Compton scattering
Since the proton is composed of quarks confined by gluons, an equivalent pressure which acts on the quarks can be defined. This allows calculation of their distribution as a function of distance from the centre using Compton scattering of high-energy electrons (DVCS, for "deeply virtual Compton scattering"). The pressure is maximum at the centre, about 1035 Pa, which is greater than the pressure inside a neutron star. It is positive (repulsive) to a radial distance of about 0.6 fm, negative (attractive) at greater distances, and very weak beyond about 2 fm.