▁str esses ") ▁to ▁ensure ▁the ▁stability ▁of ▁the ▁electrons ▁and ▁to ▁explain ▁length ▁contra ction . ▁He ▁also ▁sketch ed ▁a ▁Lor ent z - in variant ▁model ▁of ▁grav itation ▁( including ▁gravit ational ▁waves ) ▁by ▁extending ▁the ▁valid ity ▁of ▁Lor ent z - in var iance ▁to ▁non - elect rical ▁forces . ▁ ▁Event ually ▁Po inc ar é ▁( in dep end ently ▁of ▁Ein stein ) ▁finished ▁a ▁substantial ly ▁extended ▁work ▁of ▁his ▁June ▁paper ▁( the ▁so - called ▁" Pal ermo ▁paper ", ▁received ▁July ▁ 2 3 , ▁printed ▁December ▁ 1 4 , ▁published ▁January ▁ 1 9 0 6 ▁). ▁He ▁spoke ▁literally ▁of ▁" the ▁post ulate ▁of ▁relativ ity ". ▁He ▁showed ▁that ▁the ▁transformations ▁are ▁a ▁consequence ▁of ▁the ▁principle ▁of ▁least ▁action ▁and ▁developed ▁the ▁properties ▁of ▁the ▁Po inc ar é ▁str esses . ▁ ▁He ▁demonstrated ▁in ▁more ▁detail ▁the ▁group ▁characteristics ▁of ▁the ▁transformation , ▁which ▁he ▁called ▁the ▁Lor ent z ▁group , ▁and ▁he ▁showed ▁that ▁the ▁combination ▁ ▁is ▁invariant . ▁While ▁elabor ating ▁his ▁gravit ational ▁theory , ▁he ▁said ▁the ▁Lor ent z ▁transformation ▁is ▁merely ▁a ▁rotation ▁in ▁four - dimensional ▁space ▁about ▁the ▁origin , ▁by ▁introdu cing ▁ ▁as ▁a ▁fourth ▁imag inary ▁coordinate ▁( contr ary ▁to ▁Pal agy i , ▁he ▁included ▁the ▁speed ▁of ▁light ), ▁and ▁he ▁already ▁used ▁four - ve ctors . ▁He ▁wrote ▁that ▁the ▁discovery ▁of