SDF: "Relativity states clearly that you can choose the rate at which you travel forward in time. You can either slow down or speed up that rate, by living in a valley or at the top of a mountain, respectively. You can't travel back in time though because time is an artificial measure of the motions of things. It is the things that have to be returned to a previous state. There is no mechanism that would spontaneously move all the particles in the universe to their previous positions"

Hi SDF,

What you said, quoted above, is my feelings on it too.

As for a mechanism, except for talking God into doing it, the only what humans could do such a thing is on a local scale. But first, we would have to know where all the particles where of the past we want to recreate. If matter leaves some kind of path as it moves through the fabric of space, then it might just be possible.

I am working on a science fiction story based on this concept. In it, humans have created a twin Earth out of a dead world, around another star. They have matched copy-Earth atom for atom with our Earth. Every atom is tagged, and they do account for the atoms that get added to and from space.

With every atom tagged, and knowing the path every atom has taken over time, they are able to physically recreate the past. Computers control atoms and make sure they stay on their correct paths as time progresses. Things get interesting when the scientist want to see what happens when they remove this control and let the atoms go where ever they want. Will history repeat itself, or will alternate histories form? Will the humans in this simulated past become aware of what is going on (The planet is in a star system close to the Earth, so the stars are similar in the sky, but not exact) ?

In this story, it was discovered that atoms make detectable paths as they move through space. Probes were sent out to collect this data along the path that Earth tool (And later they do Mars and the other planets). The first simulators used virtual matter. They matched each virtual atom with an actual Earth atom. Each virtual atom has virtual physical properties that match the real thing (Like reflect light). By putting it all together like pixels in a picture, they get a virtual view of the past.

Later they expanded this technology to holographs, then later to real matter on a local scale. Then they moved it to planet scale.