Last night I watched a science program re-run, and in one of the segments there was a guy holding a marble (I think) in his hand, explaining that if we could shrink the mass of the earth down to the size of this (marble), then it would have the density required to create a black hole. But, is density the sole requirement for creating a black hole? We all know that the spacetime fabric is flexible, and that it takes an extreme mass in an extremely condensed form to penetrate into the spacetime fabric. But didn't Einstein say something to the effect that acceleration and gravity are the same thing? The point I'm trying to make, and the question I'm trying to ask is, isn't it possible to replace mass with velocity to a certain extent to create a black hole? In other words, a solid object doesn't have enough mass to penetrate the spacetime fabric, but if that same item was accelerated fast enough, would it possibly be able to penetrate the spacetime fabric? I read through about half of the Questions and Answers forum and couldn't find this question previously asked -- I hope I'm not resurrecting an old question here. Thanks!