On his travels he learnt that Poincaré had written a paper on quantum theory which gave similar results to those in his Annalen der Physik paper. Poincaré had not known of Ehrenfest's contribution and therefore had not referred to his work.
Sa kanyang mga travels niya learnt na Poincaré had nakasulat sa isang papel sa quantum theory na ibinigay sa mga katulad na mga resulta sa kanyang Annalen der Physik papel. Poincaré ay hindi kilala ng Ehrenfest ng kontribusyon at sa gayon ay hindi tinukoy sa kanyang trabaho.