if these theories are true, you should occassionally be running across flocks of time-travellers from the future
Backwards time-travel is very likely to be impossible in practice: time-machines are inherently highly unstable. Also, a time-machine cannot take you further back in time than the time the machine first becomes operational, so if you want to see dinosaurs, you'll need to find either a naturally occuring time-machine, or a friendly alien who will lend you his.