FORMS OF TIME TRAVEL

1. Future
   1. Changing into whatever "you" are at that point in time, even if it's a corpse.
   2. Fast-forwarded into the future (everything else changes around you).
   3. Returnable time travel (2 copies of you)
      1. Can change the future upon return (i.e. visited future is deleted)
      2. Cannot change the future upon return
2. Past
   1. Changing into whatever "you" are at that point in time, even if it's an embryo.
   2. Fast-backwarded into the back – in this case, there are 2 copies of you (CTC)
      1. Can change the past on second run (i.e. existing people are deleted)
      2. Cannot change the past on second run (i.e. past is already consistent)
   3. Returnable time travel – again, 2 copies of you
      1. Can change the past on second run (i.e. existing people are deleted)
      2. Cannot change the past on second run (already consistent)

Notes

• 1a and 2a are not actually physically distinguishable from what's normal. I've included it in case anyone's wondering.
• 1b and 2b are "special relativity" time travel. There is an asymmetry between 1b and 2b, which has to do with the fact that the object was already moving forwards in time to begin with. Also, 1b is actually possible (and happens whenever you move) while 2b probably isn't.
• 1c and 2c are "wormhole" time travel.
• Solution to the grandfather paradox – in ii options you just can't change anything (I wouldn't call it "lack of free will" but rather fall back on EY's characterization of free will as being an aspect of how an algorithm feels from the inside); in i options you just branch into a new timeline.
• In ii options, we do not have directed acyclic graphs, but rather a vast space of possible consistent timelines out of which one is randomly chosen. This would require new laws of physics to describe the probabilities of each consistent outcome to be chosen.
• Works of fiction often conflate different models of time travel, e.g. the character sees a future version of himself at some point and re-enact the event later, but then also makes decisions that change the past.