Hypothetically, if you had a gas made of really heavy atoms/molecules (although it's unlikely that they would be stable at the kind of Uranium-heavy I'm thinking of, let's say that hypothetically they are) in a liquid made of really really light molecules or atoms (e.g hydrogen), and say that temperature isn't going to change the given states of the elements (very very hypothetically), and so they can exist in the states they need to together, could the bubbles go down? Or would the relative densities make the masses of the atoms and molecules irrelevant? 

Whilst studying Mechanics (M1), you often use "light and inextensible" to describe string. So my friend posed a hypothetical question that if you had some string that was 1million light years long with me on one end and him on the other. If he pulled one end how long would it take for the my end to move. I thought it would be instantaneous but my teacher thought different.