On Fri, Apr 05, 2019 at 10:04:56PM +1300, Rupert Boleyn wrote:
> What it would do to stellar life having almost all the emitted
> energy returned to a star, I do not know.

Yes, when I was writing my reply I wondered about what if the pocket
universe had some suitable symmetry that focussed light back onto the
star.

It would be disruptive, and pretty quickly.  Unlike the "randomish"
distribution of heat in my model of a pocket universe, the extra
radiation flux would be concentrated at the star's surface.  The
star's surface temperature would increase substantially after the
first light returned, within years rather than millennia.

However, it's pretty difficult to arrange a closed spacetime to be
symmetric about every point, and impossible under usual physics to
arrange one that is also symmetric with respect to velocity.
(Though presumably Grandfather was not constrained by usual physics)

So it is very likely that the star would not remain motionless at the
focal point forever, even if it started out that way.  I suspect it
would be an unstable equilibrium: if some slight movement shifted it
to one side of the focus, then the extra heat from the focal side
would probably give it a push even further through outgassing or even
light pressure.

- Tim