"For every symmetry there comes a constraint...If physics is to look the same when the origin of time is shifted...[o]nly those processes that conserve energy are allowed. ...If physical law is to be immune to the arbitrary displacement of our spatial axes, then nature requires the conservation of . ...If the laws are to be unaffected by the arbitrary rotation of a coordinate system, then must be conserved...If the laws are to be the same for all inertial observers, then the space-time interval must be invariant...[A]nother constraint...so beautiful as to make one jaw drop in wonder...symmetry creates force...[T]he symmetry of identical particles forces matter...to be enrolled as either or ...Bosons, typified by the photon, carry the fundamental forces that cause fermions to attract and repel. Fermions, led by electrons and quarks, become constituents of ordinary matter...Gravity. Electromagnetism. The strong force. The weak force. Each is called into being by the requirements of a particular local symmetry."