▁the ▁gravit ational ▁and ▁the ▁in ert ial ▁mass ▁dependent ▁on ▁the ▁gravit ational ▁potential . ▁Nord ström ' s ▁theory ▁of ▁grav itation ▁was ▁remarkable ▁because ▁it ▁was ▁shown ▁by ▁Ein stein ▁and ▁Ad ria an ▁F ok ker ▁( 1 9 1 4 ), ▁that ▁in ▁this ▁model ▁grav itation ▁can ▁be ▁completely ▁described ▁in ▁terms ▁of ▁space - time ▁curv ature . ▁Although ▁Nord ström ' s ▁theory ▁is ▁without ▁contradiction , ▁from ▁Ein stein ' s ▁point ▁of ▁view ▁a ▁fundamental ▁problem ▁pers isted : ▁It ▁doesn ' t ▁ful fill ▁the ▁important ▁condition ▁of ▁general ▁cov ar iance , ▁as ▁in ▁this ▁theory ▁preferred ▁frames ▁of ▁reference ▁can ▁still ▁be ▁form ulated . ▁So ▁contrary ▁to ▁those ▁" scal ar ▁theories ", ▁Ein stein ▁( 1 9 1 1 – 1 9 1 5 ) ▁developed ▁a ▁" tensor ▁theory " ▁( i . e . ▁general ▁relativ ity ), ▁which ▁ful fill s ▁both ▁the ▁equivalence ▁principle ▁and ▁general ▁cov ar iance . ▁As ▁a ▁consequence , ▁the ▁notion ▁of ▁a ▁complete ▁" special ▁relativ istic " ▁theory ▁of ▁grav itation ▁had ▁to ▁be ▁given ▁up , ▁as ▁in ▁general ▁relativ ity ▁the ▁const ancy ▁of ▁light ▁speed ▁( and ▁Lor ent z ▁cov ar iance ) ▁is ▁only ▁locally ▁valid . ▁The ▁decision ▁between ▁those ▁models ▁was ▁brought ▁about ▁by ▁Ein stein , ▁when ▁he ▁was ▁able ▁to ▁exactly ▁derive ▁the ▁peri hel ion ▁pre cess ion ▁of ▁Mercur y , ▁while ▁the ▁other ▁theories ▁gave