If we believe in the multiverse hypothesis, which encompasses the ''Many Worlds'' theory, then potentially there does exist the possibility for time travel by humans in one of our alternate realities. As we have so much to learn about quantum mechanics, which is currently in its infancy, as far as scientific studies and our current understanding goes, we do not know how time travel may affect the space-time quantum. Is it not possible that a technologically advanced 'us' in another reality can not only time travel but also cross inter-realities?
We KNOW that we live in four dimensions - space width, length, height (or x, y and z) and time. However, under theoretical scientific studies, particularly retaining to the Superstring Theory, there are potentially 10 dimensions within our Universe.
According to the Superstring Theory, the fifth and sixth dimensions are where the notion of possible worlds arises. If we could see on through to the fifth dimension, we would see a world slightly different from ours that would give us a means of measuring the similarity, and the differences, between our world and other possible ones.
In the sixth, we would see a plane of possible worlds, where we could compare and position all the possible universes that start with the same initial conditions as this one. In theory, if you could master the fifth and sixth dimensions, you could travel back in time or go to different futures.
In the seventh dimension, you have access to the possible worlds that start with different initial conditions. Whereas in the fifth and sixth where the initial conditions were the same and subsequent actions were different, here, everything is different from the very beginning of time. The eighth dimension again gives us a plane of such possible universe histories, each of which begins with different initial conditions and branches out infinitely (hence why they are known as infinities).
In the ninth dimension, we can compare all the possible universe histories, starting with all the different possible laws of physics and initial conditions. In the tenth and final dimension, we arrive at the point in which everything possible and imaginable is covered.
The belief that the universe is made up of ten dimensions is an attempt to reconcile the standard model of particle physics with the existence of gravity. In short, it is an attempt to explain how all known forces within our universe interact, and how other possible universes themselves might work.
If there are indeed 10 dimensions within the Universe, then alternate realities must exist, and as such, an advanced alternate humanity would have a concept of the 5th and 6th dimensions, allowing for time travel between realities.
Now, we just gotta prove it............