Again in analogy with general relativity, the fact that the laws of nature are unaffected even if the mixtures vary from place to place and time to time makes it necessary to include a family of fields in the theory that interact with quarks, analogous to the gravitational field. There are eight of these fields; they are known as gluon fields because the strong forces they produce glue the quarks together inside the proton and neutron. Our modern theory of these forces, quantum chromo dynamics, is nothing but the theory of quarks and gluons that respects this local color symmetry.
