Board Thread:The Reference Desk/@comment-82.20.113.152-20130213135009

It's been noted how it's virtually impossible for the Doctor to cross to some other universes now, such as Pete's World where Rose now lives with the Meta-Crisis Doctor. However, recent series seem to possibly show characters easily hopping from one version of reality to another. Case in point: In THE WEDDING OF RIVER SONG, River appears in Amy and Rory's garden, having just come from the Byzantium in THE TIME OF ANGELS/FLESH & STONE. The thing is though, the incident with the Byzantium now took place in a parallel timeline to the one this episode occurs in, so River must have crossed realities. The Doctor likely does this at the end of THE ANGELS TAKE MANHATTAN, when he goes back to visit young Amy when she waited outside all night for him in the universe that was being destroyed by the Time Field. What makes instances like these easier, when Pete's World is essentially a no-go area?  