In my opinion Q and R represent Quest and Response.
In this A to Z of knowledge, he has reached Q, as soon as he gets a Response/Reply to that he'll take an unhindered ride to Z. This point in time of Mr. Ramsay's psyche represents the "thinking-block" of a scientist, a point where one is near the peak but it does not show up, this point in time is like the mist between the thinker and the truth. As soon as this mist lifts up, one sees the ultimate truth. This journey from Q to R is a little step which requires a lot of inbuilt courage and depth of knowledge. Once one steps on to R, he zooms past all the rest of alphabets and reaches Z.