When the grandfather is explaining to the son what he thinks happened to the
boy's father, the grandfather makes a comment to the effect that he doesn't understand it all because he's not 'a specialist in quantum mechanics'. This movie deals with time travel and relativity, not quantum mechanics. Relativity deals with the large scale and quantum mechanics deals with the small scale. The reconciliation of the two has been the holy grail of physics, 'Unified Field Theory'. To object that one doesn't understand time travel because one is not a quantum mechanic is nonsensical. No physics professor would make that mistake.