to be a point-like particle. We now know, however, that, instead of being point-like,
the proton has a radius of about 10-15 meter and that it is a complex dynamical
system consisting of three "quarks".
The dynamics of this bound system are still imperfectly
understood but it is well-established that the underlying theory
is the SU(3) gauge theory now known as "quantum chromodynamics".
This theory of the
strong interactions
was proposed by
Yoichiro Nambu
in 1965.
The proton is a color neutral combination of three colored quarks (one of each color). The quarks are bound together by the exchange of color gluons. The emission or absorption of a gluon causes the quarks to make a transition from one color to another. Unlike the simplified animation presented here, the gluons are associated with two colors, that of the emitting quark and that of the receiving quark. "Color" in this context is used to specify a property of quarks having three possible values and is not meant to be taken in the literal sense of color as it appears to the human eye.
Nambu's original motivation for proposing the theory was the observation that such a force would guarantee that the lowest mass states were color "neutral". It is now thought, in fact, that non-color-neutral hadrons would have infinite mass.
In the proton, two of the quarks are of 'up' type flavor while the third is 'down' type. The neutron (neutral nucleon) is identical to the proton except it has two 'down' type and one 'up' type quark.
In the most advanced theories, the quarks and gluons are string-like rather than pointlike. Experimentally, we know only that the spatial extent of quarks and gluons is less than about 10^(-18) m, i.e. less than 1/1000 the radius of the proton.