I was reading this lecture notes "Black holes from A to Z" by Andrew Strominger. In the first chapter Introduction the following statement is made:
"If we know the present, there are laws that determine the future and can be used to predict the past. This is true in Classical Physics. It is also true in Quantum Physics, where we determine the state, instead of classical variables, in the future or the past. We refer to this as the Unitarity of Quantum Mechanical systems."
I haven't understood how unitarity is related to the system being deterministic? How unitarity is related to time evolution of the system?
The link to the lecture notes is "https://scholar.harvard.edu/files/yshi/files/physics_211r_-_black_holes_final.pdf"