On Fri, Jan 10, 2014 at 12:43 PM, John Clark <johnkcl...@gmail.com> wrote:

> On Thu, Jan 9, 2014 at 5:38 PM, LizR <lizj...@gmail.com> wrote:
>
> >As a lot of people have now pointed out, physics can be local and
>> relistic if time symmetry is valid.
>>
>
> If time is symmetrical then retro-causality exists, so how can realism
> hold? How can the outcome of a coin flip today have a definite value
> independent of the observer if next year or next millennium someone can
> cause a change in today's coin flip? If realism holds under those
> circumstances then the word "realism" has no meaning.
>

There could be no "change" if the Novikov self-consistency principle holds
(see https://en.wikipedia.org/wiki/Novikov_self-consistency_principle ),
which is what physicists usually assume when analyzing theoretical
scenarios where time travel could be possible, like traversable wormholes
in general relativity. In this case the "block time" assumption holds, you
just have a fixed 4D spacetime where all cause-and-effect sequences are
already part of it, and any retrocausal influences must have been part of
history all along rather than "changing" history from one sequence of
events to an altered sequence.

Jesse

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