Daryl McCullough wrote:

> I'm a little bit confused about white holes. Many physics

> web pages, including http://casa.colorado.edu/~ajsh/schww.html

> and http://en.wikipedia.org/wiki/White_hole

> and http://cosmology.berkeley.edu/Education/BHfaq.html#q10

> all describe white holes as the time-reversal of black holes.

> It's hard for me to know what that could possibly mean, since

> the black hole metric is *symmetric* under time-reversal:

>

> ds^2 = -(1-2m/r) dt^2 + 1/(1-2m/r) dr^2 + r^2 dOmega^2
IMHO it only refers to deciding whether d/dt is future- or

past-pointing (aka. "time orientation of a Lorentz manifold"). This

flips the positively-time-oriented basis which means the metric tensor

"flips" too (becuse its components don't). Confusingly, the components

in the Eddington-Finkelstein basis do change upon t->-t. Extending

certain geodesics back in time into the interior works only if d/dt is

past-pointing, hence the singularity must be repelling.

I always had this problem with white holes in the sense that what

conceivable principle could determine WHAT comes out of them? A piano?

A sperm whale? Ketchup?

--

Jan Bielawski