  How does nature hold together protons and neutrons to form the wide variety
of complex nuclei in the universe? Describing many-nucleon systems from the
fundamental theory of quantum chromodynamics has been the greatest challenge in
answering this question. The chiral effective field theory description of the
nuclear force now makes this possible but requires certain parameters that are
not uniquely determined. Defining the nuclear force needs identification of
observables sensitive to the different parametrizations. From a measurement of
proton elastic scattering on $^{10}$C at TRIUMF and ab initio nuclear reaction
calculations we show that the shape and magnitude of the measured differential
cross section is strongly sensitive to the nuclear force prescription.
