What is the significance of Shukumar seeing the neighbours walking past?
I think that the significance of seeing Shukumar seeing neighbors walk past is a projection of where he emotionally resides at that moment. Having both heard the revelation that will fundamentally transform his being and then shared an insight with Shoba that will fundamentally transform hers, Shukumar looks out the window and sees the Bradfords walking. At that instant, the Bradfords resemble a life that Shukumar knows that he and Shoba will never inhabit together. The Bradfords, walking "arm in arm" are emotionally connected with one another and are immersed in a life together that is not "a temporary matter." Even if this is not true, given Shukumar's alienation and grief, this is the image that is conveyed to him in seeing them walk past. It is an instant where Shukumar projects his own suffering and hurt into an existence, any existence, that is not his own. Lahiri's inclusion of this detail makes Shukumar's pain and Shoba's suffering that much more intact. It is a detail that reflects how a human being reacts when encountering immeasurable pain for the first instant. There is a desire to envision someone else's life because it is not their own. It is this detail that is conveyed when Shukumar sees the neighbors walking arm in arm past the window.