The problem is fundamental and multifaceted.

1) What do we mean when we say 'go back in time'? The time we are wondering about going back to doesn't exist. It's gone. We want our memories or recollections to be real, and as those things, they are. But as realities, they aren't there anymore. Did they exist? Probably. DO they exist now, probably not. The difficulty is language. If we talk mathematically (which I am not able to do even in my wildest dreams), it might be possible to explain.

I found a method using arithmetic to 'predict' squares ten ahead. That does not mean I can 'see' into the future. If I can find a way to take that method into negative numbers (read fractions) which I'm having a go at now, that will not mean I can look into the past. The world we humans live in is four dimensional. (It's possible we live in an eleven-dimensional world, but even if we do we'd have no way to 'talk' about it other than through the use of mathematics, and if individuals who don't understand higher mathematics as a way of explaining things try to talk about it, then we're hooped at the start.)

When we say things like 'time travel' (something we're doing as we read/think/breathe) or going forward to the future/backward to the past, we admit that we perceive time to be a linear thing. It goes from here to there, wherever either or all of those things might be. To demonstrate the problem, let me pose the question, "Where is here?" Obviously it's where we are, but neither of us is the other and we do not occupy the same space simultaneously, maybe. So, is here where you are or where I am?

We invent constructs in which to 'imagine' things like time. Barium clocks, quartz crystal clocks, atomic clocks, even tick-tock clocks, all of which tell us the time. And tenses in language like past, present, present continuous, future, all of which assure us that we can think in a three dimensional ((up/down, left/right, front/back) space governed by a fourth dimensional idea (time).

[One of the reasons I suspect that Bill D is a genius--not a word I throw around lightly--is that I also suspect he thinks in more than four dimensions. Don Hackman is another, although there are moments when I am so lost with Don I feel I need a 'Hitchhikers Guide to the Moment'. And then there's Amos Jessup, a man with a vocabulary and insight that may exceed Churchill's.]

So let me ask this: if the BIG clock were to be slowing down, how would we know? The answer becomes a paradox. The problem has been alluded to in words by anthropologists who predated Heisenberg--but one group did it in words and the other person in math.

The real difficulty is found in the perception that time is a constant, and so is the speed of light. I disagree. I think it has to be one or the other.

While I get nailed to that notion, allow me to address Part Deux. Later, or earlier, if time flows in two or more directions.