He thinks this proof may have to have interesting effects on our philosophies though. If we have a time machine, some things will have to change. For example we should change our approach to free will. I want to kill myself - it doesn't happen. Free will must be restricted.
All laws of physics are restrictions of our free wills.
History as we know it is safe. We cannot change history if nature won't allow it. Physics has the last word. Not the pope.