I'm not quite sure what you're saying. I don't think there's a way to identify whether a goal is meaningless at a more fundamental level of description. Obviously Bob would be prone to say things like "today I did x in pursuit of my goal of time travel" but there's no way of telling that it's meaningless at any other level than that of meaning, i.e., with respect to language. Other than that, it seems to me that he'd be doing pretty much the same things, physically speaking, as someone pursuing a meaningful goal. He might even do useful things, like make breakthroughs in theoretical physics, despite being wholly confused about what he's doing.