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. The new model, which uses the laws of quantum mechanics, gets rid of the famous paradox surrounding time travel. Although the laws of physics seem to permit temporal gymnastics, the concept is laden with uncomfortable contradictions. The main headache stems from the idea that if you went back in time you could, theoretically, do something to change the present; and that possibility messes up the whole theory of time travel. Clearly, the present never is changed by mischievous time-travellers: people don't suddenly fade into the ether because a rerun of events has prevented their births - that much is obvious. So either time travel is not possible, or something is actually acting to prevent any backward movement from changing the present.