While trying to figure out the plot for Ra, I came up with a brand new model of time travel which I decided to call "hypertime" because it featured a second dimension of time. (This "hypertime" has no relation to the concept from DC comics.) However, I found this model of time travel so insanely complicated that I couldn't even reason logically about what would happen if the universe were to behave in such a way... let alone convey such a cosmic structure to readers in the form of a compelling story.
So I abandoned the concept and I'm now exploring a new idea. However, I don't see any reason to just bury this idea for eternity, so here it is. Make of it what you will. I suggest you draw diagrams, it'll highlight how complicated this thing is.
There are parallel universes.
Each universe is offset in time from the next, but each universe is exactly identical to the others. In our universe, it is currently 2011. In the universe "above" ours, however, it is 2012, and 2011 has already happened, just like it's scheduled to happen here. In the universe "below" ours, it's 2010, but 2011 is going to happen there just like it is happening here.
That's a simplification. In truth, the universes are not discretised and separated by one year. There is a continuum of parallel universe, offset by months and days and minutes and fractions of a second. There is a reason why all the universes are the same: it is because each particle in each universe is "dragged behind" the particles in the universe above, like curtain rings, or water flowing over some cosmic waterfall. There is a cosmic fireball at the Big Bang, spewing out each universe in turn, and each one precisely follows the path laid out by the previous one. They are separated by, perhaps, a Planck second.
When drawn out, this arrangement resembles a two-dimensional continuum. Along the horizontal axis is real time, increasing from left to right. Along the vertical axis is "hypertime", increasing from bottom to top. The top universes are further along in calendar time than the bottom universes. Horizontal lines represent single universes/timelines. Vertical lines represent lines of constant real time, with the past on the left, the future on the right, and the vertical line of "the present" constantly moving from left to right. Notice how a vertical line intersects every universe/timeline at a different calendar time. (Lines of constant calendar time, like "January 1, 2011", are diagonal, running from the upper left to the lower right. But these are less useful.) It is 2011 here, but one year "down" through hypertime it is 2010, and one year "up" through hypertime it is 2012. However, all universes are running in parallel, simultaneous in real time.
Time travel: travelling forwards in hypertime is in fact travelling to higher universes in the stack. Travelling backwards in hypertime is in fact travelling to lower universes in the stack. Neither of these strictly constitutes time travel through real time. You cannot visit an earlier point in the same timeline, there are no causal loops, you cannot alter established history, there are no paradoxes. There is a two-dimensional sheet of time, and you can dart about from point to point, but you are constantly dragged forwards in real time.
(Hypertime travel is also not, strictly speaking, "travel". All universes occupy the same physical space, but are effectively non-interacting with one another. Hypertime travel is more like shunting oneself out of phase with this timeline and into phase with a different one. They are "sterile universes", entire universes of particles coincident with ours which are completely non-interacting. This point has nothing to do with the storytelling mechanics of the universe, though.)
Now consider this. Suppose you're in one hypertimeline and you decide to jump down to another hypertimeline: say, from 00:00:00 January 1, 2011 to 00:00:00 January 1, 2001. There is another you in the hypertimeline exactly 1 second below yours: for him, it is only 23:59:59 December 31, 2010 right now, and he has not made the decision yet. But he is exactly identical to you in every way, and when, one real time second from now, his clock ticks over to 00:00:00 January 1, 2011, he will make the exact same decision as you and jump back in time to 00:00:00 January 1, 2001 himself. Therefore, he will travel through hypertime and land in the hypertimeline exactly 1 second below yours, exactly 1 real time second after you landed where you landed.
Now bear in mind that at the moment when you jumped, there were also alternate yous at 23:59:58 December 31, 2010, and at 23:59:57 December 31, 2010. In fact there is a whole continuous stack of alternate yous at every instant in time going all the way back to the alternate you at 00:00:01 January 1, 2001, who, 9.999999 real-time years from now, will make the exact same decision as you and, in turn, jump back in time to 00:00:00 January 1, 2001 in a hypertimeline 9.999999 years below where you landed. There is a ten-year stack of identical alternate hypertimelines all behaving the same: all jumping backwards in hypertime.
Next. Right now, at 00:00:00 January 1, 2011, there is also an alternate you in an alternate hypertimeline where the calendar on the wall reads 00:00:00 January 1, 2001. That's your target timeline: that's the point where you're about to arrive. And when you arrive there, the course of history in that timeline will change radically. When THAT timeline reaches 00:00:00 January 1, 2011, both of you could be dead! Maybe no time travel will occur at all for the rest of that hypertimeline's history! And because each hypertimeline from that one downwards is about to deal with a hypertime traveller from the future in the same way, we have another ten-year stack of identical alternate hypertimelines all behaving the same: all receiving a time traveller from ten years into the future.
Okay. Here's the hard part. Right now, at 00:00:00 January 1, 2011, there is an even deeper alternate hypertimeline where the calendar on the wall reads 00:00:00 January 1, 1991. And no time traveller will ever arrive in that hypertimeline. That hypertimeline will remain empty of hypertime travellers, because a traveller arriving there would have to have set off from a hypertimeline where the current real time is 00:00:00 January 1, 2001, but as we've established, that hypertimeline is about to get derailed. So there's a third ten-year stack of identical alternate hypertimelines in which history plays out exactly as it did in the first ten-year stack.
And there's a fourth ten-year stack of identical hypertimelines which is exactly the same as the second ten-year stack. And then hypertime repeats itself, in a twenty-year cycle, forever.
Got all that? I tried to draw a diagram but it didn't really work.
That's the insane nonsense that you have to deal with after one time jump.
As soon as a second person decides to travel through hypertime - from, say, 00:00:00 February 1, 2012 to 12:05:32 January 7, 2011 - it sets off another, small stack of alternating repeating identical hypertimelines within one larger otherwise homogeneous hypertimeline set. That much is not much more complicated to keep track of: it's the same story all over again.
The problem is at the cusp. Right now, at 00:00:00 January 1, 2011, consider a hypertimeline where it is currently 00:00:00 January 2, 2010. That hypertimeline will proceed like the hypertimelines immediately above it, and somebody will jump back one month or so when local time in that hypertimeline reaches 00:00:00 February 1, 2012. BUT, by jumping down one month through hypertime, that traveller arrives in a completely different hypertimeline which has already had a time travel incursion at 00:00:00 January 1, 2001. This is a universe which is not only different in the "present" but also apparently has a completely different past. Then throw in the possibility of somebody deciding to just jump back ten years and one day, thereby totally warping the pattern still further?
At this stage, even with diagrams, my head was beginning to spin. I was seriously considering writing a computer program which would be able to show me the exact consequences of everything I could think of.
I considered adding various arbitrary story limits on what can happen. Notice how we still haven't considered what would possibly happen if someone travelled forwards through hypertime. I considered fixing time-jumps to a whole number of years (Earth years? That doesn't make any sense!), or only permitting time jumps at one specific moment on the calendar, or making it so that you can only jump to a hypertimeline which has had the foresight to build a specific piece of receiving equipment for you to jump into. It still didn't become remotely comprehensible to me. As for conveying it to you, the reader? And keeping you entertained rather than completely lost?
So, yeah. Fiction, unlike realistic quantum physics, has to make sense.