Thelastthirtyyearshaveseenanenormousadvanceinourunderstandingofthemicroscopicworld.Wenowhaveaconvincingpictureofthefundamentalstruc-tureofobservablematterintermsofcertainpoint-likeelementaryparticles.Wealsohaveacomprehensivetheorydescribingthebehaviourofandtheforcesbetweentheseelementaryparticles,whichwebelieveprovidesacompleteandcorrectdescriptionofnearlyallnon-gravitationalphysics.
Matter,soitseems,consistsofjusttwotypesofelementaryparticles:quarksandleptons.Thesearethefundamentalbuildingblocksofthematerialworld,outofwhichweourselvesaremade.Thetheorydescrib-ingthemicroscopicbehaviouroftheseparticleshas,overthepastdecadeorso,becomeknownasthe'StandardModel',providingasitdoesanaccurateaccountoftheforceofelectromagnetism,theweaknuclearforce(responsibleforradioactivedecay),andthestrongnuclearforce(whichholdsatomicnucleitogether).TheStandardModelhasbeenremarkablysuccessful;upuntilayearortwoagoallexperimentaltestshaveverifiedthedetailedpredictionsofthetheory.
TheStandardModelisbasedontheprincipleof'gaugesymmetry',whichassertsthatthepropertiesandinteractionsofelementaryparticlesaregovernedbycertainfundamentalsymmetriesrelatedtofamiliarconservationlaws.