▁He ▁added ▁that ▁a ▁dynamics ▁of ▁the ▁rig id ▁body ▁must ▁be ▁created ▁in ▁the ▁framework ▁of ▁SR . ▁Event ually , ▁Max ▁Born ▁( 1 9 0 9 ) ▁in ▁the ▁course ▁of ▁his ▁above - ment ioned ▁work ▁concerning ▁acceler ated ▁motion , ▁tried ▁to ▁include ▁the ▁concept ▁of ▁rig id ▁bodies ▁into ▁SR . ▁However , ▁Paul ▁Ehren fest ▁( 1 9 0 9 ) ▁showed ▁that ▁Born ' s ▁concept ▁lead ▁the ▁so - called ▁Ehren fest ▁par ado x , ▁in ▁which , ▁due ▁to ▁length ▁contra ction , ▁the ▁circum ference ▁of ▁a ▁rot ating ▁disk ▁is ▁short ened ▁while ▁the ▁radius ▁stays ▁the ▁same . ▁This ▁question ▁was ▁also ▁considered ▁by ▁Gustav ▁Her gl ot z ▁( 1 9 1 0 ), ▁Fritz ▁No ether ▁( 1 9 1 0 ), ▁and ▁von ▁La ue ▁( 1 9 1 1 ). ▁It ▁was ▁recognized ▁by ▁La ue ▁that ▁the ▁classic ▁concept ▁is ▁not ▁applicable ▁in ▁SR ▁since ▁a ▁" rig id " ▁body ▁poss esses ▁infinitely ▁many ▁degrees ▁of ▁freedom . ▁Yet , ▁while ▁Born ' s ▁definition ▁was ▁not ▁applicable ▁on ▁rig id ▁bodies , ▁it ▁was ▁very ▁useful ▁in ▁describing ▁rig id ▁mot ions ▁of ▁bodies . ▁In ▁connection ▁to ▁the ▁Ehren fest ▁par ado x , ▁it ▁was ▁also ▁discussed ▁( by ▁Vladimir ▁Vari ć ak ▁and ▁others ) ▁whether ▁length ▁contra ction ▁is ▁" real " ▁or ▁" app ar ent ", ▁and ▁whether ▁there ▁is ▁a ▁difference ▁between ▁the ▁dynamic ▁contra