You know, I can't help but wonder just what exactly mass is. I mean, it's one of those things that seems obvious until you actually stop and think about it, like time.
Well, take a black hole. Once something passes the event horizon, it acts exactly the same (from our perspective) as it would if it were going faster than the speed of light. And it's already theorized that if you go extremely fast, time progresses very slowly for you. That suggests that time has an inverse relationship to speed.