I really think this paper http://arxiv.org/abs/1212.1087 which Chronos mentioned in another thread deserves its own thread. Has anyone had a chance to look at it? what do you think? This has been a lot of noise about this issue raised by people like Roger Penrose and Sean Caroll . They have come up with the some pretty , shall we say, creative ideas such as CCC and the Caroll/Chen model to deal with this issue. But if this paper is right they are not required.