▁equal ▁to ▁that ▁of ▁light . ▁Similar ▁models ▁to ▁that ▁of ▁Po inc ar é ▁were ▁discussed ▁by ▁M ink owski ▁( 1 9 0 7 b ) ▁and ▁Sommer feld ▁( 1 9 1 0 ). ▁However , ▁it ▁was ▁shown ▁by ▁Abraham ▁( 1 9 1 2 ) ▁that ▁those ▁models ▁belong ▁to ▁the ▁class ▁of ▁" vector ▁theories " ▁of ▁grav itation . ▁The ▁fundamental ▁defect ▁of ▁those ▁theories ▁is ▁that ▁they ▁implicitly ▁contain ▁a ▁negative ▁value ▁for ▁the ▁gravit ational ▁energy ▁in ▁the ▁vic inity ▁of ▁matter , ▁which ▁would ▁viol ate ▁the ▁energy ▁principle . ▁As ▁an ▁alternative , ▁Abraham ▁( 1 9 1 2 ) ▁and ▁Gustav ▁Mie ▁( 1 9 1 3 ) ▁proposed ▁different ▁" scal ar ▁theories " ▁of ▁grav itation . ▁While ▁Mie ▁never ▁form ulated ▁his ▁theory ▁in ▁a ▁consistent ▁way , ▁Abraham ▁completely ▁gave ▁up ▁the ▁concept ▁of ▁Lor ent z - cov ar iance ▁( even ▁locally ), ▁and ▁therefore ▁it ▁was ▁irre con cil able ▁with ▁relativ ity . ▁ ▁In ▁addition , ▁all ▁of ▁those ▁models ▁viol ated ▁the ▁equivalence ▁principle , ▁and ▁Ein stein ▁argued ▁that ▁it ▁is ▁impossible ▁to ▁form ulate ▁a ▁theory ▁which ▁is ▁both ▁Lor ent z - cov ari ant ▁and ▁satisfies ▁the ▁equivalence ▁principle . ▁However , ▁G unn ar ▁Nord ström ▁( 1 9 1 2 , ▁ 1 9 1 3 ) ▁was ▁able ▁to ▁create ▁a ▁model ▁which ▁ful filled ▁both ▁conditions . ▁This ▁was ▁achieved ▁by ▁making ▁both