▁in ▁ 1 9 5 6 ▁under ▁Hermann ▁Bond i . ▁ ▁Scient ific ▁work ▁Pir ani ▁performed ▁post - do ctor al ▁research ▁at ▁the ▁Institute ▁for ▁Advanced ▁Studies ▁in ▁Dublin . ▁In ▁ 1 9 5 8 ▁he ▁started ▁teaching ▁at ▁King ' s ▁College ▁London ▁( where ▁Bond i ▁was ▁teaching ) ▁and ▁in ▁ 1 9 6 8 ▁became ▁professor ▁of ▁rational ▁mechan ics ▁there . ▁ ▁In ▁ 1 9 5 7 ▁Pir ani ▁independently ▁discovered ▁what ▁was ▁later ▁called ▁the ▁Pet rov ▁classification ▁( also ▁Pet rov – P ir ani – P en rose ▁classification ) ▁and ▁separately ▁discovered ▁by ▁Pet rov ▁in ▁ 1 9 5 4 . ▁ ▁In ▁ 1 9 5 9 ▁Bond i , ▁Pir ani ▁and ▁I vor ▁Robinson ▁published ▁a ▁fundamental ▁paper ▁on ▁gravit ational ▁wave ▁solutions ▁in ▁general ▁relativ ity ▁and ▁showed ▁the ▁existence ▁of ▁plane ▁gravit ational ▁wave ▁solutions . ▁Pir ani ' s ▁work ▁with ▁Bond i ▁and ▁Robinson ▁resulted ▁in ▁correspond ence ▁between ▁Pir ani ▁and ▁Albert ▁Ein stein , ▁some ▁of ▁whose ▁partially ▁expressed ▁views ▁on ▁the ▁subject ▁had ▁been ▁challeng ed ▁by ▁the ▁paper . ▁ ▁In ▁ 1 9 7 2 ▁Pir ani , ▁J ürgen ▁E hl ers ▁and ▁Alfred ▁Sch ild ▁showed ▁that ▁the ▁space - time ▁geometry ▁of ▁general ▁relativ ity ▁can ▁be ▁constructed ▁from ▁simple ▁meas uring ▁processes ▁with ▁light ▁be ams ▁and ▁free - fall ing ▁particles . ▁ ▁Popular ▁books ▁In ▁ 1 9 6 0