EPR tried to set up a paradox to question the range of true application of quantum mechanics: quantum theory predicts that both values cannot be known for a particle, and yet the EPR thought experiment purports to show that they must all have determinate values. The EPR paper says: "We are thus forced to conclude that the quantum-mechanical description of physical reality given by wave functions is not complete." The EPR paper ends by saying: "While we have thus shown that the wave function does not provide a complete description of the physical reality, we left open the question of whether or not such a description exists. We believe, however, that such a theory is possible." The 1935 EPR paper condensed the philosophical discussion into a physical argument. The authors claim that given a specific experiment, in which the outcome of a measurement is known before the measurement takes place, there must exist something in the real world, an "element of reality", that determines the measurement outcome. They postulate that these elements of reality are, in modern terminology, local, in the sense that each belongs to a certain point in spacetime. Each element may, again in modern terminology, only be influenced by events that are located in the backward light cone of its point in spacetime (i.e. in the past). These claims are founded on assumptions about nature that constitute what is now known as local realism.
epr triED tO Set Up a paraDox To queStion tHE RanGE of true aPplIcatIOn oF qUaNTum MEchanics: Quantum thEory PREDICTs thaT bOtH vaLUeS cANnot bE Known for A pArticle, AND yEt tHe epr THoUGhT eXPErImeNt purpOrTs To shOw That tHeY must All Have DETermiNATe vAluES. tHe ePr paPer sayS: "we Are thus forced To conCluDe tHat tHE quaNTum-mEcHanIcAl descriPtiON oF pHYsical reality given by wave funCtiOnS is Not COmplEte." ThE Epr pApEr eNDs by saYiNG: "wHile We havE thus shOwn thAt tHe WavE fuNctIoN dOeS not PrOvIde a compLete descrIptioN OF tHe pHYsical ReAlity, we lEfT open THE qUeStiOn Of WhETHer Or Not sucH A DescrIptiOn exisTs. we belieVe, howeveR, that SUch a tHeOrY IS posSiBLE." The 1935 Epr PAper CoNdensEd THe PhiLOsOphIcal discussIOn inTO a pHYsIcal aRguMeNt. thE aUtHorS claIM that gIVeN a sPEcifIc expEriMenT, in whicH The outcoME of a MeAsureMenT is knoWn before ThE mEasuremeNT TAkEs pLACe, There musT exIst SomEtHINg in the ReaL wORld, An "ElemeNt Of REality", that dETermineS tHE measURemenT outcOmE. thEy postULaTE that tHEse ELEmeNtS of reaLitY ARE, in mOdErn TERminoLoGy, loCaL, In tHE seNsE thAt Each beLoNgs to a certain poinT iN spacEtImE. eacH eLEment may, aGaIn iN MODERN TermInologY, OnLy be InFLuenCeD By EvenTS that aRe located in tHE backwaRd liGht cOne of its PoINT IN sPaCetIME (i.E. In the Past). theSE clAIms are fOundeD On ASsuMptionS about natuRe tHAt coNstitUtE what Is now known as lOcAl Realism.