I don’t have a very sophisticated idea of ‘spacetime’, so this conjecture is undoubtedly naive.
I imagine spacetime as a sort of vertical column of clear plastic, about 6"x6” square in section (representing a 2d projection of 3d space) but potentially infinitely high to represent time. A timeline is a ‘wavy pipe’, something like a colored bendy drinking straw, embedded in the plastic. The present (or any other time) would be a plane cutting through the column.
I am not sure if my timeline (for my future) is “already there” embedded in the plastic block and I move inexorably along it or if my timeline grows as time passes. If it already there then everything would be pre-determined, so I prefer the latter.
Right, you’ve put your finger on the issue. Here’s the problem though: if the block grows with time, it won’t just grow for you, it should grow along a plane at right angles to the timeline for the whole universe at once, for all simultaneous events, as ‘the present’ continues constructing itself into the past. OK?
But we know from Einstein that simultaneity is relative. So what is in the past for you will be in the present or future for someone else, simply in virtue of their motion relative to you. There is no single, objective “present” for the entire universe. That plane at right angles to the timeline is dependent upon one’s reference frame, which in turn is dependent upon one’s accelerated motion. (Or on one’s proximity to gravitational fields, which is the same thing).
So in Einstein there is no way to make sense of a single, objective present moment for the universe.
In either case, if I were to travel back in time I would create a totally separate time line for myself. I suppose that the ‘old’ Keithproser2 timeline would still be embedded in the plastic but a totally new KP2 timeline would appear (and start to grow) at some point in the plastic block.
If I subsequently interact with an object, that object’s timeline would ‘bifurcate’, in the sense that its existing timeline would remain ‘fixed’ in the plastic, but a new ‘branch’ would be created.
So in effect there would be two sets of timelines - the ones that were “already there” (ie the timelines that led up to me making the trip) and a set of newly produced timelines (produced directly and indirectly by the action of the time-machine) that constitute a (potentially) different reality.
As I see it, it wouldn’t matter if I killed my grand-father in the new reality - he would still have existence in the original reality.
So I can’t affect history by going back in time - I can only create a different parallel history. So if you sent me back in time to kill Hitler, WW2 wouldn’t happen for me in my reality but it would still happen for you.
I don’t know how the two realities are kept apart, but if the multiple universe hypothesis has any basis in reality then perhaps that is not an insuperable problem.
If that is as clear as mud, I wouldn’t be at all surprised.
I think this is quite clear, and it is one of the standard options for making sense of (a hypothetical notion of) time travel. When you “travel into the past” you actually aren’t traveling into your own past, but into the past of a very similar you in a very similar parallel universe. When you kill your grandfather, you aren’t actually killing the person who was in your direct timeline, but someone very similar, in a slightly different universe.
The problem with all this is that although it’s neat philosophy and neat for writing strange science fiction stories, we have no reason to believe any of it is true or possible.