I came to provide some reason to this thread, but I see it has enough already.
Note: There is some serious physics simulation software (see GADGET-2) that has some horribly named variables. It's very readable to a physicist, but makes a CS major bawl. Different area, different rules.
To state the obvious: There just isn't a 'global' set of rules on how we should name variables, even something like 'readability' and understandability differs from group to group. Physics equations fall apart when you start naming things "velocity_x" and then using it all over the place and being able to find an error in the code gets worse the less it looks like an equation and the more like code it looks. Tracking down these kinds of errors is a bitch.
But as someone reasonably stated: The wider the scope, the longer the name should be. Or, fuck, leave a comment that says "Look up the equation in such and such's paper, or book on page x"