and far from trivial at the time, and implies something radical: that the proton, far from being a uniform entity, contains inside it point-like, essentially free constituents, off which the electron bounces as in a miniature Rutherford scattering experiment. Bjorken convinces the SLAC experimental team, in long conversations during mountaineering trips with Henry Kendall and other colleagues, to test this idea with SLAC's newly inaugurated linear accelerator. Between 1968 and 1969, experiments by Jerome Friedman, Henry Kendall, and Richard Taylor conclusively confirm Bjorken scaling, providing the first experimental evidence that the proton has a substructure of point-like charged particles — the quarks that Gell-Mann and Zweig had proposed in 1964 as a useful mathematical construct, but whose real physical existence remained unconfirmed. Richard Feynman soon after reformulates Bjorken's idea as the 'parton model', widely adopted to interpret the quark composition of hadrons at high energies. Bjorken scaling catalyzes the search for a fundamental quantum theory explaining why quarks behave as nearly free particles at very short distances within the proton — a property that, resolved in 1973 through the discovery of asymptotic freedom (Gross, Wilczek, and Politzer, Nobel 2004), gives rise to quantum chromodynamics (QCD), the accepted theory of the strong interaction.