When Gabriel knocks on Einstein's door the neighbour complains "Why don't you people go to his office?" This indicates a number of people have been knocking on Einstein's door. Were there other time travellers, Visiting Einstein for similar reasons to Gabriel? This isn't unreasonable. Once one scientist has made a breakthrough, other scientists, often unconnected with the first, start to make similar breakthroughs.
When the professor dismisses his class and is talking to his grandson, a bashful male student hurries between them and surreptitiously places an apple on the professor's desk. This is an inside reference to a similar bit in 'Raiders of the Lost Ark'.