[ExI] Time Travel Reconsidered

[Adrian Tymes]

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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