If a time machine does allow for changing history, it’s easy to imagine how to compute it; you could easily write a computer program which would simulate that universe and its time travel, given sufficient computing power. You would store the state of the universe in RAM and simulate it under the programmed ‘laws of physics’. Every nanosecond, say, you’d save a copy of the universe’s state to disk. When the Time-Changer was activated at 9pm, you’d retrieve the saved state of the universe from one hour ago at 8pm, load it into RAM, and then insert the Time-Changer and its user in the appropriate place. This would, of course, dump the rest of the universe from 9pm into oblivion—no processing would continue onward from that point, which is the same as ending that world and killing everyone in it.
Is there a word for time travel that works like this? I’m writing a novel that has it, and would like to be able to succinctly describe it to people who ask what it’s about or how the time travel works.
(I’m not invoking computer simulation, but the effects as far as the characters see are like this—or rather, the characters see time travelers from the future but never get to see the versions of the universe where they get to remember seeing someone leave to travel to the past.)
My understanding is that Stable Time Loops work differently: basically, the universe progresses in such a way that any and all time traveling makes sense and is consistent with the observed past. Under the above model, you will never witness another copy of yourself traveling from the future, though you might witness another copy of yourself traveling from an alternate past future that will now never have been. With STL, you can totally witness a copy of yourself traveling from the future, and you will definitely happen to travel back in time to then and do whatever they did. That’s my understanding, at least.
Of course, there’s no reason to strictly believe that what you thought was a future version of yourself wasn’t either lying or a simulacrum of some kind, or that any note you receive after intending to send a note back to yourself hasn’t been intercepted and subverted.
Which leads to interesting stories when those expectations are subverted, but only after they’ve been established.
What makes you think that elaborate passphrases are uncountably infinite? Any loop that includes a ‘we await confirmation that the plan has succeeded before we implement it’ clause at the beginning is virtually foolproof. In order to foil such a plan, one needs to overcome the adversary, prevent them from signaling failure (ever!), and then manage to signal success. (So that the plan is set into motion.)
Is there a word for time travel that works like this? I’m writing a novel that has it, and would like to be able to succinctly describe it to people who ask what it’s about or how the time travel works.
(I’m not invoking computer simulation, but the effects as far as the characters see are like this—or rather, the characters see time travelers from the future but never get to see the versions of the universe where they get to remember seeing someone leave to travel to the past.)
Yes, that’s a type 3 plot.
Such numbering isn’t however very meaningful or intuitive… I’d just say “timeline-overwriting”.
This is the standard model of time travel / prophecy in Greek myths, isn’t it? Maybe I’m overgeneralizing from Cassandra.
[edit] Eliezer calls it Stable Time Loops, which is a term I’ve seen before.
My understanding is that Stable Time Loops work differently: basically, the universe progresses in such a way that any and all time traveling makes sense and is consistent with the observed past. Under the above model, you will never witness another copy of yourself traveling from the future, though you might witness another copy of yourself traveling from an alternate past future that will now never have been. With STL, you can totally witness a copy of yourself traveling from the future, and you will definitely happen to travel back in time to then and do whatever they did. That’s my understanding, at least.
Of course, there’s no reason to strictly believe that what you thought was a future version of yourself wasn’t either lying or a simulacrum of some kind, or that any note you receive after intending to send a note back to yourself hasn’t been intercepted and subverted.
Which leads to interesting stories when those expectations are subverted, but only after they’ve been established.
True! That’s why every twelve-year-old establishes elaborate passphrases for identifying alternate / time-displaced selves.
What makes you think that elaborate passphrases are uncountably infinite? Any loop that includes a ‘we await confirmation that the plan has succeeded before we implement it’ clause at the beginning is virtually foolproof. In order to foil such a plan, one needs to overcome the adversary, prevent them from signaling failure (ever!), and then manage to signal success. (So that the plan is set into motion.)