Here is the nature and definition of time in physics
"Time appears to be more puzzling than
space because it seems to flow or pass or else people seem to advance through it. But the passage or advance seems to be unintelligible. The question of how many seconds per
second time flows (or one advances through it) is obviously an absurd one, for it suggests that the flow or advance
comprises a rate of change with respect to something else—to a sort of hypertime. But if this hypertime itself flows, then a hyper-hypertime is required, and so on, ad infinitum. Again, if the world is
thought of as spread out in
space-time, it might be asked whether human
consciousness advances up a timelike direction of this world and, if so, how fast; whether future events pop into existence as the “now” reaches them or are there all along; and how such changes in space-time can be represented, since time is already within the picture. (Ordinary change can, of course, be represented in a space-time picture: for example, a particle at rest is represented by a straight line and an oscillating particle by a wavy line.)
In the face of these difficulties, philosophers tend to divide into two sorts: the “
process philosophers” and the “philosophers of the manifold.” Process philosophers—such as
Alfred North Whitehead, an Anglo-American mathematician, scientist. and metaphysician who died in 1947—hold that the flow of time (or human advance through it) is an important
metaphysical fact. Like the French intuitionist
Henri Bergson, they may hold that this flow can be grasped only by nonrational
intuition. Bergson even held that the scientific concept of time as a
dimension actually misrepresents reality. Philosophers of the manifold hold that the flow of time or human advance through time is an
illusion. They argue, for example, that words such as
past,
future, and
now, as well as the tenses of verbs, are indexical expressions that refer to the act of their own utterance. Hence, the
alleged change of an event from being future to being past is an
illusion. To say that the event is future is to assert that it is later than this utterance. Then later yet, when one says that it is in the past, he or she asserts that it is earlier than that other utterance.
Past and
future are not real
predicates of events in this view; and change in respect of them is not a genuine change."
time | physics
To me the question is if time is a direction, the we can only travel in the positive direction. Some physicists think they can travel back in time because of the its dimensional properties, but there is no evidence of this even with the definition above. One can't travel back in time to change a past event. God hasn't allowed us to do it even though these physicists claim multiuniverses pop up just because of a free will decision. God says that because of free will that one can change their future even if it is determined. Time travel into the future is possible and it is done via Einstein's special theory of relativity.
