From The BBC News:
If you went back in time and met your teenage parents, you could not split them up and prevent your birth – even if you wanted to, a new quantum model has stated. Researchers speculate that time travel can occur within a kind of feedback loop where backwards movement is possible, but only in a way that is “complementary” to the present. In other words, you can pop back in time and have a look around, but you cannot do anything that will alter the present you left behind.
More here.
