h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

i have a theory about the consequences of time travel, it is very rough and i present it more for philosophical discussion than as a scientific theory. any feedback would be welcomed.

we perceive time as a line, a line on which we can only move forward. in the future there exist an infinite number of possible events, as time moves forward some events happen and those that dont get snuffed out. (i see this time line like a zip closing on all the possibilities.)

travelling forward
if i were to invent a time machine at point A on this "zip" all future possibilities would then include the fact that a time machine exists so travelling forward in time would not be a problem, but would be random, i could end up at any random event, even if i could specify how far into the future i wanted to go. once at this point, the "zip" would be closed up until this point (point B) behind me, with a history between A and B which i hadn't experienced.

travelling backward
this would be more dangerous as in the act of forcing my way back through time, back down the "zip", i reopen all the possible futures that existed at the point i returned to (point 1) between point A and point 1 the timeline would still exist but as separate unconnected events which each have infinite alternatives. as the possibility that i invent the time machine at point A still exists within these infinite events, the time machine could still exist at point 1, and i could theoretically move as far back as i wanted (points 2,3,4 etc) however if i then attempted to move forward in time again i would jump to a random event, one most likely in which i have never invented the time machine. i would then be stuck in that time and whatever history and future it had. in this timeline there may not be the technology or resources to remake the time machine.

travelling between points of time travel knowledge.
even if moving back down the "zip" is too risky, it may be possible to move back in time within certain limitations. say i have invented the time machine at point A and travelled forward to point B. i should be able to travel back in time from point B as long as i do not go past point A (when the time machine was invented). as long as i stay in the future from point A i could travel through time at will. the condition being that each trip into the future would be random and each trip into the past (from that point) would erase that portion of the timeline.
this would explain why people from the future do not just pop up everywhere, if we have not yet reached a point where time travel is possible, it is too risky to return to this time.