s ▁accept ance ▁of ▁Fe yn man ' s ▁work . ▁J . ▁Robert ▁O ppen heimer , ▁in ▁particular , ▁was ▁persu aded ▁by ▁D ys on ▁that ▁Fe yn man ' s ▁new ▁theory ▁was ▁as ▁valid ▁as ▁Schw inger ' s ▁and ▁Tom on aga ' s . ▁ ▁Also ▁in ▁ 1 9 4 9 , ▁in ▁related ▁work , ▁D ys on ▁invent ed ▁the ▁D ys on ▁series . ▁It ▁was ▁this ▁paper ▁that ▁inspired ▁John ▁Ward ▁to ▁derive ▁his ▁celebrated ▁Ward – T ak ah ashi ▁identity . ▁ ▁D ys on ▁joined ▁the ▁fac ulty ▁at ▁Corn ell ▁as ▁a ▁physics ▁professor ▁in ▁ 1 9 5 1 , ▁though ▁he ▁still ▁had ▁no ▁doctor ate . ▁In ▁December ▁ 1 9 5 2 ▁O ppen heimer , ▁the ▁director ▁of ▁the ▁Institute ▁for ▁Advanced ▁Study ▁in ▁Pr inc eton , ▁New ▁Jersey , ▁offered ▁D ys on ▁a ▁lifetime ▁appointment ▁at ▁the ▁Institute , ▁" for ▁prov ing ▁me ▁wrong ", ▁in ▁O ppen heimer ' s ▁words . ▁D ys on ▁remained ▁at ▁the ▁Institute ▁until ▁the ▁end ▁of ▁his ▁career . ▁In ▁ 1 9 5 7 ▁he ▁became ▁a ▁US ▁citiz en . ▁ ▁From ▁ 1 9 5 7 ▁to ▁ 1 9 6 1 ▁D ys on ▁worked ▁on ▁Project ▁Or ion , ▁which ▁proposed ▁the ▁possibility ▁of ▁space - fl ight ▁using ▁nuclear ▁pul se ▁prop ulsion . ▁A ▁prototype ▁was ▁demonstrated ▁using ▁conventional ▁explos ives , ▁but