Time in Quantum Mechanics and General Relativity

In both quanta mechanics and general relativity, there's nothing to stop your going back in time. But we don't see those sort of processes in happening er, on larger scales. And i'm still not clear really why what time is in fondo mechanics and how it ought to mean. In ther consis what we call a unitary theory. Things happen a, b, c, d but they have to be time reversible as well. So you have to be gable to go aback the other way. We just frown upon it and hope that there is no such thing. Foins, nobody's arrived from the future. Is o? Not a good practicalor ano os