Imagine you and your friend are on a huge sphere like the moon (radius r). You start at two oposite sides of the sphere and you move with a constant speed (v) yet in a completely random way (not the best example, but somehow like molecules). How long does it take until you and your friend meet eachother (the distance between you becomes less than d? And what about this question in just 2 dimensions (2 points moving on a circle)?
Interesting to know would also be a function that shows the probability of both friends having met eachother after a certain amount of time has passed. ( p(t) = ...)
I know that my question is similar to this problem: "Fastest way to meet, without communication, on a sphere?" but I couldn't find the answer to my question in the comments.