Editing Grand Unified Theory (section)

== Motivation ==
The fact that the [[electric charge]]s of [[electron]]s and [[proton]]s seem to cancel each other exactly to extreme precision is essential for the existence of the macroscopic world as we know it, but this important property of elementary particles is not explained in the Standard Model of particle physics. While the description of [[strong interaction|strong]] and weak interactions within the Standard Model is based on gauge symmetries governed by the [[simple Lie group|simple symmetry groups]] {{math|[[SU(3)]]}} and {{math|[[SU(2)]]}} which allow only discrete charges, the remaining component, the [[weak hypercharge]] interaction is described by an [[Abelian group|abelian symmetry]] {{math|[[U(1)]]}} which in principle allows for arbitrary charge assignments.<ref group=note>There are however certain constraints on the choice of particle charges from theoretical consistency, in particular [[gauge anomaly|anomaly cancellation]].</ref> The observed [[charge quantization]], namely the postulation that all known [[elementary particle]]s carry electric charges which are exact multiples of one-third of the [[elementary charge|"elementary" charge]], has led to the idea that hypercharge interactions and possibly the strong and weak interactions might be embedded in one Grand Unified interaction described by a single, larger simple symmetry group containing the Standard Model. This would automatically predict the quantized nature and values of all elementary particle charges. Since this also results in a prediction for the relative strengths of the fundamental interactions which we observe, in particular, the [[weak mixing angle]], grand unification ideally reduces the number of independent input parameters but is also constrained by observations.

Grand unification is reminiscent of the unification of electric and magnetic forces by [[Maxwell equations|Maxwell's field theory of electromagnetism]] in the 19th century, but its physical implications and mathematical structure are qualitatively different.