In one writers group I read an unpublished story titled the Gods Of Time. It was a science fiction story on the outside but really a philosophic examination of time. Time is like God, an omnipresent force that gives shape to the whole universe and is still total mystery.
It has been long understood how gravity and time have an inverse relationship in Einstein's Theory of General Relativity. Time slows down in gravitational fields. The equations in General Relativity keep on working if you were traveling greater than the speed of light if you have either negative time of negative gravity -but not both together.
In one theory about the crush of matter inside a black hole came the question if time still existed? Does infinite gravity obliterate time? The theory had the interesting conjecture that in the singularity of the black hole everything gets crushed down to the two elemental particles of the universe gravity and time. Both particles would be at the very smallest size dictated by the Plank Constant. The two particles are slightly asymmetrical and if they were physical spaces we could see, one would have 12 dimensional sides the other 14. This initial asymmetry is what divides matter from energy and limits super-symmetry, there are subatomic particles that do not have symmetrical partners.
In another theory of dimensional gravity, similar to the idea of Time-Space, each graviton is actually like a cell of space that expands or shrinks dependant on the gravitational field it is in. That the real measure of distance in the universe is the number cells, the size of each cell is effected by the gradational field it is in. What we see as the vacuum of space is really a sea of free gravitons (each a distinct and undividable space). The speed of light in a vacuum is a photon passing a set number of gravitons per second. When a photon is in a gravitational field that's more tightly packed with gravitons then light bends and travels a greater distance.
Time on the scale of human lives or how we perceive the passing of time is like a multilevel thought experiment that has no answer because there are so many contradictions involved. Science Fiction writers have long played with the paradoxes of time travel. Traveling forward in time at a slower rate that the rest of the world is possible but traveling backwards in time is still an unlikely maybe.
As another writer I meet once said since the universe is expanding outward from the Big Bang then time is really the location of how far out from the Big Bang we are at any given moment. Time travel backwards is probably impossible because to travel back in time would mean you would have to reverse the whole expansion of the universe to get back to the location of the past. That thought doesn't eliminate parallel universes but that a whole other idea.
Suddenly I'm in the mood to write a little Sc-fi today.