How do physicists know the strong force becomes weaker at short distances?

Physicists know the strong force weakens at very short distances from measurements in particle accelerators and from theoretical calculations in quantum chromodynamics. In deep inelastic scattering experiments, such as those at SLAC in the late 1960s and later at CERN, high-energy probes revealed that protons contain pointlike quarks. When particles are smashed together at higher energies, quarks and gluons inside hadrons interact less strongly than they do at low energies. This counterintuitive behavior is called asymptotic freedom and was explained in 1973 by David Gross, Frank Wilczek, and David Politzer. It is one of the key reasons QCD is so different from everyday forces.