Sentence:  If a twin goes off on a spaceship for a long trip at nearly the speed of light, that twin would be much younger on return to earth than the twin who <WSD>stayed behind</WSD> (Fock, 1964; French, 1968; Hawking, 1988; Young, 1992). 
Meaning of the word marked with <WSD> tags:  To remain where one is, whilst others leave.