I've done a lot of thinking about this sort of thing, and have come to the conclusion that either time branches into many multiple flows of time, like you are discussing, or time travel is not possible at all. But since theoretically 'everything' is possible one way or another, including time travel, we'll be accepting the first choice.
At first, I found the whole idea of multiple parallel flows of time difficult to grasp - In one word, I thought it was ridiculous. But we must remember that the way of the universe stretches far beyond our common plane of thinking.
The whole 'parallel' reality thing always seems a far-out, science fictiony thing, when in fact it could very well be possible from what we know so far, and is the only way to explain time travel without encountering any paradoxes or other conceptual snags.
First of all, one thing we have probably got to do is change our way of thinking of dimensions. e.g 1st, 2nd, 3rd, 4th etc... In a sense all these things exist, but they are not exactly the same. In fact, all these things as a whole perhaps do not fit exclusively into the category 'dimensions' and a new term may be required to label these all as a whole.
Really, dimensions are commonly what we refer to for the volume and space side of things, hence our three dimensions - Length, Width and height. However, these three dimensions are intertwined as one, and cannot be broken down individually - Well, not in this existance anyway. So perhaps the three spacial dimensions are really just one entity, space alone, with length, height and width being the sub-components we use to comprehend it.
As for time, is it really the 'fourth dimenion'? Of course, it is definitely not a spacial dimension, rather it is an entity, part of the universes clockwork with governs how space and all within it propogates and progresses at a relatively constant rate.
So we have space, which confines 3d volume and mass, and time, which 3D space flows through. Beyond this there are infinite timeflows through which flow the events of infinite possibilities of events of this universe. And who knows... Beyond this there may be things completely different altogether.
When it comes to time travel and its effects on a single flow of time, the answer simply is we are no longer dealing with that single flow of time. When we travel to the past we have instantly branched off the flow of time into a new reality, through which we are now flowing through.
If you picture time flow as splitting like a forked road, with the original flow of time on the right and the new altered flow of time upon our arrival on the left, we are travelling down the left path from where we arrived. To travel back to the future would mean not back to the timeflow we left, but rather that same time period, of this altered flow. Our very arrival and presence of ourself and our time machine had branched off time from the first moment.
With this concluded, it is theoretically possible to affect past events however much, even if it leads to preventing your own existance, and it would cause no paradox, because your interference would only affect this new flow of time, not the original, abandoned flow which still, in a sense is flowing its own way paralell, and perhaps, if the know-how were aquired you could even go back to your own original flow somehow.
As for the reduction/increase in amounts of matter upon leaving and travelling to times, really matter is not being destroyed or created, it is just being transferred. The bucket theory probably does not apply to just one single flow of time, but all infinite time flows of a universe combined.
As for 'past', 'present', 'future', like space and velocity, this is relative. Of course the year 2050 to us now is the future. But, to someone in the year 2100, it is the past. It all depends where you are looking at. Of you leave this year, 2001, to visit 1901, to you this year is no longer the present, 1901 will become your present because that will be when you are, simple as that.
Upon every arrival and leaving in time, a new flow of time is created. In a sense, it's like changing lanes on a freeway that donot combine again, they stay branched off forever.
If you travel say 50 years into the future, you will arrive in a time where you have been missing for 50 years, you won't see your older self! You are in a sense still on the same flow of events of time that were from when your time machine left the original time, so to everyone you have been missing for 50 years.
Of course, once you decide to return back to the time you left, you will instantly branch off a new flow of time and it will be like you never left, and you won't go missing after all. To someone observing your time travels in this year, in one reality they never see you return, in another you do return.
It is this second reality that takes place for them instantly, so long as you do return from that future year, so in a sense you only ever go missing for 50 years from your point of reference.
-Raze
