Quote Originally Posted by Viking
And how often would this universe split up? Every nanosecond there is a chemical reaction in a chemical equilibrium that could go both ways...
Well the theory is that there are an infinite number of paralell universe with new ones created at every instant based on the potential outcome of any choice/chance in the universe.

As for why we haven't had any time travellers from the future, I forgot where I read about this, but there is a proposition that even if time travel is possible, you could only travel back in time to the exact point when the machine was made and not further back