As it turns out, the color field that holds quarks together to form hadrons, such as protons and neutrons, and mesons, such as pions, muons, and kaons, acts like a magnetic field in a superconductor. This slide shows the quarks and color force field lines in a proton, which is made of 2 up quarks and a down quark. Single field lines bind each pair of differently colored quarks together. At the low energies that these particles typically experience on Earth, the field lines are relaxed and only bind the quarks loosely, which allows the quarks to move freely within the particles they comprise. When quarks become further separated at higher energies, the field lines become stretched, which causes them to bind the quarks strongly, drawing any wayward quark back in toward the other quarks. Unlike the electromagnetic force, this restoring force does not diminish as the separation between quarks increases. The original string theory was a theory that described the color force field lines as strings connecting the quarks that acted in these ways. Another way to describe this behavior is to model the field lines as quantized loops of electric flux. This approach inspired a similar approach to describing the gravitational field called "loop quantum gravity".