The closest physicists are able to come to explaining this puzzle involves the related concept of entropy
. Time's unidirectional flow onwards to the future and the fact of entropy in our world seem to be intimately linked. For some reason, "things break down," the natural progress of the universe is to break down, wind down, unwind, fall apart. It's almost certain that no time travel into the past is possible. But no one is able to decisively prove it, in large part due to the whole mystery over just what time is.