If "matter cannot be created nor destroyed" then wouldn't traveling backwards in time violate this rule? If the sum of all the contents in the universe = x, and you sent n material back in time, wouldn't x + n make the universe pop in some manner?
And even if it were possible, wouldn't sending something backwards in time, even if it were on atom, cause a paradox because that one atom would interact with the older universe?
These kind of questions have always led me to believe time travel backwards is impossible.