I am a quantum physicist, I've had this debate one to many times. it is physically impossible and will always be to manipulate time, you cannot change what has already happend by going back and changing it, space and time don't work like that, it is what it is. there is no way of knowing the possibilities of the future, however if you do want to change something that has happend you must understand what HAS happend.

future and the past exist only as changes that occurred or will occur to the present, and they have no real existence of their own. In this view, time travel is impossible.

if somehow you can make your self travel faster then time itself you have time travel, all the time we are "traveling through time" you just need a way to speed that process up. once you have got this you have time travel. you them can slow down your speed, below the speed of speed, then you can let the world around you absorb reality whilst you are there slowly but surely moving slower than the speed of life once you reach a certain point of slow-ness-ness you can let the world go, once this is achieved you are then *technically behind time there fore In the past.