Thanks to Alexander Wales joining Tumblr, I learned about qntm’s 13-year-old “hypertime” idea: the third model of time travel I’ve ever encountered that isn’t completely loosey-goosey.

The idea is that there’s a bunch of initially-identical timelines, each one a (nearly) closed, deterministic system, each following an identical deterministic path through time; but they were initialized at slightly different times, so they’re offset slightly from each other. You can visualize this as a stack of horizontal lines, each horizontal line being a timeline, and “calendar times” like “Jan 1” being diagonal:


(src)

“Time travel,” in this model, consists of, not going back in real time – impossible, that would create causal loops – but rather, going down in hypertime.

An example: suppose, when it’s “Jan 18” in the top timeline (call it TL-0), there’s some sort of bloody revolution, so, on Jan 19, somebody in that timeline (“Arnold-0”), naturally, wants to travel back in time to prevent it. He can travel “back in time” by going down to the timeline that’s lagging four days behind him (call it TL-4):

So far, so cute. But here’s some extra cuteness: TL-1, the timeline immediately below Arnold-0’s home timeline, has an exact copy of him, doing exactly the same stuff; so shortly after Arnold-0 jumps “down” by four days, Arnold-1 jumps down by four days too, landing in TL-5, the timeline immediately below Arnold-0’s destination timeline; and then Arnold-2 jumps down to TL-6; and so on.

But the gnarly part is: when Arnold-0 arrives in TL-4, he derails it from the original timeline: the original timeline didn’t have an Arnold just magically appear out of nowhere on Jan 15! And so, in TL-4, maybe Arnold-0 averts the disaster, and Arnold-4 never jumps down to TL-8 like he would’ve!

…but that means that TL-8 never diverges from the original, so the disaster happens, and Arnold-8 jumps down to TL-12 to prevent it…

…and you end up with this periodic striping pattern, which, if we were to draw it with a continuum of hypertimes offset infinitesimally, rather than one hypertime per day, would look like:

And if there are multiple jumps, things can get much wilder! Suppose, in the original timeline, the day after Arnold-0 departs, one of the revolutionaries (“Kyle-0”) grabs the time-travel device and tries to go back to the day before Arnold-0, to prepare and try to stop him. Then…

…some Kyles arrive on Jan 14, and prepare and defeat the Arnolds who arrive on Jan 15. But other Kyles arrive on Jan 14, then then… there’s… no Arnold? Just… nothing happens!

Maybe those Kyles, baffled, figure they missed somehow, and try going even further back, to Jan 3:

Some of them arrive in timelines where Arnold never appears; some in timelines where Arnold appears on Jan 15 as originally expected; some in timelines where Arnold never shows up but Kyle-from-his-earlier-jump does!

I wrote this hypertime simulator to explore these wacky antics.