in quatum physics there r scientists who claim that we r actually living in a multiverse therefore when the guy travel back in time he actually reach another world and the world he came from wont have anything in differences...bcoz they claimed something like if the guy change the history by putting a letter in a box in the past then before he goes the letter thing will pop in their mind automatically so the paradox is wether they want to put the letter 1st or the letter thing which pop in their mind make them do that