One of the most emotional moments of the Doctor Who revival (hell, probably in the whole of the long history of the show) was when David Tennant's Tenth Doctor finally said goodbye to Billie Piper's Rose Tyler. He used a supernova to send his message across dimensions, explaining that they can never see each other again as it'd mean very bad things for the fabric of space and time. And so, Rose remained trapped in a parallel dimension, albeit with the companionship of a 'fake' half-human Meta-Crisis Doctor (it's complicated, okay).