[ExI] Time Travel Reconsidered

[Keith Henson]

You could enter the branch simulation as a new instance based on your later
state, or you could just update the memory and state of your previous
existence. That would be weird. There you are in high school and suddenly
you have the memories of being 85 years old.

Oh, the things we would do differently if only we had accurate foresight.

Keith


On Wed, Nov 4, 2026 at 3:12 PM Adrian Tymes <atymes at gmail.com> wrote:

> If you've fully checkpointed, how would you know you restored from
> checkpoint?  The checkpoint would include all your memories and
> sensory data as of that moment.
>
> On Wed, Nov 4, 2026 at 3:32 PM Keith Henson <hkeithhenson at gmail.com>
> wrote:
> >
> > I tend to think that FTL and time travel are impossible in this universe.
> >
> > However, in a simulation, you can checkpoint and store the simulation
> from time to time and reenter the simulation at a later time.
> >
> > An SF story based on this might be rather amusing.
> >
> >
> > Keith
> >
> >
> > On Wed, Nov 4, 2026 at 9:16 AM Adrian Tymes via extropy-chat <
> extropy-chat at lists.extropy.org> wrote:
> >>
> >> On Wed, Nov 4, 2026, 10:54 AM BillK via extropy-chat <
> extropy-chat at lists.extropy.org> wrote:
> >>>
> >>> That would not be a counterexample to Zhang’s theorem. It would mean
> that the spacetime does not satisfy the theorem’s hypothesis. Specifically,
> it would mean that there is no globally defined single-valued function u
> with everywhere-null gradient that remains well behaved along the relevant
> region.
> >>> That is exactly where the “escape routes” come in:
> >>>
> >>> Twist: the null direction field twists so that no global null
> foliation exists.
> >>> Topology: the spacetime is identified in such a way that the would-be
> null coordinate is not globally single-valued.
> >>
> >> Put that way, Zhang's theorem seems to reduce to almost a tautology: if
> no one can bend their future personal timeline to return to the global
> past, then you cannot return to the global past.  In other words: if no one
> can go back in time, then you can't either.
> >> _______________________________________________
> >> extropy-chat mailing list
> >> extropy-chat at lists.extropy.org
> >> http://lists.extropy.org/mailman/listinfo.cgi/extropy-chat
>
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