Nature of time

The physical universe embodies change. It (and that means everything that makes it up) is in one state; it changes, and is then in another state. It changes again, and we have yet another state. And so on.

Time is what we conscious humans call the perception of that change. Time doesn't have an existence outside of our perception.

Einstein's Special Theory of Relativity showed that time appeared to run at a different rate within different frames of reference, where each frame was travelling at a different velocity, or was within the effect of a gravatational field.

The reason for this is that the rate of change of our physical universe occurs at a different rate depending upon how close to the speed of light the matter undergoing that change is travelling, or whether the matter is within a gravitational field. In both cases, the rate of change is less: it's changing more slowly. My own mental imagery for this is that things get "gluggy", as if immersed in mollases. Things can move, can change, but at a slower rate.

For a human undergoing such an effect, our perception of time remains unchanged, because we ourselves are based upon that same changing matter. Our conscious processes, and the devices we use to measure the passage of time, are affected as much as anything else.

So, time does not exist.

It's why time travel is impossible: the physical universe is constantly changing, but what it was no longer exists. The past is gone. There is only what it is now, which is immediately followed by another now. Likewise, the future does not exist, because the changes haven't happened to get there yet.

If the past state of the universe was somehow recorded (e.g., in the Akashic Records), then it could be possible to observe the past. Seeing the future is more a matter of exploring possibilities, though the future is not fixed. Some lines of probability may be more likely to occur, but nothing is guaranteed.

An analogy (oft used) is the weaving of a tapestry, one row at a time. Each row is a "now", a snapshot of the current state of the universe. Another row is added, and we have a new now. You cannot change what has already been woven; the pattern that has so far emerged is immutable. You cannot see what the pattern in what is yet to be woven, though perhaps you could anticipate what might emerge based upon the pattern leading up to the latest row.

Time travel is a nice SF device, but nothing more than that. Matter transporters may prove to end up in the same category (unfortunately).