I think that time is still a very poorly understood concept. That is because we are beings trapped in time. We can try to imagine what it might be like to be a being outside of or above time, but so far I think that has never been more than conjecture since nobody really has any experience being anything other than a being trapped in time.
Why is it that in the other three dimensions, we can move around more-or-less at will, but in time we seem to be stuck going forward at a generally constant rate? If we were to travel at extreme velocities (close to the speed of light), relativity says we could travel forward in time a bit slower. However, most of us will never have that opportunity. Earth-bound people have aged less than a second compared to astronauts using this method. But who wants to get older? We're all doing that anyway! Traveling backwards in time would be a much greater trick (physicists don't even know if we can do it), and nobody seems to know how to stop themselves in time either.
There are similarities, I think, with gravity. We feel as though we have control of our movements in the standard 3 dimensions, but in fact gravity does limit our up-down movement, also thanks to gravity we are stuck to the Earth, constantly rotating at the same velocities, both about the Earth's axis and about the sun. We don't even feel this movement, but we are actually stuck moving in the current 3 dimensions, as well as time. If you could take a spaceship out of orbit of anything, and just float around in a space suit, then perhaps you could actually be at rest relative to all the reference points I can imagine. However, most of us will never have the opportunity to do that - even astronauts stay in orbit in their missions (so far as I'm aware).
Kurt Gödel provided certain solutions to Einstein's relativity equations that describe a universe where the geometry of time is a circle, meaning that it is possible to travel all the way around the circle, and revisit the same point again. The relativity equations have been amazingly accurate at predicting various phenomena, so most physicists are hesitant to question those equations. Also, Gödel (a friend of Einstein's) was a notoriously thorough person, and no problems have been found with his solutions. These premises imply that time travel must be possible. We are quite certain that our universe does not have time running in a circle, however from the point of view of modal logic (Gödel was a logician), that does not matter. Because, if the equations can allow a universe to exist with such a geometry, then time travel cannot be logically forbidden in any universe resulting from the equations (including our universe). So, thanks to Gödel, time travel cannot be expunged from the realm of possibility.
So now there are many speculations about how one could travel back in time. Electrons and other sub-atomic particles may actually be doing it all the time! Positrons seem to be nothing more than electrons traveling backwards in time. In fact, electrons are all indistinguishable from one another, causing some to posit that the entire universe may in fact be nothing more than a single electron zig-zagging back and forth through time interacting with itself! However, just because an electron can do this does not mean that a human body could. If we think of space-time as a fabric, then it is theoretically possible to twist and tear this fabric into a geometry that folds back and touches itself at certain points, thereby allowing time travel. It would probably take a large amount of gravity and/or energy to warp space-time in this way, thus creating a time portal. We do not currently know how to create this much energy or gravity, though perhaps some black-holes may already be doing this. Could a human survive passage through a black hole? In most cases no, but there might be certain sub-classes of black holes that we could travel through and survive. Maybe.
But then, supposing we could go back in time: Perhaps you've heard of the Grandfather Paradox? This can take different forms, for instance if you go back in time and kill your grandfather, then you should never be born, which means you could never go back in time to kill your own grandfather. Or, suppose you go back in time and marry your grandmother (assuming you are a man), and thus become your own grandfather? How could such things be logically possible? Many physicists insist that they cannot be possible, and that is a large part of why time travel is not usually held in high esteem by physicists. Also, if time travel is ever going to be invented, why don't time travelers ever come here? Stephen Hawking has proposed the idea of a "Chronology Protection Agency," so that even if time travel is allowed, paradoxes cannot be created. Or, what if free-will is truly an illusion and everything is in fact deterministic? That would mean that even if you could go back in time, you would not be able to change a thing. Anyone able to travel back in time would simply experience Nietzsche's Eternal Recurrence.
However, all of this is still looking at time as something linear, something to be traversed either forwards or backwards. But what if their are multiple time dimensions? What if each possible series of events is a timeline, and we simply live in one of these? What of lateral time-travel, or timeline jumping? What if you traveled diagonally in time, that is to the past of a timeline that has an identical past with your current timeline, but a different future? Within that timeline, everything would still be deterministic, but determined to a different end (or at least a different end for your lifetime) than the timeline you had just left. The "many worlds" interpretation of quantum physics might allow just such scenarios. Perhaps each quantum observation is a moment of choice, an inter-dimensional-handshake between the deterministic endpoints of first and final causes, and each individual free will is in fact a lateral time journey, where we are free to move to any adjacent timeline at any present moment, even though they all originate and terminate in singularities? In that case, free will cannot change the ultimate fate of the universe, and all past choices limit your present choices - but it is still totally up to you, of the options available to you in the present moment, whether you take a road less traveled (less probable) or more traveled (more probable).
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