[ExI] Time Travel Reconsidered

[Keith Henson]

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:
>>
>>    1. *Twist:* the null direction field twists so that no global null
>>    foliation exists.
>>    2. *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.
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